Northeast Fisheries Science Center Reference Document 02-11
Status
of the Northeast U.S. Continental Shelf Ecosystem:
A Report of the Northeast Fisheries Science Center's
Ecosystem Status Working Group
by Jason S. Link1 and Jon K.T. Brodziak1, editors;
with contributions from (listed alphabetically): Jon K.T. Brodziak1,
David D. Dow1, Steven F. Edwards1, Mary C. Fabrizio2,
Michael J. Fogarty1, Devorah Hart1, Jack W. Jossi3,
Joseph Kane3, Kathy L. Lang1, Christopher M.
Legault1, Jason S. Link1, Sharon A. MacLean3,
David G. Mountain1, Julia Olson1, William J.
Overholtz1, Debra L. Palka1, and Tim D. Smith1
1 National Marine Fisheries Serv.,
166 Water St., Woods Hole, MA 02543
2 National Marine Fisheries Serv.,74 Magruder Rd., Highlands,
NJ 07732
3 National Marine Fisheries Serv., 28 Tarzwell Dr., Narragansett,
RI 02882
Print
publication date August 2002;
web version posted September 9,
2002
Citation: Link, J.S.; Brodziak, J.K.T., editors, and Brodziak, J.K.T.; Dow, D.D.; Edwards, S.F.; Fabrizio, M.C.; Fogarty,
M.J.; Hart, D.; Jossi, J.W.; Kane, J.; Lang, K.L.; Legault, C.M.; Link, J.S.; MacLean, S.A.; Mountain, D.G.;
Olson, J.; Overholtz, W.J.; Palka, D.L.; Smith, T.D., contributors. 2002. Status of the Northeast U.S.
Continental Shelf Ecosystem: a report of the Northeast Fisheries Science Center's Ecosystem Status Working
Group. Northeast Fish. Sci. Cent. Ref. Doc. 02-11; 245 p.
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Executive
Summary
We describe trends and conditions within the Northeastern
U.S. continental shelf ecosystem. Conceptual models of ecosystem processes
and working hypotheses about their interrelationships are identified.
While interpreting information on the status of various ecosystem attributes
is a complex process, we believe the documentation within this report
provides a useful first step towards implementing ecosystem-based fisheries
management (EBFM) within this ecosystem. The principal objective of
this report is to characterize the state of the northeastern continental
shelf ecosystem using a vast array of available data.
Most of the data in this report were collected by the Northeast
Fisheries Science Center (NEFSC). The NEFSC conducts long-term scientific
monitoring of trends in living marine resources, ranging from zooplankton
to fish to whales, and of abiotic conditions (e.g., physical oceanography),
within the Northeastern U.S. continental shelf ecosystem. The NEFSC
bottom trawl survey (BTS) has been conducted since the 1960s. The BTS
has used a single, standardized, depth-stratified random design to
measure the distribution, abundance, and size-, and age-compositions
of fish populations as well as collect oceanographic data during spring
and fall seasons. Fish stomachs have been sampled during BTSs since
the early-1970s to examine trophic ecology. Several other surveys (e.g.,
Marine Resources Monitoring, Assessment, and Prediction [MARMAP], Ecosystem
Monitoring [ECOMON]) were also initiated in the 1970s to provide information
on chlorophyll a levels, 14C primary production, zooplankton and ichthyoplankton
abundance, and inorganic nutrients along transects perpendicular to
the coastline. Data collected during these surveys were augmented with
data from the Ships of Opportunity Program (SOOP), which used continuous
plankton recorders aboard commercial vessels steaming from Boston,
MA to Cape Sable, NS, Canada and from New York City to Bermuda to measure
plankton and hydrographic variables. Other information was gathered
from resource surveys for sea scallops, surf clams, whales, benthos,
and special projects that have been conducted over the past four decades.
In addition to these fishery-independent survey data, the NEFSC has
collected fishery-dependent data from catch sampling at port, at-sea
observer sampling, fishery logbook reports, and commercial and recreational
fishery landings statistics since the 1960s. These fishery-dependent
data provide the basis for many of the socio-economic factors we examine.
Substantial changes occurred within this ecosystem during
the late-1970s to early-1980s when many abiotic, biotic, and human
metrics exhibited coincident increases or decreases. Potential mechanisms
for the observed changes are identified, with multiple working hypotheses
provided where appropriate. For example, there appears to have been
a shift in relative biomass between the demersal and pelagic fish communities,
as demersal abundance has declined and pelagic abundance increased.
Potential changes due to a shift from a cooler to a warmer temperature
phase and due to a shift from low to high fishing effort may also be
important.
These observations should provide the basis for future
process-oriented research or multivariate approaches to further examine
potential causal relationships between biotic, abiotic, and socioeconomic
variables. We conclude with a list of working hypotheses which, if
addressed, should help to quantify the status of this ecosystem for
EBFM.
II. Introduction
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A. Why this topic?
Ecosystem-based fisheries management (EBFM) has generated a lot of
scientific interest in recent years (see Link 2002b for an overview).
Many factors have contributed to the recent focus on EBFM, including
conflict among stakeholders, conflict between legislative requirements,
ongoing debate over the most important processes in marine ecosystems,
and recognition of the limitations of single species management. The
relative effects of multi-species predator-prey interactions, intra-
and interspecific competition, and changing oceanographic conditions
are important scientific issues that could hinder the near-term application
of EBFM. These ongoing issues are certainly not novel (e.g., Baird 1873;
Lankester 1884; Lotka 1925; Volterra 1926). Further, while several approaches
to address broader considerations in a fisheries context were proposed
in the 1970s and 1980s (e.g., Steele 1974; Andersen and Ursin 1977; May
et al. 1979; Mercer 1982; Kerr and Ryder 1989; Daan and Sissenwine 1991),
many basic issues still remain unaddressed.
Recently, some progress has been made in defining terms for EBFM, providing
rationale for using a more holistic management approach, and in particular,
answering when, why, and how EBFM can be practically implemented in a
fisheries context (e.g., Larkin 1996; Jennings and Kaiser 1998; Hall
1999; ICES 2000; Link 2000; NMFS 2000; Link 2002a, 2002b; Brodziak and
Link 2002). To date, there are few empirical descriptions of fisheries
ecosystems (see, for example, AFSC; Livingston 1999, 2000). Yet the direct
implementation of broader ecosystem considerations has not become widespread
in fisheries management even though they have been advocated (NMFS 1999;
NRC 1999; ICES 2000) and even mandated in recent years (NOAA 1996). There
are no clear protocols for actually implementing EBFM and some questions
of feasibility and definition are still unaddressed. However, implementation
will be via iteration and sequential improvement. To this end, the Group
has focused on documenting the status of the northeast U.S. continental
shelf ecosystem as an essential first step to facilitate the development
of an operational approach to ecosystem-based fisheries management.
B. General background of the Working Group
The core of our Ecosystem Status Working Group (hereafter, the Group)
started out approximately in mid-1998 as a reading group for interested
staff from the NEFSC to keep abreast of current issues in fisheries science
and management. After reading and discussing and numerous literature
articles on the subject, including Steve Hall's (1999) book on the topic,
the Group realized that we could make a positive contribution towards
the implementation of ecosystem-based fisheries management. Since the
NEFSC has some of the world's premier time series of fisheries independent
data, on subjects ranging from species abundance to zooplankton biomass
to food habits to temperature, the Group thought it would be very useful
to assemble these data to document the current status and recent history
of the northeastern U.S. continental shelf ecosystem.
Our first objective was to assemble the diverse, multi-disciplinary
sets of time series that exist at the NEFSC in detail (Table
2.1). This document describes those abiotic, biotic and human metrics.
For a list of these metrics, see Table 2.1. Our
second objective was to compare these metrics. We compiled these diverse
datasets in common formats amenable for easy comparison. From this compilation,
we produced a set of simple, common, general observations. Our third
objective was to synthesize the information into a set of working hypotheses
that can serve as a basis for future detailed examinations.
C. New England fisheries: Case study for ecosystem-based
fisheries management
The substantial changes in New England fisheries over the past several
decades, and in particular groundfish fisheries, have been associated
with excessive fishing pressure (Serchuk
et al. 1994; Murawski
et al. 1997; Boreman et al. 1997; NEFSC 1998a; Fogarty and Murawski 1998).
The abundance of commercially desirable gadids (Atlantic cod, Gadus
morhua, and haddock, Melanogrammus aeglefinus) as well
as flatfish (yellowtail flounder, Limanda ferruginea, American
plaice, Hippoglossoides platessoides, and winter flounder, Pseudopleuronectes
americanus) has declined with a concurrent increase in the abundance
of elasmobranchs (spiny dogfish, Squalus acanthias, and skates, Raja spp.)
and small pelagic fishes. Changes
in the fish community structure began occurring in the 1950s and 1960s
with the arrival of distant water fleets and subsequent increase in fishing
pressure exerted on the major gadid and flatfish stocks. As a result
of the dramatic increase in landings (and presumably high discards),
the estimated total biomass of these stocks declined by at least 50%.
After the foreign fleets were displaced from the U.S. Exclusive Economic
Zone (EEZ), moderate increases in stock sizes were observed in the late-1970s
to early-1980s. Capacity and efficiency of the domestic fleet increased
during the 1980s, however, and this led to subsequent declines in groundfish
abundance. Groundfish abundance plummeted to historic lows in the 1990s,
although abundances of some stocks have increased in recent years under
restrictive fishery management measures. Yet some groundfish stocks,
such as cod, have remained at low abundance. Many
groundfish stocks on Georges Bank exhibited classic signs of overfishing
in recent decades, including declines in abundance, faster growth, earlier
age-at-maturity, and a truncated size structure (NEFSC 1998a, 1998b;
reviewed in Jennings and Kaiser 1998). However, much less in known about
the indirect and secondary effects of intense fishing pressure on the
fish community in this and most marine ecosystems (Hall 1999; ICES 2000).
Further, how fishing pressure affects other aspects of the northeast
U.S. continental shelf ecosystem are generally not known. In this context,
we hope to provide some insights on the issue of indirect effects, particularly
in the context of the fishing and environmental changes that have occurred
in this ecosystem.
D.
Spatial delineation of northeastern U.S. continental shelf ecosystem
We use ecosystem to refer
to the combination of physical processes and organisms existing within
the spatial range of the system, taken together as a whole. The spatial
range of the northeastern U.S. continental shelf ecosystem includes
the estuarine and oceanic waters to depths of approximately 200 m from
a southern boundary at Cape Hatteras, North Carolina to a northeastern
boundary at the beginning of the Scotian Shelf (<100 m depth) in
the northeastern Gulf of Maine through the Northeast Channel separating
Georges Bank from Browns Bank and the Scotian Shelf (Figure
2.1). It is also commonly referred to as the Northeast Large
Marine Ecosystem (LME; Sherman1991, Sherman et al. 1993). This ecosystem
is an open oceanic system that is part of the northwestern Atlantic
continental shelves province, which is a much larger oceanic region
consisting of continental shelf and slope water from Florida to the
Grand Banks of Newfoundland (Longhurst 1998). Within this ecosystem,
we define four subdivisions with distinct hydrography and biota: the
Mid-Atlantic Bight, Southern New England, Georges Bank, and the Gulf
of Maine-Bay of Fundy. We will provide metrics to describe system attributes
at several spatial scales, ranging from individual estuaries to subdivisions
to the entire northeastern U.S. continental shelf ecosystem.
E.
Temporal extent and resolution
Many of the metrics we examined
are derived from the NEFSCs spring and fall bottom trawl survey (Grosslein
1969; Azarovitz 1981; NEFC 1988). These extend back to 1968 and 1963,
respectively, and are maintained to the present time. Several other
time series (e.g., MARMAP, SOOP, food habits, vessel landings) are
available for the same time period. We present a suite of over 100
metrics, many of which span 25 - 40 years (Table 2.1).
Metrics with short time series have been included even though they
may represent only snapshots of particular system attributes, however,
most of the metrics provide information on annual or interannual time
scales. Although some data were available to examine seasonal contrasts,
we did not require this level of resolution to document the status
of the ecosystem.
F.
References
Andersen,
K.P., and Ursin, E.. 1977. A multispecies extension to the Beverton
and Holt theory, with accounts of phosphorus circulation and primary
production. Meddelelser fra Danmarks Fiskeri-og Havundersogelser N.S.,
7:319-435.
Azarovitz,
T.R. 1981. A brief historical review of the Woods Hole Laboratory trawl
survey time series. In: Bottom Trawl Surveys. Doubleday W.
G. and Rivard D. Canadian Special Publications in Fisheries and Aquatic
Sciences 58:62-67.
Baird, S.F. 1873. Report
on the condition of the sea fisheries of the south coast of New England
in 1871 and 1872. Report of the United States Fish Commission, Vol.
1. GPO, Washington, D.C.
Boreman, J., Nakashima,
B.S. Wilson, J.A., and Kendall, R.L. (Eds), 1997. Northwest Atlantic
groundfish, perspectives on a fishery collapse. American Fisheries
Society, Bethesda, MD.
Brodziak, J. and Link, J.
2002. Ecosystem Management: What is it and how can we do it? Bull.
Mar. Sci. 70(2):589-611.
Daan, N., and Sissenwine,
M.P. (eds). 1991. Multispecies models relevant to management of living
resources. ICES Marine Science Symposium 193.
Fogarty, M.J., Murawski,
S.A., 1998. Large scale disturbance and the structure of marine systems,
fishery impacts on Georges Bank. Ecol. Appl., 8(1):Supplement S6-S22.
Grosslein. M.D. 1969. Groundfish
survey program of BCF Woods Hole. Comm. Fish. Rev. 31(8-9):22-25.
Hall, S.J. 1999. The effects
of fishing on marine ecosystems and communities. Blackwell Science,
Oxford, England.
ICES (International Council
for the Exploration of the Sea). 2000. Ecosystem effects of fishing.
ICES Journal of Marine Science Volume 57, Number 3. 791 pp.
Jennings, S., Kaiser, M.J.
and Reynolds, J.D. 2001. Marine Fisheries Ecology. Blackwell Science,
Oxford, England.
Jennings, S. and Kaiser,
M.J. 1998. The effects of fishing on marine ecosystems. Advances in
Marine Biology 34:201-352.
Kerr, S.R. and Ryder, R.A.
1989. Current approaches to multispecies analysis of marine fisheries.
Canadian Journal of Fisheries and Aquatic Sciences 46:528-534.
Lankester, E.R. 1884. The
scientific results of the exhibition. Fisheries Exhibition Literature
4:405-42.
Larkin, P.A. 1996. Concepts
and issues in marine ecosystem management. Reviews in Fish Biology
and Fisheries 6:139-164.
Link,
J. 2000. Fisheries management in an ecosystem context: what does this
mean, what do we want, and can we do it? Proceedings of the 6th National
Stock Assessment Workshop: Incorporating Ecosystem Considerations into
Stock Assessments and Management Advice. NOAA Tech. Memo. NMFS-F/SPO-46.
pp. 5-11.
Link, J.S. 2002a. Ecological
Considerations in Fisheries Management: When Does It Matter? Fisheries
27(4):10-17.
Link, J.S. 2002b. What Does
Ecosystem-Based Fisheries Management Mean? Fisheries 27(4):18-21.
Livingston, P.A. (ed.).
1999. Ecosystem Considerations for 2000. Appendix: Stock Assessment
and Fishery Evaluation Report for the Groundfish Resources of the Eastern
Bering Sea, Aleutian Islands, and Gulf of Alaska. North Pacific Fishery
Management Council, 605 W. 4th Ave, Suite 306, Anchorage,
AK, 99501. 140 pp.
Livingston, P.A. (ed.).
2000. Ecosystem Considerations for 2001. Appendix: Stock Assessment
and Fishery Evaluation Report for the Groundfish Resources of the Eastern
Bering Sea, Aleutian Islands, and Gulf of Alaska. North Pacific Fishery
Management Council, 605 W. 4th Ave, Suite 306, Anchorage,
AK, 99501. 119 pp.
Longhurst, A. 1999. Ecological
geography of the sea. Academic Press, New York, 398 p.
Lotka, A.J. 1925. Elements
of physical biology. Williams and Wilkins, Baltimore, MD.
May, R.M., J.R. Beddington,
C.W. Clark, S.J. Holt, and R.M. Laws. 1979. Management of multispecies
fisheries. Science 205-267-277.
Mercer, M.C. 1982. Multispecies
approaches to fisheries management advice. Canadian Special Publication
in Fisheries and Aquatic Science 59.
Murawski,
S.A., Maguire, J.J., Mayo, R.K., Serchuk, F.M. 1997. Groundfish stocks
and the fishing industry. pp. 27-70, In: Boreman J., Nakashima
B.S. , Wilson J.A., Kendall R.L. (eds.) Northwest Atlantic groundfish:
perspectives on a fishery collapse. American Fisheries Society, Bethesda,
Maryland, USA.
Murawski, S.A. 2000. Definitions
of overfishing from an ecosystem perspective. Symposium proceedings
on the ecosystem effects of fishing, ICES Journal of Marine Science
57:649-658.
NEFC
(Northeast Fisheries Center). 1988. An evaluation of the bottom trawl
survey program of the Northeast Fisheries Science Center. NOAA Tech.
Memo. NMFS-F/NEC-52.
NEFSC, (Northeast Fisheries
Science Center) 1998a. Status of the Fishery Resources off the Northeastern
United States. NOAA Tech. Memo. NMFS-NE-115. 149 pp.
NEFSC. 1998b. 27th Northeast
Regional Stock Assessment Workshop. NEFSC Center Reference Document
98-15. 350 pp.
NRC (National Research Council).
1999. Sustaining Marine Fisheries. National Academy Press, Washington,
D.C.
NMFS (National Marine Fisheries
Service). 1999. Ecosystem-based fishery management. A report to Congress
by the Ecosystems Principles Advisory Panel. US Department of Commerce,
Silver Spring, MD.
NMFS. 2000. Proceedings
of the 6th National Stock Assessment Workshop: Incorporating
Ecosystem Considerations into Stock Assessments and Management Advice.
NOAA Tech. Memo. NMFS-F/SPO-46.
NOAA
(National Oceanic and Atmospheric Administration). 1996. Magnuson-Stevens
Fishery Management and Conservation Act amended through 11 October
1996. NOAA Technical Memorandum NMFS-F/SPO-23.
Serchuk, F.M., Grosslein,
M.D., Lough, R.G., Mountain, D.G., and O'Brien, L. 1994. Fishery and
environmental factors affecting trends and fluctuations in the Georges
Bank and Gulf of Maine Atlantic cod stocks: an overview. ICES Mar.
Sci. Symp. 198, 77-109.
Sherman, K. 1991. The large
marine ecosystem concept: research and management strategy for living
marine resources. Ecological Applications 1:349-360.
Sherman, K., Alexander,
L.M. and Gold, B.D. (eds). 1993.
Large marine ecosystems: stress, mitigation and sustainability. AAAS
Press, Washington, D.C.
Steele, J.H. 1974.
The structure of marine ecosystems. Blackwell Scientific Publ., Oxford,
U.K.
Volterra, V. 1926. Fluctuations
in the abundance of a species considered mathematically. Nature 118:558-560.
III. ABIOTIC METRICS
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A. Geology, Chemistry
Geologic and chemical features significantly influence the physical
and biological components of this ecosystem. Although data on these factors
exists, few time series are available.
We do not include geological metrics in this report because the extant
data and expertise in this area reside with the United States Geological
Survey. From an ecosystem perspective, we need definitions of major geologic
regions, including the distributions of major sediment/bottom types,
and delineations of high/low energy areas in the ecosystem. Some information
on the marine geology of the region is summarized in Backus (1987). Time
series of geological characteristics may not be essential for understanding
ecosystem dynamics, particularly in the context of living resources.
Because these issues are beyond our expertise, they should be considered
(and currently are) in collaboration with the USGS.
In the case of chemical metrics, we need to identify key chemical indicators
from an array of important nutrients, metals, and toxins. We also need
to be able to track their concentrations through time and space. Few
time series data exist for these chemicals. Some chemicals have been
sampled by our Highlands, NJ Laboratory over time at particular locations.
However, we do not know the spatial extent and resolution of sampling
needed to synoptically understand how these chemicals influence ecosystem
dynamics. Important questions to address include:
how often do we need to sample, what selection of representative chemicals
should we monitor, and what are the major gaps of information? These
questions need to be addressed before we can develop chemical metrics
for this ecosystem.
B. Physical
1. NAO Index
Time: 1823-2000
Spatial: North Atlantic Ocean
Contributed by: Brodziak
Figures A.1 and A.2
Methodology and Data Source
The NAO index is calculated as the air pressure difference between
sites in Iceland and southern locations at the Azores or Gibralter (Jones
et al. 1997). The NAO index time series was computed using data available
from the Climate Research Unit at the University of East Anglia. This
data may be accessed at http://www.cru.uea.ac.uk/.
The NAO winter index is reported here. In year y, the NAO winter index
is the arithmetic average of monthly NAO values for December in year
y and January-March in year y+1. The winter index is available for 1823-2000.
The five-year moving average of the NAO index in year y is computed as
the arithmetic average of NAO values in years y-2, y-1, y, y+1, and y+2;
the five-year average is available for 1825-1998.
Key Points and Major Observations
The North Atlantic Oscillation (NAO) is one of the major features of
the global climate system. There is an upward trend in the NAO from the
1960s to the early 1990s. The NAO index is highly variable and the largest
recorded interannual change in the NAO index occurred from 1994 to 1995.
The latitude of the Gulf Stream has been correlated with the NAO over
the last 30 years (Taylor et al. 1998). Large positive NAO values are
associated with colder air and stronger winds over the North Atlantic
and a larger cold intermediate water layer on the Labrador Shelf. Large
negative NAO values are associated with warmer air and weaker winds over
the North Atlantic and a smaller cold intermediate water layer on the
Labrador Shelf.
2. Shelf wide Temperature anomaly
Time: 1963-2000
Spatial: Shelf wide
Contributed by: Mountain
Figure A.3
Methodology and Data Source
These are the surface and bottom temperature anomalies for NMFS fall
bottom trawl survey, averaged over the whole shelf region from Cape Hatteras
through the Gulf of Maine (Holzwarth and Mountain 1992; Taylor and Bascunan
2001). For each temperature observation made on a survey, its anomaly
was determined by comparison with annual cycles of temperature derived
from the MARMAP program (1978-1987). This procedure takes into account
the day of the year on which the observation was made and its specific
location. All of the anomaly values for a survey were averaged on an
area weighted basis to determine the values plotted.
Key Points and Major Observations
The variability of 2-4 degrees C has been consistently observed over
the past four decades. The late 1960s were a particularly cold period.
The 1990s appear to be slightly warmer than preceding decades.
3. MAB Volume, Salinity & Temperature
anomaly
Time: 1977-2000
Spatial: Mid-Atlantic Bight
Contributed by: Mountain
Figure A.4 (a-c)
Methodology and Data Source
The volume and average temperature and salinity of Shelf Water in the
MAB have been determined for each NEFSC cruise that made temperature
and salinity observations through the MAB area (Mountain 1991). Shelf
Water is defined as water with salinity < 34 PSU, and is in contrast
to the oceanic Slope Water that is found seaward of the shelf/slope front.
From the surveys averaged values for the volume, temperature and salinity
of Shelf Water in the MAB, annual cycles were derived for each variable.
Anomalies for each variable were determined relative to these characteristic
annual cycles
Key Points and Major Observations
There is very large variability in the amount of Shelf Water in the
MAB. Additionally, there is large variability in the salinity of the
Shelf Water in the MAB. The Shelf Water volume in the 1990s was substantially
higher than in the 1980s and the salinity in the 1990s was lower than
in the 1980s. The source of the volume and salinity variations is largely
advective from the Gulf of Maine - and from variation in the inflows
to the Gulf.
4. Surface and Bottom Temperature anomalies
Time: 1963-2000
Spatial: All the major subregions
Contributed by: Mountain, Brodziak
Figure A.5 (a-h)
Methodology and Data Source
These are the surface and bottom temperature anomalies for NMFS bottom
trawl survey, averaged for each of the major subregions (Holzwarth and
Mountain 1992; Taylor and Bascunan 2001). For each temperature observation
made on a survey, its anomaly was determined by comparison with annual
cycles of temperature derived from the MARMAP program (1978-1987). This
procedure takes into account the day of the year on which the observation
was made and its specific location. All of the anomaly values for a survey
were averaged on an area weighted basis to determine the values plotted.
Key Points and Major Observations
There is large variability in the surface and bottom temperatures in
each region. The late 1960s were a particularly cold period. Little trends
are observed in any region through the 1970s and 1980s, although there
may be slightly warmer waters in the 1990s for a few regions. The differences
between the regions show no consistent pattern.
5. MAB Temperature anomalies, by 5 provinces
Time: 1990s, Annual, composite average
Spatial: Mid-Atlantic Bight
Contributed by: Mountain
Figure A.6
Methodology and Data Source
The shelf water temperature anomalies during the 1990s for five regions
of the MAB (from north to south) have been averaged for three periods
of the year (in essence, for thirds of the year) (Mountain 2001). The
anomalies are relative to the MARMAP period (1978-1987). The methods
for determining the shelf water anomalies were describe earlier.Key
Points and Major Observations
During the winters of the 1990s the MAB became progressively warmer
from north to south as compared to the MARMAP period. The summer period
exhibited some cooling in the central MAB. The fall period was generally
a bit warmer than the MARMAP period. Overall, the MAB was about 1 C warmer
in the 1990s than during the MARMAP period.
6. Massachusetts Bay Surface Temperature,
Surface Salinity, Bottom Temperature Anomalies
Time: 1978-2000
Spatial: Massachusetts Bay
Contributed by: J. Jossi
Figure A.7
Methodology and Data Source
These data were collected as part of the MARMAP Ships of Opportunity
Program (Benway et al. In Review; Jossi et al. In Review). Expendable
bathythermograph and surface salinity measurements were taken monthly
by merchant vessels between Boston, MA and Cape Sable, NS. Values were
gridded in time and space (distance along transect). Grids of long term
means and standard deviations; and single year conditions, anomalies,
and standardized anomalies are produced. Grids were sliced through time
at a distance representing Massachusetts Bay for this portrayal, which
also shows a smooth curve based on a 15 month running average (Benway
et al 1993). The location chosen to represent Massachusetts Bay was at
48 km reference distance, or approximately 70o 20'W, along
the transect.
Key Points and Major Observations
Surface Temperature- With the exception of isolated monthly departures
near, or in excess of two standard deviations, the period 1978 through
1988 exhibited no enduring anomalous surface temperatures. From 1992
to mid-1994 mostly colder than average conditions prevailed. No trend
during the time period was apparent.
Surface Salinity- Salinities generally increased from 1978 through
1980, declined through 1984 to a period minimum, rose sharply in 1985,
were below average in 1987, and after 1990 they again declined to the
end of the sampling period in 1993. The longest sustained anomalous period
was that of low salinities in 1983 and 1984.
Bottom Temperature- From 1978 to 1981 values were near normal. Higher
temperatures occurred during 1982 and 1983 followed by near average values
in the mid-1980s. From 1987 through 1990, and 1992 to 1994 values were
generally negative, after which departures became inconsistent, with
several significantly warm months. Departures in the late 1990s were
more excessive than in the earlier period, and might result in a warming
trend for these data.
7. Mid-Atlantic Bight Surface Temperature,
Surface Salinity, Bottom Temperature Anomalies
Time: 1978-2000
Spatial: Mid-Atlantic Bight and mid-Continental Shelf
Contributed by: J. Jossi
Figure A.8
Methodology and Data Source
These data were collected as part of the MARMAP Ships of Opportunity
Program (Benway et al. In Review; Jossi et al. In Review). Expendable
bathythermograph and surface salinity measurements taken monthly by merchant
vessels along a transect from New York City towards Bermuda to the Gulf
Stream. Values were gridded in time and space (distance along transect).
Grids of long term means and standard deviations; and single year conditions,
anomalies, and standardized anomalies are produced. Grids were sliced
through time at a distance representing the continental shelf, generally
unaffected by river runoff and/or slope water, for this portrayal. The
portrayal also shows a smooth curve based on a 15 month running average
(Benway et al. 1993). The location chosen to represent the Middle Atlantic
Bight was at 101 km reference distance, or approximately 40o N;
73o W, along the transect.
Key Points and Major Observations
Surface Temperature- Isolated months through the 1978-2000 time period
exhibit significant departures from the 1978-1990 means. Departures in
excess of 2 standard deviation were more numerous in the 1990s than in
the previous years, even after adjustments to account for the 1990s not
being included in the base period. Sequential, monthly positive or negative
departures were more consistent in the 1990s than in previous years.
Finally, the surface temperatures appear to be trending upwards between
1978 and 2000.
Surface Salinity- Isolated months exhibit significant departures during
the time period, and are more prevalent in especially the late 1990s
than earlier periods. There is more month-to-month consistency of the
surface salinity departures than of the surface temperatures. Uninterrupted
positive departures of two years (1980-1981; 1985-1986; 1994-1995), and
negative departures of two to three years (1996-1998; 1998-1999) occurred.
No trend during the time period was apparent.
Bottom Temperature- Greater departures in the 1990s also occurred in
the bottom temperature data. Aside from beginning the time period in
a negative phase and ending in a positive phase, a possible trend is
not as clear as with surface temperature. However, the phase changes
of the smoothed values are quite similar through the time period for
these two features.
8. W. Gulf of Maine Surface Temperature,
Surface Salinity, Bottom Temperature Anomalies
Time: 1978-2000
Spatial: Gulf of Maine
Contributed by: J. Jossi
Figure A.9
Methodology and Data Source
These data were collected as part of the MARMAP Ships of Opportunity
Program (Benway et al. In Review; Jossi et al. In Review). Expendable
bathythermograph and surface salinity measurements taken monthly by merchant
vessels along a transect from Boston, MA to Cape Sable, NS.. Values were
gridded in time and space (distance along transect). Grids of long term
means and standard deviations; and single year conditions, anomalies,
and standardized anomalies are produced. Grids were sliced through time
at a distance representing approximately Wilkinson Basin for this portrayal.
The portrayal also shows a smooth curve based on a 15 month running average
(Benway et al. 1993). The location chosen to represent the western Gulf
of Maine was at 165 km reference distance, or approximately 68o 55'
W along the transect.
Key Points and Major Observations
Surface Temperature- Variations from 1978 through 1990 followed a similar
pattern to those for surface temperature in Massachusetts Bay, except
that they were of slightly larger magnitude. High values occurred from
1983 to 1985, and low values occurred in 1982, for a fairly prolonged
period from 1986 to 1991, and again from mid-1991 to 1994. This was followed
in 1996 and 1997 by the lowest temperatures of the period, from which
point temperatures began increasing to reach the highest values of the
period by 2000. Neither of these last two conditions were seen to any
extent in Massachusetts Bay. No trend was apparent, although the last
four years of the period exhibited a dramatic increase.
Surface Salinity- The western Gulf of Maine surface salinity pattern
follows that of Massachusetts Bay very closely. The only major exception
was that in the western Gulf of Maine the 1985 high persisted to the
beginning of 1987. No trend was apparent during the time period.
Bottom Temperature- Patterns here were also very similar to those for
bottom temperature in Massachusetts Bay, although the departures were
of less magnitude. Time period low occurred in late-1994 followed by
the series high in 1995. Similarly, variations were larger in the late-1990s
than earlier in the period. No trend was apparent.
9. Relationships
Among NAO, Salinity, Plankton, and Cod on Georges Bank
Time: 1970-1996
Spatial: Georges Bank
Contributed by: Mountain
Figure A.10
Methodology and Data Source
The early spring plankton displacement volume on Georges Bank is compared
with a detrended, inverted NAO series and with salinity variability on
the bank (Mountain et al. 2000). A cod survival index (ratio of the number
of recruits to the spawning stock biomass, with both series hanned before
the ratio was taken) is also compared with the detrended NAO series.
The plankton displacement volume series was determined by J. Kane from
the Center's plankton survey data. The salinity anomalies were derived
from the Center temperature and salinity data, relative to annual cycles
of salinity derived from the MARMAP data set. The cod series were from
stock assessment documents. The NAO was from a NAO website. The method
was straight forward of plotting the predetermined series.
Key Points and Major Observations
The displacement volume appears to follow the detrended NAO and the
salinity variability quite well. There are large interannual differences
in the displacement volume. The cod survivorship series also seems to
follow the NAO quite well. There are no obvious processes that connect
these series.
C. Summary of Abiotic Metrics
Various graphics of temperature and salinity data from Ship-of-Opportunity
(SOOP) data and shelfwide research cruise data were examined. Preliminary
examination of the average of surface and bottom temperatures from the
Autumn Bottom Trawl data, shelf-wide for all regions from Cape Hatteras
to Nova Scotia, showed the 1960s were cold and the remaining years were
variable without any apparent trend. It is questionable if the 1990s
were slightly warmer than preceding decades. When these data are sorted
out spatially into subregions, they exhibit a similar pattern.
Data on the volume of water, salinity and temperature were examined
for the Mid-Atlantic Bight (MAB) shelf water inside the shelf/slope boundary.
In the 1990s, the following were observed: 1) a 25-30% increase in the
amount of shelf water volume in the Bight was apparent over that of the
long term mean; 2) salinity was lower in the 90s; similar to observations
made for northwestern Georges Bank; and 3) temperature was about 1 degree
warmer in the 90s.
The MAB data were broken out into shelf sectors (SNE, NYB1, NYB2, SS1,
SS2, north to south orientation). It was noted that the apparent warming
in the MAB in the 1990s concentrated in the southern regions (SS1, SS2)
during the winter. Atmospheric heat flux seems the likely source and
needs to be investigated. Further, there is some indication that advective
events present in the Gulf of Maine (GOM) have affected SNE and NYB temperature
and salinity. For example, GLOBEC data indicates a shift in the basic
circulation into GOM from 1 part Scotian Shelf water and 2 parts Oceanic
current, to 2 parts Scotian Shelf and 1 part Oceanic water. Documentation
of changes in the major inflows into the GOM is needed.
Given the extent of the variability in the data, what metrics are useful
to see system-wide changes? Several data sets were examined relative
to the detrended North Atlantic Oscillation (NAO) which shows significant
3-5 year variability over a strong 30 year trend. Large changes in zooplankton
volume occurred over the 1970-1995 period. Volumes decreased in the early
1980s, followed by a large increase in 1985-1990 period. Plankton volume
fluxes correlated with the detrended, inverse of the NAO (see chapter
4 for further details). Plankton volume and salinity anomalies may have
a relationship and other covarying parameters may exist. These relationships
merit further examination. Additionally, an index of cod recruitment
and standing stock biomass (SSB) data correlate with the detrended, inverted
NAO data. Possible relationships between the cod survival anomaly, the
SSB and detrended NAO data also merit examination. Chlorophyll data is
also needed to help corroborate production, particulary of the plankton
(i.e., volume) and the NAO trends.
No linkage is apparent between offshore waters and the NAO events of
the 1960s through the 1990s, however, the linkage between coastal water
temperatures and NAO needs to be examined.
D. References
Backus, R.H. 1987. Georges Bank. MIT Press, Cambridge, Massachusetts.
Benway, R.L. and Jossi, J.W. In Review. Ships of opportunity (SOOP)
sampling. In: Jossi, J.W. and Griswold, C.A. (eds.) In Review.
MARMAP Ecosystem Monitoring: Operations Manual. NOAA Technical Memorandum
NMFS-F/NEC.
Benway, R.L., Jossi, J.W., Thomas, K.P., and Goulet, J.R. 1993. Variability
of temperature and salinity in the Middle Atlantic Bight and Gulf of
Maine. NOAA Technical Report NMFS, 112.
Holzwarth, T. and Mountain, D. 1992. Surface and bottom temperature
distributions from the Northeast Fisheries Center spring and fall bottom
trawl survey program, 1963-1987, with addendum for 1988-1990. National
Marine Fisheries Service, Northeast Fisheries Science Center, Center
Reference Document 90-03.
Jones, P.D., Jonsson, T. and Wheeler, D. 1997. Extension of the North
Atlantic Oscillation using early instrumental pressure observations from
Gibralter and southwest Iceland. Int. J. Climatol. 17:1433-1450.
Jossi, J.W., Benway, R.L., and Goulet, J.R. In Review. MARMAP Ecosystem
Monitoring: Program Description. NOAA Technical Memorandum NMFS-F/NEC.
Mountain, D. 1991. The volume of shelf water in the Middle Atlantic
Bight: seasonal and interannual variability, 1977-1987. Continental Shelf
Res., 11, 251-267.
Mountain, D. 2001. Variability in the properties of Shelf Water in
the Middle Atlantic Bight, 1977-1999. JGR (submitted).
Mountain, D., Kane, J. and Green, J.. 2000. Environmental forcing and
variability in zooplankton abundance and cod recruitment on Georges Bank.
ICES CM 2000/M:15.
Taylor, A.H. and Stephens, J.A. 1998. The North Atlantic Oscillation
and the latitude of the Gulf Stream. Tellus, 50A: 134-142.
Taylor, M.H. and Bascunan, C. 2001. Description of the 2000 oceanographic
conditions on the northeast continental shelf. National Marine Fisheries
Service, Northeast Fisheries Science Center, Center Reference Document
01-01.
IV. BIOTIC METRICS
(Click here for PDF Version)
A. Phytoplankton
1. US Northeast Continental Shelf Ecosystem, Chlorophyll-a
Time: 1977-1988
Spatial: US Northeast Shelf Ecosystem (Shelf wide)
Contributed by: J. Jossi and J.E. O'Reilly
Methodology and Data Source
These data were collected as part of the MARMAP Program. Six to twelve
research vessel surveys/year undertook water column sampling of phyto-pigments
in the euphotic zone (O'Reilly and Zetlin 1998).
Key Points and Major Observations
Fifty-seven thousand eighty-eight measurements were made during 78
oceanographic surveys from 1977 through 1988. Extensive horizontal, vertical,
and seasonal distributions are portrayed. No time series per se has been
constructed. Not much inter-annual change in chlorophyll a is
observed.
B. Birds
We recognize that birds are an important part of this ecosystem, but
few time series data are available for these species. Although there
is some extant data, no one from the group provided data for this report.
Certainly this is an important issue to consider for some species, and
merits further examination in the future. In fact, basic questions such
as "what are the trends in abundance of major species?" remain unanswered.
How often do we need to sample to better answer these questions? What
spatial extent and resolution do we need? What are the most cost effective
methodologies?
C. Turtles
We also recognize that turtles are an important part of this ecosystem,
but few time series data are available for these species. Although there
is some extant data, no one from the group provided data for this report.
See Palka et al. (In review) for some estimates of turtle abundance for
selected years in the 1990s. Certainly this is an important issue to
consider for some species, and merits further examination in the future.
D. Benthos
In general, few time series data are available for the benthos. Classic
shelf-wide studies were conducted by Theroux and Wigley (1998). Other
studies have covered smaller areas, and synoptic, shelf-wide information
is generally lacking. However, a few components of the benthic community
are surveyed regularly.
1. Georges Bank, Mid-Atlantic Bight Scallop
Biomass, Landings, and Survey Indices
Time: 1962-1999 (Landings & Survey), 1980-2000 (Biomass)
Spatial: Georges Bank, Mid Atlantic Bight
Contributed by: Hart
Figures B.1, B.2, B.3, B.4
Methodology and Data Source
These data were collected from the NMFS sea scallop survey and landings
database. Biomass was poststratified into open and closed areas. For
further details see NEFSC (2001) and Murawski et al. (2000).
Key Points and Major Observations
Biomass was at low levels through 1994 due to increasingly severe overfishing.
This resulted in highly variable landings well below optimal levels,
driven primarily by sporadic recruitment events. After area closures
(December 1994 in Georges Bank, April 1998 in Mid-Atlantic), there was
a rapid buildup of biomass in the closed areas. The limited amount of
fishing permitted in the closed areas in 1999-2000 does not appear to
have substantially impacted the biomass there. Biomasses in open areas
have increased recently due to effort reductions and good recruitment.
Recent good recruitment on both Georges Bank and Mid-Atlantic may be
related to the increased levels of spawning-stock biomass in the closed
areas.
2. Sculpin abundance from fall bottom trawl
survey
Time: 1963 - 1998
Spatial: Southern New England and Georges Bank
Contributed by: Link
Figure B.5
Methodology and Data Source
These data were collected as part of the NEFSC Habitat Research Program
and standard bottom trawl survey. The stratified mean trawl catch per
tow (Azarovitz 1981) was calculated for this species. See Link and Almeida
(2002) for further details.
Key Points and Major Observations
Longhorn sculpin abundance peaked in the mid 1960s and then exhibited
a relatively steady period for the first 15 years of the survey. This
was followed by a period of lower abundance during the mid 1980s and
an increasing trend in the 1990s. In most years sculpin abundance ranged
from 10 to 20 fish per tow. The years with highest index of sculpin abundance
were 1966 and 1998. Relative to the several preceding years, the index
of sculpin abundance notably increased during 1966, 1987 and 1998.
3. Blue crab abundance
Time: July 1996 - October 2000 (spring, summer, and fall)
Spatial: Navesink River and Sandy Hook Bay in the mid-Atlantic region
Contributed by: Fabrizio
Figure B.6
Methodology and Data Source
These data were collected as part of the Behavioral Ecology Survey
of Demersal Species in Navesink River. Three seasonal collections were
made in the spring, summer, and fall beginning in the summer of 1996.
Demersal species were collected by replicate tows of a 1-m beam and a
5- m otter trawl at 84 stations throughout the Navesink River and Sandy
Hook Bay. Beginning in July 1998, only 24 stations were sampled throughout
this system. All fish and decapod crustaceans were enumerated and environmental
characteristics were measured. The data in the figure represent the mean
number of blue crabs per m2 across all stations in the Navesink
River and Sandy Hook Bay (Meise and Stehlik In Press).
Key Points and Major Observations
Blue crab abundance increased in 1998-1999 in the Navesink River-Sandy
Hook Bay estuarine system, but declined by 2000. These data are from
a short time series with limited spatial coverage, but are important
to the local estuarine dynamics.
E. Zooplankton
1. Central Gulf of Maine Calanus finmarchicus,
c.1-4, c.5-6 anomalies
Time: 1961-1990
Spatial: Central Gulf of Maine
Contributed by: Jossi
Figure B.7 (a-b)
Methodology and Data Source
These data were collected as part of the MARMAP Ships of Opportunity
Program (Benway et al. In Review; Jossi et al. In Review). Continuous
Plankton Recorders were towed monthly by merchant vessels along a transect
from Boston, MA to Cape Sable, NS. Zooplankton and larger phytoplankton
were captured, identified and enumerated. Abundance values were gridded
in time and space (distance along transect). Grids of long term medians,
means and standard deviations; and single year conditions, anomalies,
and standardized anomalies are produced. Grids were sliced through time
at a distance representing the central Gulf of Maine in this portrayal.
The portrayal also shows a smooth curve based on a 15 month running average
(Jossi and Goulet 1993; Pershing et al. 2001).
Key Points and Major Observations
A biphase pattern has been found in this, and several other of the
dominant zooplankton taxa of the Gulf of Maine during the 1961-1990 period
(Jossi and Goulet, 1993), and also an uptrend for the adult stages of Calanus
finmarchicus. Also, the adult stages of this taxon have exhibited
a positive (with lag) correlation with the winter North Atlantic Oscillation
(Pershing et al. 2001).
2. Anomalies of major zooplankton during
spring
Time: 1977 - 1996, Spring (15 Feb- 15 May)
Spatial: Georges Bank
Contributed by: Jossi
Figure B.8
Methodology and Data Source
These data were collected as part of the MARMAP Surveys (Benway et
al. In Review; Jossi et al. In Review). Zooplankton and larger phytoplankton
were captured, identified and enumerated. Abundance values were gridded
in time and space (distance along transect). Single year conditions,
anomalies, and standardized anomalies are produced.
Key Points and Major Observations
The community composition has changed notably over time. Yet there
are no apparent trends in total zooplankton abundance and no major departures
from zero even though predator biomass has changed greatly during the
time period.
3. Time and space conditions of Centropagus
typicus across the continental shelf
Time: 1976 - 1990, averaged
Spatial: transect from New York to Bermuda
Contributed by: Jossi
Figure B.9
Methodology and Data Source
These data were collected as part of the MARMAP Ships of Opportunity
Program (Benway et al. In Review; Jossi et al. In Review). Continuous
Plankton Recorders were towed monthly by merchant vessels along a transect
from New York to Bermuda. Zooplankton and larger phytoplankton were captured,
identified and enumerated. Abundance values were gridded in time and
space (distance along transect). Grids of long term medians, means and
standard deviations; and single year conditions, anomalies, and standardized
anomalies are produced. Key Points and Major Observations
An impressive color figure captures seasonal and local spatial dynamics
well, although this is not a time series per se.
4. Calanus abundance by day of
year over time
Time: 1961-1998
Spatial: transect from Boston, Mass. to Cape Sable
Contributed by: Jossi
Figure B.10
Methodology and Data Source
These data were collected as part of the MARMAP Ships of Opportunity
Program (Benway et al. In Review; Jossi et al. In Review). Continuous
Plankton Recorders were towed monthly by merchant vessels along a transect
from Boston, MA to Cape Sable, NS. Zooplankton and larger phytoplankton
were captured, identified and enumerated. Abundance values were gridded
in time and space (distance along transect), and in this case, gridded
in time (years) vs time (days of year). This portrayal shows changes
of seasonality for the Gulf of Maine as a whole during the 38 year time
span.
Key Points and Major Observations
During the mid 1980s, Calanus finmarchicus shows up later
and leaves earlier. In the early1990s there is an even earlier appearance
of this species. Can these timing changes be related to the changing
oceanographic conditions over this time period?
5. The overall zooplankton biomass and
abundance trends of two dominant copepods: Calanus finmarchicus and Centropages
typicus
Time: 1977 - 2000
Spatial: Georges Bank and Gulf of Maine
Contributed by: Kane
Figures B.11 and B.12
Methodology and Data Source
These data were collected as part of the MARMAP Surveys (Benway et
al. In Review; Jossi et al. In Review). Zooplankton samples were collected
at approximately bimonthly intervals throughout the region with a 0.333-mm
mesh net fitted on one side of a 61-cm bongo frame. Biomass was measured
by displacement volume and individual species were sorted and counted
from sub samples. Data in the figures represent ranked departures from
the time series monthly means with a fourth order polynomial fit to the
data. See Kane (1993), Sherman et al. (1998), and Kane (1999) for further
details.
Key Points and Major Observations
Zooplankton trends in both regions were similar. Biomass was usually
high in the late seventies, low throughout most of the eighties, and
highly variable during the 1990s. The biomass trend line on Georges Bank
during the 1990s is higher because of high values recorded in 1989 and
1990, years where budget constraints prevented sampling in the GOM. Calanus
finmarchicus abundance was high in the late seventies and highly
variable during the past two decades with no persistent long-term trend. Centropages
typicus density was high from 1978-82, low throughout the remainder
of the 1980s, and above average during the past decade.
6. Total Zooplankton Biomass
Time: 1977-2000
Spatial: Shelf wide
Contributed by: Kane
Figure B.13
Methodology and Data Source
These data were collected as part of the MARMAP Surveys (Benway et
al. In Review; Jossi et al. In Review). Zooplankton samples were collected
at approximately bimonthly intervals throughout the region with a 0.333-mm
mesh net fitted on one side of a 61-cm bongo frame. Biomass was measured
by displacement volume and individual species were sorted and counted
from sub samples. Data in the figures represent ranked departures from
the time series monthly means with a fourth order polynomial fit to the
data. See Kane (1993), Sherman et al. (1998), and Kane (1999) for further
details.
Key Points and Major Observations
Biomass was generally higher in the late 1970s, with no persistant
long term trend during the past two decades. There was a lot of variability
in the data. Patterns are similar in each of the four main subregions.
F. Fish and Squids
For the majority of these organisms, we refer the reader to NEFSC (1998a,
1998b, 1998c, 2000a, 2000b, 2000c, 2001). These documents contain individual
species stock assessments and annual reports on the status of the major
or commerically valuable species.
1. Relative abundance of northeast species
groups (groundfish, pelagics, elasmobranchs, others) from combined
fall and spring bottom trawl surveys
Time: 1963 - 1999
Spatial: Shelf wide
Contributed by: NEFSC
Figure B.14 (a-d)
Methodology and Data Source
These data were collected as part of the NEFSC Bottom Trawl Survey
(Azarovitz 1981; NEFC 1988). Species were aggregated as principal groundfish,
other groundfish, principal pelagics, and elasmobranchs. A stratified
mean biomass per tow was calculated and smoothed over the time series.
Key Points and Major Observations
The abundance of principal groundfish declined through the mid 1970s,
increased slightly in the late 1970s and early 1980s, and declined thereafter,
remaining at low levels through the 1990s. The abundance of pelagic fishes
declined in the 1970s and increased substantially and continuously thereafter.
Elasmobranch abundance increased from the 1960s through the 1990s, then
declined moderately in the late 1990s. The abundance of other groundfish
has fluctuated without trend. These observations suggest a shift in community
structure and food web dominance.
2. Principal groundfish biomass for Georges
Bank from autumn bottom trawl survey
Time: 1963 - 1999
Spatial: Georges Bank
Contributed by: Brodziak
Figure B.15
Methodology and Data Source
The principal groundfish index is the sum of indices of 12 principal
(exploited) groundfish on Georges Bank. These species include Atlantic
cod (Gadus morhua), haddock (Melanogrammus aeglefinus),
redfish (Sebastes fasciatus), silver hake (Merluccius bilinearis),
red hake (Urophyscis chuss), pollock (Pollachius virens),
yellowtail flounder (Limanda ferruginea), summer flounder (Paralichthys
dentatus), American plaice (Hippoglossoides platessoides),
witch flounder (Glyptocephalus cynoglosses), winter flounder
(Pseudopleuronectes americanus), and windowpane flounder (Scophthalmus
aquosus). The individual indices are stratified mean weight per
tow during autumn, calculated with survey gear adjustment factors applied
where appropriate using NEFSC offshore survey strata 9-23 and 25. See
Brodziak and Link (2002) and Azarovitz (1981) for further details.
Key Points and Major Observations
A large decline in principal groundfish occurred during 1960s and early
1970s. A moderate increase occurred during the late-1970s and early 1980s.
Principal groundfish abundance declined through the 1990s, although recently
there has been a moderate increase.
3. Elasmobranch biomass for Georges Bank
from autumn bottom trawl survey
Time: 1968 - 2000
Spatial: Georges Bank
Contributed by: Brodziak
Figure B.16
Methodology and Data Source
The elasmobranch index is the sum of indices of 6 primary elasmobranchs
on Georges Bank. These species include spiny dogfish (Squalus acanthius),
barndoor skate (Raja laevis), thorny skate (Raja radiata),
smooth skate (Raja senta), winter skate (Raja ocellata),
and little skate (Raja erinacea). The individual indices are
stratified mean weight per tow during spring, calculated with survey
gear adjustment factors applied where appropriate using NEFSC offshore
survey strata 9-23 and 25. See Brodziak and Link (2002) and Azarovitz
(1981) for further details.
Key Points and Major Observations
Elasmobranch biomass was low in the 1970s. Elasmobranch biomass increased
to high values in the 1980s and early1990s. Elasmobranch biomass has
decreased in the late1990s.
4. Principal pelagics biomass estimates
from recent assessments
Time: 1967 - 1994
Spatial: entire range of population in the northwest Atlantic (shelf wide)
Contributed by: Brodziak
Figure B.17
Methodology and Data Source
These data were derived from the NEFSC assessments of pelagics species.
Age-structured assessments using sequential population analysis tuned
to NEFSC survey abundance-at-age indices were used. See Brodziak and
Link (2002) and NEFSC (1998a) for further details.
Key Points and Major Observations
The principal pelagics (Altantic herring Clupea harengus and
Atlantic mackerel Scomber scombrus) are migratory resources
that were heavily fished by distant water fleets in the 1960s-1970s.
Abundance of principal pelagics was high (or moderate) in the early-1970s
and declined to record lows in the 1970s and early-1980s. Abundance was
high and increasing in the late-1980s through the 1990s.
5. Cephalapod biomass for Georges Bank
from fall bottom trawl survey
Time: 1967 - 1999
Spatial: Georges Bank
Contributed by: Brodziak
Figure B.18
Methodology and Data Source
The cephalopod biomass index is the sum of indices of two principal
(exploited) cephalopods, long-finned squid (Loligo pealeii)
and northern short-finned squid (Illex illecebrosus), along
with other squid and octopuses on Georges Bank. The individual indices
are stratified mean weight per tow during autumn, calculated with survey
gear adjustment factors applied where appropriate using NEFSC offshore
survey strata 9-23 and 25. See Brodziak and Link (2002) and Azarovitz
(1981) for further details.
Key Points and Major Observations
Cephalopods are short-lived (lifespan< 1 year) and are common prey
for many species. Distribution of the two primary squids on Georges Bank
depends on seasonal changes in water temperatures. Cephalopod abundance
increased during the late-1960s to late-1970s, declined to the mid-1980s,
and increased in the late-1980s. Abundance declined during the early
1990s and has increased moderately since 1996.
6. Frequency of occurrence of parasitic
nematodes in all predators
Time: 1973 - 1998 in five year blocks
Spatial: Shelf wide
Contributed by: Link
Figure B.19
Methodology and Data Source
These data were derived from the NEFSC Food Habits Database. Live nematodes
observed in examined stomachs were noted. See Link and Almeida (2000)
for further details.
Key Points and Major Observations
There was a methodological shift between 1980 and 1981, so the apparent
trend may be misleading. Otherwise nematode occurrence may provide an
index of density dependent health in fish.
7. Winter flounder collected by beam and
otter trawls
Time: July 1996 - October 2000 (spring, summer, and fall)
Spatial: Navesink River and Sandy Hook Bay in the mid-Atlantic region
Contributed by: Fabrizio
Figure B.20
Methodology and Data Source
These data were collected in the Behavioral Ecology Survey of Demersal
Species. Three seasonal collections were made in the spring, summer,
and fall beginning in the summer of 1996. Demersal species were collected
by replicate tows of a 1-m beam and a 5- m otter trawl at 84 stations
throughout the Navesink River and Sandy Hook Bay. Beginning in July 1998,
only 24 stations were sampled throughout this system. All fish and decapod
crustaceans were enumerated and environmental characteristics were measured.
The data in the figure represent the mean number of winter flounder per
m2 across all stations in the Navesink River and Sandy Hook
Bay.
See Stehlik and Meise (2000) and Stoner et al. (2001) for further details.
Key Points and Major Observations
Beam trawls captured newly settled winter flounder, and generally not
older stages.
As indicated by the beam trawl samples, young-of-the-year winter flounder
abundance was high in the spring of 1999. These data are from a short
time series with limited spatial coverage, but are important to the local
estuarine dynamics.
8. Haddock and cod % maturity for ages
1 and 2
Time: 1963 - 1997 in five year blocks (haddock) and 1978 - 1997 in four
year blocks (cod)
Spatial: Georges Bank
Contributed by: NEFSC SARCs
Figure B.21
Methodology and Data Source
These data are from the NEFSC Age Database (SVBIO) collected as part
of the bottom trawl survey. The particular analyses for these species
can be found in NEFSC (1998b, 1998c, 2000a, 2000b, 2000c, 2001).
Key Points and Major Observations
Haddock seem to show an increase in early maturity over time. How do
changes in maturity reflect ecosystem level effects?
9. Cod survival ratio anomaly
Time: 1978 - 1998
Spatial: Georges Bank
Contributed by: Brodziak
Figure B.22
Methodology and Data Source
The cod survival ratio anomaly measures the difference between the
observed value of cod recruitment per unit of spawning biomass (survival
ratio index) and its predicted value from a fitted Beverton-Holt stock-recruitment
curve. Higher anomaly values are associated with more favorable recruitment
conditions. See Brodziak and Link (2002) for further details.
Key Points and Major Observations
The Georges Bank cod survival ratio anomaly has no apparent trend during
1978-1998, although anomaly values were negative in the late 1980s-early
1990s and have been more positive since 1995. Georges Bank cod recruitment
has been low in recent years and this data suggests that this is not
primarily due to adverse environmental conditions. Survival ratio anomaly
measures deviation of recruits per spawner from a spawner recruit relationship.
10. Haddock survival ratio anomaly
Time: 1931 - 1998
Spatial: Georges Bank
Contributed by: Brodziak
Figure B.23
Methodology and Data Source The haddock survival ratio anomaly
measures the difference between the observed value of haddock recruitment
per unit of spawning biomass (survival ratio index) and its predicted
value from a fitted Beverton-Holt stock-recruitment curve. Lower anomaly
values are associated with less favorable recruitment conditions. See
Brodziak and Link (2002) for further details.
Key Points and Major Observations
Georges Bank haddock survival ratio anomalies appear to be higher during
the 1930s-early 1960s than during the late1960s-1990. The two largest
anomalies correspond to the 1963 and 1975 year classes which were very
large based on assessment results (i.e., the two "super year classes" are
apparent). Survival ratio anomaly measures deviation of recruits per
spawner from a spawner recruit relationship.
11. Yellowtail flounder survival ratio
anomaly
Time: 1973 - 1997
Spatial: Georges Bank
Contributed by: Brodziak
Figure B.24
Methodology and Data Source
The yellowtail survival ratio anomaly measures the difference between
the observed value of yellowtail flounder recruitment per unit of spawning
biomass (survival ratio index) and its predicted value from a fitted
Beverton-Holt stock-recruitment curve. See Brodziak and Link (2002) for
further details.
Key Points and Major Observations
There appears to be an increasing trend in the survival ratio anomaly
since the mid-1980s. Since area II was closed on Georges Bank in 1994,
the survival ratio anomalies have been relatively high. Survival ratio
anomalies for Georges Bank yellowtail flounder appear to be more variable
than for cod or haddock. Survival ratio anomaly measures deviation of
recruits per spawner from a spawner recruit relationship.
G. Mammals
1. Several marine mammal trends
Time: Various years in the 1980s, 90s
Spatial: Shelf wide
Contributed by: Palka, Smith
Table 4.1
Methodology and Data Sources
Abundance of harbor seals were estimated as the total
count of hauled out animals that were estimated from aerial photos of
animals hauled out during the pupping season on the New England coast
(Gilbert and Guldager 1998). This abundance is considered a minimum estimate
because it was not corrected for animals in the water or outside the
survey area.
Data for all other species were collected during sighting line transect
surveys conducted by planes (1982, 1995, 1998, and 1999) and/or ships
(1991-1999). Shipboard data were collected using the two independent
sighting team procedure and were analyzed using the product integral
or modified direct duplicate methods (Palka 1995). These estimates were
corrected for g(0), the probability of detecting a group on
the track line and, if applicable, also for school size-bias. Standard
aerial sighting procedures with two bubble windows and one belly window
observer were used during the aerial surveys. An estimate of g(0) was
not made for the aerial portion of the surveys, except for harbor porpoises
from surveys conducted after 1990. For a brief overview of all survey
results, see CETAP (1982), Smith et al. (1993), Palka (1996), Palka (2000),
Waring et al. (2000), Mullin (In review) and Palka et al. (In review).
Key Points and Major Observations
These surveys were conducted in different areas within the US and Canadian
Northwest Atlantic Ocean, thus, it is not possible to directly compare
the reported numbers. Most of these estimates are negatively biased due
to not accounting for dive times, ship reaction, and animals outside
of the surveyed area. These biases vary by species. Estimates from1998/1999
are generally the largest, and the best recent estimates, because the
surveys covered waters from Florida to the Gulf of St. Lawrence, the
largest portion of the animal=s habitat that was ever covered.
H. Aggregate
1. Total biomass from both fall and spring
bottom trawl surveys
Time: 1963 - 2000
Spatial: Shelf wide
Contributed by: Link
Figure B.25 (a-b)
Methodology and Data Source
These data were collected as part of the NEFSC Bottom Trawl Survey
(Azarovitz 1981; NEFC 1988). Biomass of all net-caught organisms was
aggregated irrespective of species, and a stratified mean biomass per
tow was calculated over the time series. Both a mean per tow and minimum
swept area estimate of total biomass were calculated.
Key Points and Major Observations
There is no apparent trend in total biomass from the mid 1960s to 2000s.
The may reflect an overall system carrying capacity. The implication
is that if we want to simulataneously rebuild/restore all major groups,
then other components of the ecosystem will have to decline. Can fluctuations
in total biomass be linked to the physical environment? This raises the
question of examining standing stock vs productivity (changes in trophic
transfer) of the different component species. The bottom trawl is not
highly selective for pelagics, jellyfish, plankton, etc., and no corrections
for selectivity were made. The jump in biomass during the late 1960s
could be due to adding the spring survey in 1968.
2. Mean length of all species collected
in fall and spring bottom trawl
Time: 1963 - 2000
Spatial: Georges Bank
Contributed by: Link
Figure B.26
Methodology and Data Source
These data were collected as part of the NEFSC Bottom Trawl Survey
(Azarovitz 1981; NEFC 1988). Organisms were aggregated irrespective of
species, and a stratified mean length for each year was calculated over
the time series.
Key Points and Major Observations
Lengths were lower through the mid 1970s, and longer in the late 1970s
through early 1990s. Lengths were again shorter in the mid to late 1990s.
Does this infer regime shifts, or could it just be the effect of dogfish
and skates? The peak length corresponds to the period when herring and
other pelagics were low in abundance.
3. Abundance of various guilds in fall
and spring bottom trawl surveys
Time: 1963 - 2000
Spatial: Shelf wide
Contributed by: Link
Figure B.27 (a-l)
Methodology and Data Source
These data were collected as part of the NEFSC Bottom Trawl Survey
(Azarovitz 1981; NEFC 1988). Species were aggregated into appropriate
guilds (Garrison and Link 2000), and a stratified mean biomass per tow
was calculated and smoothed over the time series. Both a mean per tow
and minimum swept area estimate of total biomass were calculated.Key
Points and Major Observations
These results are similar to other graphs of grouped biomass. Do these
better convey information better than groupings by taxonomy? Guilds may
be an useful approach, and certainly provide a slightly different picture
of fish community dynamics than the taxonomic groupings.
I. Community Indices
1. Gulf of Maine total species diversity
from bottom trawl survey
Time: 1963 - 2000
Spatial: Gulf of Maine
Contributed by: Brodziak
Figure B.28
Methodology and Data Source
Total species diversity was indexed by the average number of species
per haul during the autumn bottom trawl survey in Gulf of Maine offshore
strata. See Brodziak and Link (2002) for related details, and Ludwig
and Reynolds (1988) for a further discussion of diversity.
Key Points and Major Observations
This diversity index has an increasing trend since late 1980s. The
most recent index value is the highest in time series. This measure may
have been impacted by decisions regarding recording of species during
trawl survey cruises.
2. Gulf of Maine abundant species diversity
from bottom trawl survey
Time: 1963 - 2000
Spatial: Gulf of Maine
Contributed by: Brodziak
Figure B.29
Methodology and Data Source
Abundant species diversity was indexed by the average number of abundant
species (N1) per haul during the autumn bottom trawl survey in Gulf of
Maine offshore strata. N1 was computed as the N1=eH , where
H was Shannon's diversity index evaluated in terms of the biomass proportion
within a trawl sample. See Brodziak and Link (2002) for related details,
and Ludwig and Reynolds (1988) for a further discussion of diversity.
Key Points and Major Observations
This diversity index peaked in the early 1980s. This index provides
a measure of species dominance.
3. Gulf of Maine species evenness from
bottom trawl survey
Time: 1963 - 2000
Spatial: Gulf of Maine
Contributed by: Brodziak
Figure B.30
Methodology and Data Source
This is Hill's modified evenness index (see for example, Ludwig and
Reynolds 1988). Species evenness was indexed by the average of the ratio
(N2-1)/(N1-1) during the autumn bottom trawl survey in Gulf of Maine
offshore strata. N2 was computed as the inverse of Simpson's diversity
index, evaluated in terms of the biomass proportion within a trawl sample.
See Brodziak and Link (2002) for related details, and Ludwig and Reynolds
(1988) for a further discussion of diversity.
Key Points and Major Observations
Species evenness has a decreasing trend since the early 1980s. Current
evenness values are the lowest in the time series. The decreasing trend
in evenness may be due to the abundance of large skates in some areas
of the Gulf of Maine.
4. Georges Bank total species diversity
from bottom trawl survey
Time: 1963 - 2000
Spatial: Georges Bank
Contributed by: Brodziak
Figure B.31
Methodology and Data Source
Total species diversity was indexed by the average number of species
per haul during the autumn bottom trawl survey in Georges Bank strata.
See Brodziak and Link (2002) for related details, and Ludwig and Reynolds
(1988) for a further discussion of diversity.
Key Points and Major Observations
This diversity index appears to trend up and down throughout the observed
time series. Total species diversity on Georges Bank has trended upward
since the early 1990s after declining to a time series low during the
1980s. This measure may have been impacted by decisions regarding recording
of species during trawl survey cruises.
5. Georges Bank abundant species diversity
from bottom trawl surveys
Time: 1963 - 2000
Spatial: Georges Bank
Contributed by: Brodziak
Figure B.32
Methodology and Data Source
Abundant species diversity was indexed by the average number of abundant
species (N1) per haul during the autumn bottom trawl survey in Georges
Bank strata. N1 was computed as the N1=eH , where H was Shannon's
diversity index evaluated in terms of the biomass proportion within a
trawl sample. See Brodziak and Link (2002) for related details, and Ludwig
and Reynolds (1988) for a further discussion of diversity.
Key Points and Major Observations
This species dominance index was higher during the 1960s-1970s than
during the 1980s. In recent years, abundant species diversity has exhibited
an increasing trend. This metric is a measure of dominance.
6. Georges Bank species evenness from bottom
trawl surveys
Time: 1963 - 2000
Spatial: Georges Bank
Contributed by: Brodziak
Figure B.33
Methodology and Data Source
This is Hill's modified evenness index (see for example, Ludwig and
Reynolds 1988). Species evenness was indexed by the average of the ratio
(N2-1)/(N1-1) during the autumn bottom trawl survey in Georges Bank strata.
N2 was computed as the inverse of Simpson's diversity index, evaluated
in terms of the biomass proportion within a trawl sample. See Brodziak
and Link (2002) for related details, and Ludwig and Reynolds (1988) for
a further discussion of diversity.
Key Points and Major Observations
Species evenness on Georges Bank peaked in the early 1970s. This index
steadily decreased during 1975-1990 and has only increased a small amount
in recent years.
7. Mid-Atlantic Bight total species diversity
from bottom trawl surveys
Time: 1963 - 2000
Spatial: Mid-Atlantic Bight
Contributed by: Brodziak
Figure B.34
Methodology and Data Source
Total species diversity was indexed by the average number of species
per haul during the autumn bottom trawl survey in Mid-Atlantic Bight
offshore strata. See Brodziak and Link (2002) for related details, and
Ludwig and Reynolds (1988) for a further discussion of diversity.
Key Points and Major Observations
This diversity index has no apparent trend.
8. Mid-Atlantic Bight Abundant species
diversity from bottom trawl surveys
Time: 1963 - 2000
Spatial: Mid-Atlantic Bight
Contributed by: Brodziak
Figure B.35
Methodology and Data Source
Abundant species diversity was indexed by the average number of abundant
species (N1) per haul during the autumn bottom trawl survey in Gulf of
Maine offshore strata. N1 was computed as the N1=eH , where
H was Shannon's diversity index evaluated in terms of the biomass proportion
within a trawl sample. See Brodziak and Link (2002) for related details,
and Ludwig and Reynolds (1988) for a further discussion of diversity.
Key Points and Major Observations
This measure of species dominance has no apparent trend.
9. Mid-Atlantic Bight Species evenness
from bottom trawl survey
Time: 1963 - 2000
Spatial: Mid-Atlantic Bight
Contributed by: Brodziak
Figure B.36
Methodology and Data Source
This is Hill's modified evenness index (see for example, Ludwig and
Reynolds 1988). Species evenness was indexed by the average of the ratio
(N2-1)/(N1-1) during the autumn bottom trawl survey in Gulf of Maine
offshore strata. N2 was computed as the inverse of Simpson's diversity
index, evaluated in terms of the biomass proportion within a trawl sample.
See Brodziak and Link (2002) for related details, and Ludwig and Reynolds
(1988) for a further discussion of diversity.
Key Points and Major Observations
Species evenness has had no apparent trend in the Mid-Atlantic Bight.
J. Food Web Indices
1. Silver hake linkage density
Time: 1973 - 1998
Spatial: Shelf wide
Contributed by: Link
Figure B.37
Methodology and Data Source
These data are derived from the NEFSC Food Habits Database. See Link
and Almeida (2000) for further details on the food habits sampling.Key
Points and Major Observations
This metric measures number of species eating and being eaten by silver
hake. Silver hake is a "canary" population because a large amount of
energy passes through this species, i.e., it eats many species and many
species eat it. The same is true for red hake (not shown). The number
of prey species consumed by silver hake declined in the mid 1980s, but
has increased through the mid 1990s. Do these changes reflect an overall
change in number of species in ecosystem?
2. Total consumption by 12 piscivores
Time: 1977 - 1997
Spatial: primarily Georges Bank
Contributed by: Overholtz
Figure B.38
Methodology and Data Source
These data are derived from both the NEFSC Bottom Trawl Survey Data
and the Food Habits Database. See Link and Almeida (2000) for further
details on the food habits sampling and Azarovitz (1981) for the bottom
trawl survey sampling. For specifics on the consumption estimation, see
Overholtz et al. (2000).
Key Points and Major Observations
Total consumption (all prey) by 12 predatory fish (pollock, goosefish,
cod-2 stocks, spiny dogfish, white hake, weakfish, winter skate, summer
flounder, bluefish, red hake, spotted hake, and silver hake) averaged
1.5 million mt and ranged between 1.3 and 2.9 million mt during 1977-1997.
Consumption peaked in the early 1980s and declined steadily through 1997.
This trend is consistent with the large biomass of elasmobranchs and
groundfish that were present during the 1980s and a subsequent large
decline in spiny dogfish, cod, white hake, and bluefish, due to fishing,
during the later period. Total annual consumption by individual predators
was lowest by goosefish and summer flounder and highest by silver hake,
and spiny dogfish. Consumption estimates for individual predator species
spanned nearly three orders of magnitude and was heavily influenced by
predator abundance. As an example, spiny dogfish consumed an average
of 619,000 mt, bluefish, 108,000 mt, and goosefish, 14,000 mt during
1977-1997.
3. Total fish consumption by six piscivores
on Georges Bank
Time: 1977 - 1998 in three year blocks
Spatial: Georges Bank
Contributed by: Link
Figure B.39
Methodology and Data Source
These data are derived from both the NEFSC Bottom Trawl Survey Data
and the Food Habits Database. See Link and Almeida (2000) for further
details on the food habits sampling and Azarovitz (1981) for the bottom
trawl survey sampling. For specifics on the consumption estimation, see
Link and Garrison (2002a).
Key Points and Major Observations
There was a peak in the early 1980s due to an abundance of extra large
cod. Consumption by silver hake and cod dominated 1977 and 1980 values;
consumption by dogfish dominated the rest of the time series. The total
consumption was relatively consistent aside from the one peak.
4. Consumption of prey species by 12 piscivores
Time: 1977 - 1997
Spatial: Shelf wide
Contributed by: Overholtz
Figure B.40 (a-f)
Methodology and Data Source
These data are derived from both the NEFSC Bottom Trawl Survey Data
and the Food Habits Database. See Link and Almeida (2000) for further
details on the food habits sampling and Azarovitz (1981) for the bottom
trawl survey sampling. For specifics on the consumption estimation, see
Overholtz et al. (2000).
Key Points and Major Observations
Consumption of pelagic fishes and squids by the 12 predators varied
greatly during 1977-1997 and was particularly large in some years on
herring and sandlance. Predation on sand lance reached high levels in
the late 1970s and early 1980s, coincident with the large biomass of
this species present at the time and major declines in Atlantic mackerel
and herring. As the Atlantic mackerel stock began to recover, predation
on mackerel increased, reaching 89,000 mt in 1988. This was followed
by an increase in herring consumption to over 200,000 mt during 1992
and 1993, declining to about 100,000 mt in 1997. Consumption of short-finned
and long-finned squid averaged 24,000 and 46,000 mt during 1977-1997,
but remained relatively constant over this period. Predation on butterfish
was more variable than the other species, but with the exception of a
few years , was relatively low. The recent decline in consumption of
these species is directly related to declines in the biomass of key predators
such as spiny dogfish, cod, white hake, and bluefish. Earlier studies
(Bowman and Michaels 1984) suggest that these prey, especially sand lance,
herring and mackerel, were important in the diets of these key predatory
fish prior to 1977.
5. Snapshot of food web for three years
in three different decades
Time: 1977, 1987, and 1997
Spatial: Shelf wide
Contributed by: Link
Figures B.41, B.42, and B.43
Methodology and Data Source
These data are derived from both the NEFSC Bottom Trawl Survey Data
and the Food Habits Database. See Link and Almeida (2000) for further
details on the food habits sampling and Azarovitz (1981) for the bottom
trawl survey sampling. For specifics on the consumption estimation, see
Overholtz et al. (2000) and Link and Garrison (2002a).
Key Points and Major Observations
The size of the circle is proportional to the size of population; the
thickness of an arrow shows how much of the population is consumed by
predator. During1977, squid and sand lance were the major prey and this
was a relatively simple food web. During 1987 and 1997, this was a much
more complex food web, with the major groundfish populations lower in
abundance and the importance of pelagics as prey more notable.
6. Fish consumption and % fish in diet
of cod
Time: 1978 - 1997
Spatial: Shelf wide
Contributed by: Link
Figure B.44
Methodology and Data Source
These data are derived from both the NEFSC Bottom Trawl Survey Data
and the Food Habits Database. See Link and Almeida (2000) for further
details on the food habits sampling and Azarovitz (1981) for the bottom
trawl survey sampling. For specifics on the consumption estimation, see
Overholtz et al. (2000) and Link and Garrison (2002a).
Key Points and Major Observations
There was a peak in the early 1980s for both how much fish comprised
the diet of cod and how much fish biomass was consumed by cod. Lower
values in the 1990s likely reflect the smaller size structure of the
cod population.
7. Fish consumption by cod at age
Time: 1978 - 1997
Spatial: Shelf wide
Contributed by: Link
Figure B.45
Methodology and Data Source
These data are derived from both the NEFSC Bottom Trawl Survey Data
and the Food Habits Database. See Link and Almeida (2000) for further
details on the food habits sampling and Azarovitz (1981) for the bottom
trawl survey sampling. For specifics on the consumption estimation, see
Overholtz et al. (2000) and Link and Garrison (2002a).
Key Points and Major Observations
There is an overall decline in the amount of total fish consumed by
cod seen here and in Figure B.44. The amount of fish eaten by cod at
different ages varied over time. Through the 1980s and into the 1990s,
the relative and absolute amount of fish eaten by age 7+ cod declined.
In early to mid 1990s older fish (ages 7+) were a smaller component
of the population and contributed a relatively smaller proportion of
the amount of fish consumed relative to age 3-5 cod.
8. Cod % diet composition of major fish
prey
Time: 1973 - 1997
Spatial: Shelf wide
Contributed by: Link
Figure B.46
Methodology and Data Source
These data are derived from both the NEFSC Bottom Trawl Survey Data
and the Food Habits Database. See Link and Almeida (2000) for further
details on the food habits sampling and Azarovitz (1981) for the bottom
trawl survey sampling. For further details see Link and Garrison (2002b).
Key Points and Major Observations
This demonstrates the transfer of energy from pelagic to benthic environment.
It also seems to show prey switching based upon prey availability.
9. Spiny dogfish % diet composition of
major fish prey
Time: 1973 - 1997
Spatial: Shelf wide
Contributed by: Link
Figure B.47
Methodology and Data Source
These data are derived from both the NEFSC Bottom Trawl Survey Data
and the Food Habits Database. See Link and Almeida (2000) for further
details on the food habits sampling and Azarovitz (1981) for the bottom
trawl survey sampling.
Key Points and Major Observations
The dogfish diet seems to track prey availability. The diet of dogfish
is comprised mainly by pelagic prey.
10. Number of predators for sand lance,
herring, hermit crab, ophiuroids, mysids, and red hake
Time: 1973 - 1998
Spatial: Shelf wide
Contributed by: Link
Figure 48 (a-f)
Methodology and Data Source
These data are derived from both the NEFSC Bottom Trawl Survey Data
and the Food Habits Database. See Link and Almeida (2000) for further
details on the food habits sampling and Azarovitz (1981) for the bottom
trawl survey sampling.
Key Points and Major Observations
This metric is a measure of food web linkage density. There are some
notable changes over time, particularly an increase in red hake and herring
predators in more recent years.
11. Silver hake % cannibalism
Time: 1973 - 1998
Spatial: Shelf wide
Contributed by: Link
Figure B.49
Methodology and Data Source
These data are derived from both the NEFSC Bottom Trawl Survey Data
and the Food Habits Database. These data represent what fraction of silver
hake diet consists of silver hake. See Link and Almeida (2000) for further
details on the food habits sampling and Azarovitz (1981) for the bottom
trawl survey sampling.
Key Points and Major Observations
When other prey are not available, silver hake are cannabilistic. This
phenomena has a consistently high occurrence, with in an increasing trend
in the mid 1990s. How this impacts population dynamics is unclear.
12. Silver hake and red hake number of
prey items
Time: 1973 - 1998 (with 4 year moving averages overlaid)
Spatial: Shelf wide
Contributed by: Link
Figure B.50
Methodology and Data Source
These data are derived from both the NEFSC Bottom Trawl Survey Data
and the Food Habits Database. See Link and Almeida (2000) for further
details on the food habits sampling and Azarovitz (1981) for the bottom
trawl survey sampling.
Key Points and Major Observations
There was a decrease in the number of prey consumed by silver hake
in mid 1980s, with an increasing number of prey throughout the 1990s.
The number of prey of red hake has increased continuously until the mid
1990s. The two hakes show similar patterns and also exhibit similar diets.
13. Herring consumption to landings ratio
Time: 1977 - 1997
Spatial: Shelf wide
Contributed by: Overholtz
Figure B.51
Methodology and Data Source
These data are derived from both the NEFSC Bottom Trawl Survey Data
and the Food Habits Database. See Link and Almeida (2000) for further
details on the food habits sampling and Azarovitz (1981) for the bottom
trawl survey sampling. For specifics on the consumption and landings
information, see Overholtz et al. (2000).
Key Points and Major Observations
Consumption of Atlantic herring was below 50,000 mt from 1977-1987
and then increased in the 1990s to over 200,000 mt in some years. Landings
for this species averaged 82,000 mt during 1977-1997. As herring increased
in the 1990s, consumption to landings ratios increased dramatically in
the early 1990s and then declined. If predator fish biomass is allowed
to recover we would expect consumption of this species to increase and
greatly exceed landings in the future.
14. Mackerel consumption to landings ratio
Time: 1977 - 1997
Spatial: Shelf wide
Contributed by: Overholtz
Figure B.52
Methodology and Data Source
These data are derived from both the NEFSC Bottom Trawl Survey Data
and the Food Habits Database. See Link and Almeida (2000) for further
details on the food habits sampling and Azarovitz (1981) for the bottom
trawl survey sampling. For specifics on the consumption and landings
information, see Overholtz et al. (2000).
Key Points and Major Observations
Consumption and landings of Atlantic mackerel by 12 predatory fish
were fairly similar during 1977-1997 and both were well below established
reference points for this species (MSY 326,000 mt). Consumption to landings
ratios for this species were relatively constant during 1977-1997. This
suggests that a recovery in predator biomass may not cause any large
increases in consumption on this species, with the exception perhaps
of a large recruiting year-class. Several factors such as fast swimming
speed and enhanced growth rates, allowing for a larger body size, probably
make Atlantic mackerel less available or suitable to this suite of 12
predators.
15. Loligo consumption to landings
ratio
Time: 1977 - 1997
Spatial: Shelf wide
Contributed by: Overholtz
Figure B.53
Methodology and Data Source
These data are derived from both the NEFSC Bottom Trawl Survey Data
and the Food Habits Database. See Link and Almeida (2000) for further
details on the food habits sampling and Azarovitz (1981) for the bottom
trawl survey sampling. For specifics on the consumption and landings
information, see Overholtz et al. (2000).
Key Points and Major Observations
Consumption of long-finned squid exceeded landings and MSY (24,000
mt) in all years except 1993 and 1994. Consumption to landings ratios
for this species were relatively high throughout the 1977-1997 period,
averaging 2.36 and ranging from 0.58-4.88. This suggests that any increase
in predator biomass will translate into an immediate increase in consumption
of this species by predatory fish. Consumption will probably always be
in excess of sustainable landings for this species.
K. System Level Indices
We recognize that there are also several system level indices that
one could estimate to ascertain the status of this ecosystem. For example,
what are the values for emergy, exergy, free energy, information content,
energy flows, system level consumption, metabolism, and production, total
production, total biomass, and flux rates across time? Similarly, how
strong is the resilience, persistence, resistance, or stability of the
system? Not much is known in general or in a time series sense for these
measure, but these emergent metrics could be estimated in future efforts.
L. Summary of Biotic Metrics
We examined biotic metrics ranging from single species to ecosystem
level.
The early to mid 1980s seem to have a consistent "blip" in many of
the graphs. The cause of these peaks or troughs are currently unknown.
Some potential hypotheses include a change in the "environmental condition" (not
specified), removal of the foreign fishing fleets in 1976 and changes
in management during the late 1970s and early 1980s, predatory release
due to changes in overall selectivity, changes in the trophic linkages,
alteration of habitat, or some combination thereof.
Total biomass (as measured by the trawl survey time series) has been
remarkably consistent from the late 1960s to present given the large
changes observed in biomass of individual species.
Changes in the abundance and diversity of commercially important species
and associated bycatch species should be interpreted in light of changing
management measures over time. In particular, the implementation of the
closed areas since 1995 may influence these trends.
Are systematic (taxonomic) or trophic (functional) groupings more important
for providing information? Would plotting fishing pressure on graphs
of fish biomass improve our understanding? Similarly, would a similar
plot against environmental variables improve our understanding? These
and a suite of related questions merit examination in the future.
M. References
Azarovitz, T.R. 1981. A brief historical review of the Woods Hole Laboratory
trawl survey time series. In: Bottom Trawl Surveys. Doubleday
W.G., Rivard D. Canadian Special Publications in Fisheries and Aquatic
Sciences 58:62-67.
Brodziak, J. and Link, J. 2002. Ecosystem-base
