Northeast Fisheries Science Center Reference Document 04-06
Stock Assessment of the
Gulf of Maine - Georges Bank
Atlantic Herring Complex, 2003
by W.J. Overholtz1, L.D. Jacobson1,
G.D. Melvin2, M. Cieri3,
M. Power2, D. Libby3, and K. Clark2
1National Marine Fisheries Serv., Woods Hole Lab., 166 Water St.,
Woods Hole, MA 02543
2Dept. of Fisheries and Oceans Canada, St. Andrews Biological Sta., 531
Brandy Cove Rd., St. Andrews, NB E5B 2L9
3Maine Dept. of Marine Resources, Boothbay Harbor Lab., P.O. Box 8, West
Boothbay Harbor, ME 04575
Print
publication date February 2004;
web version posted March 5, 2004
Citation: Overholtz, W.J.; Jacobson, L.D.; Melvin, G.D.; Cieri, M.; Power, M.; Libby, D.; Clark, K. 2004. Stock
assessment of the Gulf of Maine - Georges Bank Atlantic herring complex, 2003. Northeast Fish. Sci.
Cent. Ref. Doc. 04-06; 290 p.
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EXECUTIVE SUMMARY
The Transboundary Resource Assessment Committee (TRAC) met during
10-14 February, 2003 in the Conference Center, Biological Station,
St Andrews, NB, Canada, to assess the Gulf of Maine-Georges Bank
Atlantic herring complex. Some data were preliminary (i.e. 2002 landings)
at the time of the meeting and all analyses were completed with these
data. Two assessments were presented at the meeting, a forward projection
analysis (Chapter 10) and an ADAPT assessment (Chapter
14). The review
committee did not reject either assessment and therefore, the results
of both approaches are contained in this document. However, much progress
was made on many other facets of the status of the herring complex.
For example, both counties agreed that an assessment of the overall
complex was warranted, that historic tagging information was still
relevant, that multiple research survey time-series should be used
in the analysis, and that the new acoustics results should be used.
The assessment focused on the fishery during 1959-2002, but historically
landings of herring in coastal Maine have occurred over several centuries.
The fishery on Atlantic herring in the region shifted from fixed gear
with landings dominated by juvenile herring in the 1950s and 1960s
to an intense foreign trawl fishery that occurred offshore (Georges
Bank) by ICNAF countries in the mid 1960s through the late 1970s. In
recent years, the fishery captures adult herring and landings are dominated
by mid-water trawlers. Landings during the last 15 years have averaged
slightly over 100,000 mt, and almost 123,000 mt during 1998-2002.
The herring assessment utilized research survey data from
a variety of sources. Indices are available from NMFS research bottom
trawl surveys (winter (1992-2002), spring (1968-2002), autumn 1963-2002),
Canadian research bottom trawl surveys (winter 1986-2002), US and Canadian
larval herring surveys (US 1971-1994, Canada 1987-1995), US herring acoustic
surveys on Georges Bank (1998-2002) and Maine DMR inshore herring acoustic
surveys (1999-2002). Trends from US and Canadian bottom trawl surveys
indicate a decline in herring during the late 1960s through the 1970s,
a very low period of abundance during the late 1970s through the late
1980s, and recovery during the 1990s. Both larval herring surveys indicate
an increasing trend during the late 1980s and early 1990s. The US herring
acoustic survey on Georges Bank indicates that a major recovery of herring
has occurred on Georges Bank and a large herring biomass is present,
while the acoustic survey in Maine inshore waters indicates a relatively
stable biomass for the inshore component.
The forward projection analysis suggests that a major recovery of the
entire herring complex occurred during the 1990s. Fishing mortality increased
steadily to about F=0.8 during the late 1960s and then increased further
to above F=1.0 in the mid to late 1970s and early 1980s. Fishing mortality
declined in the late 1980s and 1990s and has remained low during recent
years (F2002=0.06). Total stock biomass declined from a high of 1.4 million
mt in 1962 to a low of 87,000 mt in 1982. Stock biomass increased gradually
thereafter to 1.0 million mt in 1994 and 1.8 million mt in 2000. Trends
in spawning biomass are very similar to the pattern observed in total
stock biomass, reaching about 1.6 million mt in 2001. Recruitment was
very poor during the late 1970s and 1980s, but steadily improved in the
1990s with two very large year-classes, the 1994 and 1998 cohorts.
Results for the ADAPT assessment also suggest that Atlantic herring
from the Gulf of Maine-Georges Bank complex have also recovered from
low biomass in the 1980s. Fishing mortality increased stead lily from
the late 1960s through the late 1970s, reaching F=1.1 in 1980. After
1980 fishing mortality declined and averaged about F=0.3 during 1983-1997.
Recent F’s have averaged about F=0.2 and F in 2002 was 0.18. Stock
biomass declined from a high of about 1.2 million mt in 1967 to less
than 100,000 mt in 1982. Total stock biomass recovered very slowly during
1983-1994 to about 220,000 mt and then more quickly to about 700,000
mt in 2002. Spawning biomass followed the same pattern, reaching about
600,000 mt in 2002. Recruitment was relatively low during 1972-1994 and
two large year-classes occurred in 1994 and 1998.
Yield per recruit reference points were re-estimated and results were
Fmax=0.40, F0.1=0.18, and F40%=0.15. Biomass dynamics based reference
point estimates were obtained with a Fox (1975) model and results were
Fmsy=0.25, MSY=222,000 mt, and Bmsy=896,000 mt. An Fmsy proxy (F95% Fmsy)
was estimated in the ADAPT assessment from parametric and non-parametric
stock-recruitment relationships and results were F95% MSY=0.20-0.22.
The prognosis from forward projection model results suggests that fishing
the stock at an F of 0.1 would produce a catch of 170,000 mt in 2004
and a 2+ biomass of about 1.79 million mt in 2005. An F of 0.2 in 2004
would produce a catch of 323,000 mt in 2004 and a beginning year stock
size of 1.64 million mt in 2005. Corresponding projections with the ADAPT
results produce a 2004 catch of 60,000 mt (F=0.1) and a 3+ biomass of
550,000 mt. Fishing the stock at an F=0.2 would produce a 2004 catch
of 100,000 mt and a 2005 biomass of 500,000 mt in 2005.
Assessment results from the two modeling approaches suggest a three
fold difference in 2002 biomass (1.8 million mt vs. 600,000 mt and) and
F (0.06 vs. 0.18). These results were not reconciled by the TRAC working
group during the February 10-14, 2003 meeting, and future work was suggested.
1.0. Overview of Atlantic Herring
in the Region
1.1. Introduction
Atlantic Herring exhibit a high degree of "population richness" with
a number of separate spawning areas and discrete egg and larval distributions
throughout their range in the northwest Atlantic (Sinclair 1988,
Sinclair and Iles 1988). The population structure of Atlantic herring
has been described as a metapopulation (McQuinn 1997) and fitting
the population complex models of Stephenson et al. (2001) and Smedbol
and Stephenson (2001). In these models, it is recognized that herring
form identifiable, relatively discrete and self-sustaining populations
that persist both spatially and temporally.
In recent years there has been increasing emphasis on preserving
all aspects of biodiversity, including within species diversity (Stephenson
and Kenchington 2000). The biological rationale for preserving this
diversity is that such variation allows adaptation to changing conditions
(Smedbol and Stephenson 2001). The economic rationale is that the
decrease or elimination of population richness may lead to the loss
of fisheries, such as occurred during the mid 1970s when the Georges
Bank herring stock collapsed.
Most fishery management units for herring are at the scale of the
stock complex (Stephenson et al. 2001) rather than at the level of
the individual spawning ground. The three recognized spawning groups
within the Gulf of Maine-Georges Bank Atlantic herring stock complex
present a unique challenge to management. Given the intermixing
of these spawning groups, and the timing of the index surveys, it
is currently not possible to assess each spawning group separately. At
the same time, it is recognized that conspecific populations often
differ in productivity and may not support equal levels of exploitation
(Smedbol and Stephenson 2001). Thus, appropriate fishing levels
may not be the same for the different populations within the stock
complex and individual spawning components must be monitored to ensure
that they are not eroded by overfishing.
1.2. Herring stocks
Western Atlantic herring (Clupea harengus) range geographically
from Labrador to Cape Hatteras, with major spawning areas restricted
to the northern regions of this distribution (Scott and Scott 1983;
Collette and Klein-MacPhee 2002). In the Gulf of Maine/Bay of Fundy
region there are three separate stock components recognized; Southwest
Nova Scotia-Bay of Fundy (4WX), coastal waters of the Gulf of Maine
(5Y) and Georges Bank (5Z), the latter including Nantucket Shoals
(Figure 1.1). However, our the perception
of stock structure has varied over time and the delineation of stock
boundaries has been challenging due to the degree of inter-seasonal
mixing among the components. The movement and the seasonal distribution
of fish has also had a significant impact on the assessment of stock
status, on how fishing effort has been assigned, on the development
of a catch-at-age matrix and on management.
A fishery for adult and juvenile herring in the coastal waters
of the Gulf of Maine has been conducted for several centuries, but
it wasn’t until the mid to late 1960s that the fisheries in the region
markedly increased (Collette and Klein-MacPhee 2002). In 1961 an
international fishery for herring developed in the offshore waters
of Georges Bank and southern New England. The Georges Bank fishery
peaked in 1968 with a catch of 373,000t, but the fishery collapsed
in 1976 due to overfishing and poor recruitment. Only 2500t of herring
were harvested from Georges Bank in 1977. An excellent summary of
the Gulf of Maine fisheries is provided by Anthony and Waring (1980).
Assumptions regarding the seasonal movement, intermixing, and spawning
of the individual components of the Gulf of Maine/Georges Bank herring
stock have changed over the years. During the 1970s when herring
in the Gulf of Maine region were assessed and managed by the International
Commission for the Northwest Atlantic Fisheries (ICNAF), the three
components (4WX, 5Y and 5Z) were assessed independently with the
basic assumption of little intermixing among them (Figure 1.1). Stock
boundaries were based on ICNAF (later NAFO) statistical reporting
areas, and catches assigned accordingly. Today, we recognize that
catches in US waters may contain a mixture of herring, and that the
distribution/mixing of fish varies from season to season. In this
section of the report the information on the distribution, seasonal
movement, and stock structure is summarized and reviewed.
1.3. Scientific and Management Units
Delineation of the Northwest Atlantic into statistical areas for
reporting of fish landings began in US waters in the late 1800’s,
but it was not until 1932 that statistical areas were first agreed (Halliday
and Pinhorn 1990). Over time, and several institutional changes,
reporting boundaries and management units, were redefined and renamed.
Presently, the statistical reporting areas developed by the International
Commission for the Northwest Atlantic Fisheries (ICNAF) are used
to define stock and management units. ICNAF divisions were an attempt
to define, based on the knowledge at the time, the entire stock unit
while sub-divisions were used to define fisheries and fish densities
(Halliday and Pinhorn 1990). Sub-areas and statistical areas within
sub-divisions are used today to compile fishery data (Figure 1.1).
Important modifications to the ICNAF statistical areas included the
changing of the boundaries between Divisions 4X and 5Y to correspond
with the US/Canada maritime boundary delineated in October 1984 and
the recording and reporting of separate fisheries statistics for
the Canadian and US portions of Georges Bank beginning in 1986.
1.4. Stock Structure
Perceptions of herring stock structure in the Gulf of Maine-Georges
Bank region have changed over time. Mackenzie and Tibbo (1960) identified
the main spawning groups in the Gulf of Maine based on distribution
of <10mm larvae (Figure 1.2). Although
they did not define stocks, their observations are generally consistent
with later views of stock structure. In 1971 ICNAF defined the stock
structure of herring in the region based on the available information
(Figure 1.3). The stock structure, as revised
in 1971, is today with few changes the foundation for the assessment
and management of herring in the Gulf of Maine-Georges Banks region
(Figure 1.4).
1.5. Spawning Locations
Herring in the Gulf of Maine and Georges Bank region are mixed
during much of the year, except during spawning when they separate
and return to their spawning grounds.
Documented spawning locations include:
- Inshore Coastal Areas of the Gulf of Maine: Scots
Bay, Southwest shore of Grand Manan, off Eastern Maine, off Penobscot
Bay, Western Gulf off Wood Island, Jeffreys Ledge, Stellwagen Bank (Figure
1.5) (Clark et al. 1999, Power et al. 2002, Reid et al. 1999,
Tupper et al. 1998)
- Georges Bank (including Nantucket Shoals): Varied
with time – contracted and protracted around Nantucket Shoals. Major
grounds Northeast Peak (pre and post collapse), Cultivator Shoals
and the Nantucket Shoals (Figure
1.5) (Melvin et al. 1996, Reid et
al. 1999). Currently, spawning appears to be continuous from Massachusetts
Bay into Great South Channel and along the northern fringe of Georges
Bank to the Northeast Peak.
1.6. Larval Distribution
Larvae produced by the major spawning stocks in the Gulf of Maine/Georges
Bank region remain discrete during the early part of the larval stage
(Sinclair and Iles 1985; Tupper et al. 1998). Therefore, the distribution
pattern of young larvae (<10mm) provides information on stock structure
(Figure 1.6a to 1.6d).
Both Canada and the US have conducted larval surveys on Georges Bank
and in the Gulf of Maine. In 1956 the Fisheries Research Board of Canada
and the US Fish and Wildlife Service initiated a co-operative program
to identify herring spawning grounds and nursery areas in the Gulf of
Maine/Bay of Fundy region (Tibbo et al., 1958). A broad scale survey
design was implemented to cover most of the offshore waters. Based on
the distribution of 4-9mm larvae, the study concluded that the largest
herring spawning area in the Gulf of Maine occurred on the northern edge
of Georges Bank (Figure 1.7). Annual larval surveys
were conducted throughout the 1960s in the Gulf of Maine (Boyar et al.
1973a, Boyar et al. 1973b; Tibbo and Legare, 1960). Again, the studies
found that the largest herring spawning component occurred on the northeastern
portion of Georges Bank. In 1971 ICNAF initiated an international larval
survey that concentrated in NAFO sub-division 5Z (i.e., Massachusetts
Bay, Nantucket Shoals, and Georges Bank) and which formed the foundation
for the future US larval programs summarized by Smith and Morse (1993).
1.6.1. US Larval Survey
The ICNAF larval herring survey in the Gulf of Maine ended in 1977,
but was followed during 1977 to 1987 by a comprehensive US fisheries
ecosystem study known as the Marine Resource Monitoring, Assessment,
and Prediction program (MARMAP). Following the completion of the MARMAP
program, larval surveys of the area were continued by the Northeast Fisheries
Science Center (NEFSC) under a herring/sand lance interaction study.
US larval surveys ceased in January 1995.
The information collected by the ICNAF and US larval programs provide
an overview of herring abundance and distribution during the pre-collapse,
the collapse, the post-collapse, and the recovery stages of the Georges
Bank herring stock. Smith and Morse (1993) clearly illustrate the transition
from pre-collapse to the recovery. Larval distribution figures from their
study are reproduced here as Figure 1.6a to 1.6d. During the early
to mid 1970’s, recently hatched herring larvae (4-8mm) on Georges Bank
were concentrated on the northeastern portion of the bank (consistent
with previous studies) and in the Great South Channel, just south east
of Cape Cod, but west of 69o longitude. Only small concentrations
were observed in the Massachusetts Bay region. Later stage larvae, 2
to 5 weeks of age, were dispersed from these epicenters of spawning and
were distributed over most of the bank by the time they were 5 to 8
weeks of age (Figure 1.6a).
During 1976-1984, when the Georges Bank herring stock had collapsed,
the distribution and abundance of larval herring contracted to the west
with the only strong signs of newly hatched larvae occurring in the vicinity
of Massachusetts Bay. The large and dense aggregations of young larvae
on the eastern portion of the bank had all but disappeared. Older larvae
were restricted to a much narrower geographical range in the western
portion of the bank, with only small and sparse occurrences of 13+mm
larvae on the eastern half of Georges Bank (Figure 1.6b).
During 1985-1987, herring on Georges Bank began to show signs of a
recovery. While the newly hatched larvae were still restricted to the
Massachusetts Bay/ Nantucket Shoals area, older larvae were found over
more of the bank (Figure 1.6c). This time frame represents the transition
period back to the pre-collapse distribution By 1988-1990, young herring
larvae were still concentrated in the Massachusetts Bay/Nantucket Shoals
area, but occurred across the bank to almost the Hague Line (Figure 1.6d). Large
aggregations of newly hatched larvae first appeared on the Canadian portion
of Georges Bank in 1992 (Figure 1.7). By the time the US larval surveys
ended in 1994-1995, large aggregations of newly hatched larval herring
were distributed throughout most of the Bank with dense aggregations
characteristic of the pre-collapse era occurring on the northeastern
portion. Dense concentrations of 4-7mm larvae were also found in the
Nantucket Shoals area during this same time period (SARC, 1996).
1.6.2. Canadian Larval Survey
Canadian larval surveys on Georges Bank were initiated in 1987 to monitor
the distribution and abundance of herring larvae during what appeared
to be the early stages of recovery. Annual larval surveys were conducted
from 1987 to 1995 with expansion of the survey area occurring in 1990
and 1992 to provide better coverage of potential spawning areas. Information
pertaining to the timing of each survey and the number and size range
of larvae caught is presented in Table 1.1.
The spatial distribution of larvae <10 mm total length (size generally
considered to reflect spawning area) sampled in 1988, 1990, 1992, and
is presented in Figure 1.7. Detailed annual plots of sampling stations
and total larval distribution are provided in Melvin et al. (1996).
During 1992-1995, the Canadian larval survey was conducted during late
October to early November. During 1992-1995, the surveys were conducted 2-3
weeks later, from mid to late November. In these latter surveys, the
number and size range of larvae caught markedly increased.
Canadian survey results clearly show herring spawning during 1986-1991
spawning, as reflected by aggregations of larvae <10mm in length,
was concentrated west of the Hague line in the vicinity of Georges and
Cultivator shoals. By 1992, however, the distribution of larvae <10
mm expanded well into Canadian waters. This pattern continued annually
through the last survey in 1995. Interestingly, in October 2001 a plankton
tow conducted just east of the Hague line collected over 50,000 5-7mm
larvae.
Annual distributions of herring larvae in the Canadian surveys were
generally consistent with the those in US larval surveys through 1990,
the last year for which detailed US data are available.
1.6.3 Summary of Information on Larval Distributions
Herring larvae produced on spawning grounds in eastern Maine and New
Brunswick are transported in a westerly direction and recruit to the
juvenile herring population along the Maine coast (Tupper et al 1998).
Larvae from spawning grounds in the western Gulf of Maine recruit to
the juvenile herring populations along the coast of central and western
Maine and along the coast of New Hampshire and Massachusetts (Lazzari
and Stevenson 1992, Tupper et al. 1998). Larvae produced in the Jeffreys
Ledge area move inshore and disperse in all directions (Tupper et al
1998).
Georges Bank larvae may be retained in a clockwise current gyre for
several months (Boyar et al. 1973a, Reid et al 1999). However, larvae
from Georges Bank and Nantucket Shoals may also migrate inshore (herring
younger than two years of age are not usually found on Georges Bank)
(Anthony and Waring, 1980). This would most likely occur when the Georges
Bank and Nantucket Shoals spawning populations are large (Tupper et al,
1998). Graham et al. (1972) report herring larvae entering the Sheepscot
estuary of Western Maine in the early fall, soon after hatching. In
the spring, additional larvae also entered the coastal area. The authors
postulate that the spring larvae originated from Georges Bank because
when the Georges Bank component declined so to did the abundance of spring
larvae along the coast.
1.7. Distribution of Herring
The distribution of adult/juvenile herring on Georges Bank and in adjacent
areas has changed dramatically since 1961. Figures 1.8a
to 1.8d provide a chronological overview, in 5-year intervals, of
the distribution of herring in the Gulf of Maine and on Georges Bank,
as indicated in Canadian (1986-1995) and the US (1966-2002) fall research
bottom trawl surveys. Annual plots of herring distribution (up to 1995)
are presented in Melvin et al. (1996). During the early and peak years
of the Georges Bank fishery, 1961-1970, adult and juvenile herring were
sparsely scattered throughout the Gulf of Maine and Georges Bank, with
concentrations in the vicinity of known spawning areas (i.e., northern
reach of Georges Bank, Nantucket Shoals and in Massachusetts Bay) (Figure
1.8a to 1.8c). However, the survey abundance indices were relatively
low during 1961-1970 compared to recent years.
Between 1971 and 1977 the abundance of herring declined sharply and
the distribution of the resource contracted to a few areas on the northwestern
flank of the Bank and around Nantucket Shoals (Figures 1.8c and 1.8d). By
1979 herring had all but disappeared from Georges Bank and Nantucket
Shoals during the traditional spawning season (Oct/Nov) and only a single
immature herring (total length 21cm) was taken in the USA 1979 autumn
bottom trawl survey on Georges Bank and in the Gulf of Maine. This trend
continued into 1980 where no herring were caught in 121 survey tows and
into 1981 when only two mature herring (26 and 33 cm) were collected
at two stations just north of Cultivator Shoals on Georges Bank.
In 1982 adult herring began to appear again in limited numbers in the
survey. The distribution of herring on Georges Bank was, however, restricted
to survey sampling stations in the vicinity of Little Georges and Cultivator
Shoals. Trawl stations near Nantucket Shoals and in Massachusetts Bay
generally showed a wider distribution and greater number of herring during
1982-85. The first herring were collected on the Canadian portion of
the Bank occurred in 1985, but it wasn't until 1986 that significant
amounts of adult/juvenile herring were sampled east of the Hague line.
Between 1985 and 1989, trawl survey catches of herring increased substantially
on Georges Bank and the surrounding area, especially in Massachusetts
Bay. Survey catches from 1988 onward exceeded those taken in the 1960s
when the stock was heavily exploited. The expanded spawning distributions
and increases in abundance continued through the 1990s and into the new
millennium (Figures 1.8b, 1.8c), however, it wasn’t until 1992 that spawning
was detected on the Canadian portion of Georges Bank. In recent years
spawning herring have been consistently taken in the autumn surveys in
Massachusetts Bay, throughout Nantucket Shoals and along northern flank
of Georges Bank from the Great South Channel to the Northeast Peak in
an almost continuous band.
Although survey coverage of the inshore waters of the Gulf of Maine
is generally poor, increasing numbers of herring have been collected
in the coastal areas of Maine since about 1990.
Herring from the Gulf of Maine and Georges Bank overwinter between
Cape Cod and Cape Hatteras, with major aggregations occurring in coastal
and shelf waters off Long Island. Distributions patterns of herring
from the US Spring bottom trawl survey series, which began in 1968, illustrate
the winter distribution of Atlantic herring along the US coast and depict
spatial changes over time. During the late 1960’s and early 1970s, herring
primarily occurred south of Cape Cod in both the inshore and offshore
waters (Figure 1.9a). Limited numbers were also
found east of Cape Cod and in the Gulf of Maine proper. Between 1976
and 1984 (Figure 1.9b), after the Georges Bank spawning component had
collapsed, very few herring were found in the offshore waters of southern
New England or the Mid-Atlantic. Herring aggregations occurred in Massachusetts
Bay just north of Cape Cod, but elsewhere in the Gulf of Maine, herring
were found only sporadically. This led some researchers to speculate
that the herring from the inner Gulf of Maine overwinter in near-shore
coastal areas, while those originating from Georges Bank overwinter offshore.
During 1986 to 1990 (Figure 1.9c), the spring distribution of herring
expanded; more fish occurred offshore and in Massachusetts Bay, and also
became more common on Georges Bank, Nantucket Shoals and along the coast
of Maine. Since 1990, herring have continued to broaden their winter
distribution and increase in abundance in both coastal and offshore waters
from Cape Cod to Cape Hattaras (Figures 1.9c and 1.9d).
1.8. Seasonal migration
Tagging studies and fisheries data provide the background source of
information on seasonal movements of adult and juvenile herring from
each of the three spawning components (4WX, 5Y and 5Z). Conclusions
based on this information may only apply in a general sense because herring
from this region are extremely migratory, are known to inter-mix throughout
most of the year, vary their migration patterns from year to year, and
the majority of the tagging programs were undertaken more than 20 years
ago. Furthermore, most of the tagging was conducted when the Georges
Bank component had collapsed, and so little information is available
on the seasonal movement or intermixing of this group.
1.9. Juveniles
Larval herring move into the inshore Gulf of Maine waters from southeast
New Brunswick to southern Massachusetts during fall and winter and metamorphose
into juvenile (brit) herring the following spring. During the early
brit stage, herring are weak swimmers and probably do not travel long
distances once they reach shore. During the first year of life there
is probably little mixing between the different spawning groups along
the New England and New Brunswick coasts (Tupper et al. 1998). In late
summer and fall, first year brit move farther offshore and overwinter
close to the bottom. They return inshore the following spring at age
two when they are large enough to be recruited to the sardine fishery.
The movements of juvenile herring from the Georges Bank component are
not well known. Significant numbers of age 1 and 2 fish were sampled
in Canadian surveys during 1988 to 1995 (Melvin et al. 1996). Davis
and Morris (1976) found brit distributed widely in the open waters of
the Gulf of Maine and suggested that these might have originated from
Georges Bank since there was no evidence of spawning in the offshore
Gulf of Maine.
Tagging studies indicate that juvenile herring migrate little during
the summer (Speirs 1977; Anthony and Waring 1980; Waring 1981; Stobo
1983a), but move into deep bays or offshore areas to overwinter (Reid
et al. 1999).
Prior to the collapse of the Georges Bank stock, meristic evidence
indicated that the coastal Maine and New Brunswick juvenile herring populations
were augmented by juveniles from Georges Bank (Anthony and Waring 1980). However,
since the coastal juvenile population did not seem to be seriously affected
when the Georges Bank component collapsed, the juvenile contribution
may have been small. Aggregations of juvenile herring along the coast
of Maine and New Brunswick are therefore likely derived from a variety
of spawning grounds.
Tagging studies conducted in the late 1970s and early 1980s by the
Maine Department of Marine Resources found that juvenile herring migrate
westward in the late autumn and overwinter as far south as Massachusetts
Bay (Tupper et al. 1998). In spring, as the waters warm, they return
back east. Juveniles tagged in the Passamquoddy Bay area, however, seemed
to remain in the Bay of Fundy throughout the year (Creaser et al. 1984). Tagging
studies are currently underway in New Brunswick weirs that should provide
further knowledge of juvenile and young adult movement and migration
patterns.
After moving inshore from the deeper waters of the Gulf of Maine/Bay
of Fundy in May, juvenile herring move very little during the summer
feeding season. With the onset of fall, the young fish move offshore
into deeper water where they remain until the next season. As the fish
grow they tend to mix with adults and undergo more extensive migrations.
Juvenile herring tagged along the Maine coast tended to move eastward
as the feeding season progressed, while those tagged in the Passamaquoddy
area over winter in the Bay of Fundy. There is no indication from any
of the tagging results that the juveniles observed along the coast of
Maine make a significant contribution to other spawning stocks such as
those in Southwest Nova Scotia. However, Anthony and Waring (1980) found
a relationship between age 3 herring recruiting to Georges Bank and catches
of age 2 fish in Maine weirs.
1.10. Adults
Adult herring from all three spawning components (5Y, 5Z, and 4WX)
undertake extensive summer feeding and over-wintering migrations and
intermix with stocks other than their own. At spawning, each stock seems
to home to its individual spawning group. Unfortunately, neither the
degree of seasonal intermixing nor the integrity (fidelity) of individual
stocks to spawning grounds are known. However, there is strong evidence,
both historical and recent, that the stocks fluctuate independently,
demonstrate different size and age structures, and undertake distinct
but overlapping migrations.
Three general migratory patterns have been recognized for herring in
the Gulf of Maine: (Figure 1.10)
- Most herring that spend the summer and fall in southwest Nova
Scotia overwinter primarily off Chedabucto Head and in Chedabucto
Bay in eastern Nova Scotia.
- Most herring in the western Gulf of Maine migrate southwest
along the coast and overwinter in Massachusetts Bay and off southern
New
England (Tupper et al 1998).
- Georges Bank herring overwinter along the Mid Atlantic coast and
spend the summer and fall on Georges Bank. Some adults move into
the Gulf of Maine in the summer and return to spawning grounds
on the Bank
and Nantucket Shoals in the fall (Tupper et al 1998).
1.11. Tagging
The annual life cycle of the herring can be divided into five seasonal
phases: overwintering, spring migration, summer feeding, spawning and
fall migration. Tagging of herring at each of these stages has previously
been undertaken to characterize movements and identify stocks (Stobo
1983a,b, Tupper et al 1998). Gulf of Maine and Georges Bank herring
components are mixed to various degrees during all phases of their annual
life cycle, except during spawning.
A brief summary of information derived from various tagging studies
is provided below.
1.11.1 Gulf of Maine
Herring tagged in the summer and fall along the Maine coast tend to
move southwest and overwinter in Massachusetts Bay, although a few move
south of Cape Cod and some move across the Bay of Fundy to Nova Scotia
(Stobo 1983a; b; Tupper et al. 1998). Adult herring tagged off Cape
Cod and the western Gulf of Maine move north and east from the central
coast of Maine to southwest Nova Scotia during spring and summer (Grosslein
1986). Summer feeding adults and older juveniles (age 3) tagged in eastern
Maine from 1976 to 1982 were recaptured on overwintering grounds in Massachusetts
and Cape Cod Bays and in Southern New England (Creaser and Libby 1988).
1.11.2 Great South Channel and Jeffreys Ledge
Herring tagged in 1977 in the Great South Channel and on Jeffreys Ledge
were recovered all along the northeast coast from Ipswich Bay, Massachusetts
into the Bay of Fundy and along southwest Nova Scotia in the summer and
autumn herring fisheries. Tagged fish were also returned during the
winter fisheries in Chedabucto Bay, Cape Cod Bay and Block Island Sound
(Almeida and Burns 1978, Anthony and Waring, 1980).
1.11.3 Canadian Tagging Studies
Herring tagged in the autumn in the Bay of Fundy and off Nova Scotia
migrated north to Chedabucto Bay and south to Cape Cod Bay and Block
Island Sound to overwinter (Stobo et al. 1975; Stobo 1976; 1982). During
the feeding and pre-spawning period, the Bay of Fundy contained a large
mixture of Gulf of Maine and Scotian Shelf stocks (Stobo 1982).
1.12 Other Stock Structure Studies
Studies of meristics, otolith characteristics, and genetics have also
been used to investigate the distinctness of herring stocks in the Gulf
of Maine. Pectoral fin ray counts were used in the past to distinguish between herring
from the Maine coast, Georges Bank and Nova Scotia (Anthony and Waring
1980). However, the number of pectoral fin ray is related to water temperature
and is determined at an early age. Adult herring from Georges to Cape
Cod are expected to have fewer fin rays than adults from further north
since they inhabit warmer waters (Reid et al. 1999). Pectoral fin ray
counts from juvenile fish from the Maine coast were found to be similar
to adults from Georges Bank to Cape Cod (Anthony and Waring 1980).
Libby (cited in Tupper et al.1998) examined a number of otolith size
and shape characteristics from recently hatched larvae from southwest
Nova Scotia, western Georges Bank and mid-coast Maine. Eighty four percent
of 38 otoliths were classified to the correct spawning area.
Genetics have provided little conclusive evidence of discrete stock
structure (Tupper et al. 1998). Biochemical methods for distinguishing
herring populations in the Northwest Atlantic have been conducted since
the 1970s. The U.S. and USSR biochemical and serological studies of
the 1970s were considered flawed and thus no conclusions could be reached
based on their information (Anthony and Waring 1980). Kornfield and
Bogdonowicz (1987) found no evidence of genetically distinct herring
populations in the Gulf of Maine based on mtDNA RFLP analysis.
More recently, McPherson (2002) found evidence for four semi-isolated
groupings of herring. These groupings were herring from the Bras d’Or
Lakes, Eastern Passage, Southwestern Nova Scotia and the interior Bay
of Fundy/Georges Bank.
1.13. Conclusions
The Gulf of Maine and Georges Bank contain three major (and perhaps
additional smaller) distinct but seasonally intermixing components from
Georges Bank, Nantucket Shoals (Great South Channel area) and the coast
of Gulf of Maine. As a result of mixing outside of the spawning season,
much of the fishery takes place on mixed aggregations.
Intermixing of components in the fishery and during resource surveys
precludes separate assessment and management of the components. It is
therefore necessary (as in recent years) to evaluate the entire complex,
with subsequent consideration of the individual components.
Summary Statement
- Atlantic herring generally exhibits complex stock structure
- Gulf of Maine and Georges Bank support three major distinct but
seasonally intermixing components
- Major components spawn on Georges Bank, Nantucket Shoals and the
coastal Gulf of Maine
- Objectives related to preserving stock structure require consideration
of individual components
- Intermixing of components in the fishery and in surveys preclude
separate assessment and management
- The most robust strategy for evaluation and management of this resource
is an assessment of the entire complex with subsequent consideration
of individual components.
2.0 Management
of the Stock Complex
2.1. Management
Atlantic herring stocks in the international waters off the US coast
were first managed in 1972 by ICNAF, which set quotas and country allocations
during 1972-1976. With the passing of the Magnuson Fishery Conservation
and Management Act in 1976 and the extension of jurisdictional waters
in 1977, the New England Fishery Management Council (NEFMC) developed
a management plan for Atlantic herring that was approved in December1978.
During 1977 and 1978 the Atlantic herring fishery (in US waters) was
regulated by a NMFS prepared preliminary fishery management plan. The
1978 management plan had two main objectives (NEFMC 1999):
- “to manage the Gulf of Maine and Georges Bank adult herring stocks
so as to achieve levels of spawning biomass providing continued and
relatively stable recruitment,” and
- “to manage the Gulf of Maine juvenile herring fishery resources
to stabilize and rebuild the sardine industry.”
Since most of the herring fishery took place in state waters, an Interstate
Sea Herring Management Plan for Maine, New Hampshire, Massachusetts,
and Rhode Island was developed in 1983 by the Atlantic States Marine
Fisheries Commission (ASMFC). The ASMFC plan had a main objective and
two sub-objectives as follows:
- “To acquire information that will allow development and facilitate
implementation of management approaches designed to minimize prospects
of a collapse of herring stocks on which New England fishermen depend,”
- “to protect spawning herring,”
- “to promote complementary management of all components of sea herring
fisheries throughout the range of the stocks of interest to U.S. fishermen,
including relevant Canadian waters.”
During the early 1990s, the increase in the abundance of herring in
the Gulf of Maine, Nantucket Shoals and Georges Bank created a situation
in which the majority of catches shifted from state to federal waters.
This, combined with other changes, promoted the adoption of another management
plan in 1994, which defined Atlantic herring as an inter-jurisdictional
resource. As the resource continued to expand, there was a need to address
changing fishing patterns and the interests of new stakeholders. This
eventually led to the development of the current Management Plan submitted
jointly by the NEFMC and the ASMFC in 1999. The primary goals of the
plan are:
- To achieve, on a continuing basis, optimum yield (OY) for the United
States fishing industry and to prevent overfishing of the Atlantic
sea herring resource
- To provide for the orderly development of the offshore and inshore
fisheries, taking into account the variability of current participants
in the fishery
- To provide controlled opportunities for fishermen and vessel in the
other mid-Atlantic and New England fisheries
The FMP defined the management unit to include all the Atlantic herring
within the US territorial sea and the Exclusive Economic Zone (EEZ). Three
management areas were delineated to accommodate current knowledge of
stock structure and existing fishing patterns (Figure
2.1), recognizing that changes might occur in the future due to new
information. Area 1 includes the Gulf of Maine, Area 2 Nantucket Shoals
and south, and area 3, Georges Bank east of the Great South Channel. Area
1 was further subdivided into Area 1a, the inshore waters and Area 1b,
the offshore waters. In Canada, herring from the Gulf of Maine occur
in two Canadian management areas: the Bay of Fundy Region of 4X, and
the Canadian portion of Georges Bank (5Z).
The relative contribution of herring from each of the major spawning
components (coastal Maine, Nantucket Shoals and Georges Bank) to the
overall stock complex was evaluated using swept area estimates of minimum
population size ( both in number and weight) derived from NEFSC autumn
bottom surveys within the three management areas. Based on these estimates
during the ten-year period 1988 to 1997, the coastal Maine area accounted
for 27% of the total herring biomass and 26% by number, Nantucket Shoals
accounted for 63% of the total (in both biomass and number) and Georges
Bank accounted for 10% of the herring biomass and 11% of the total abundance.
Based on the five-year period, 1993-1997, the Coastal Maine and Nantucket
Shoals areas accounted for slightly less of the total than during the
ten-year period and Georges Bank accounted for slightly more (Table
2.1). In
the 1999 FMP, the relative contributions (portion of biomass in each
area during spawning season) of herring in the Gulf of Maine, Nantucket
Shoals, and Georges Bank areas were assumed to be 25%, 55%, and 20% respectively.
Autumn survey data since 1997 show an increase in the relative contribution
of herring in the Gulf of Maine (Figure 2.2). The
most recent 5-year survey data (1997-2001) indicate that herring in the
Gulf of Maine comprise 38% of the total biomass of the complex. Furthermore,
the abundance of the Georges Bank/Nantucket Shoals population appears
to be declining (Figure 2.3). Further investigation
may reveal whether a different a survey strata set would better reflect
changes in the relative abundance of herring among the three areas.
The seasonal distribution of herring in the Gulf of Maine, as reflected
by patterns in the US spring and fall surveys, has also varied substantially
over time. Table 2.2 summarizes the current view of how herring the herring
components are seasonally distributed among the three management areas. These
percentages are used to allocate the TAC for each of the sea herring
management areas. An allocation of 20,000t is provided for Canadian weir
landings, but herring catches on the Canadian portion of Georges Bank
are not currently addressed in the US Atlantic herring fishery management
plan.
3.0. A general overview of the fishery
3.1. Introduction
Atlantic herring which spawn in the Gulf of Maine (GOM) and on Georges
Bank are harvested in five major fisheries: coastal Maine, New Hampshire,
and Massachusetts (5Y); Nantucket Shoals/Georges Bank (5Z); Southern
New England (5Zw); Mid Atlantic (SA 6); and along the New Brunswick coast
(4X). An unknown portion of GOM herring are also caught in the Canadian
Bay of Fundy/Southwest Nova Scotia (4WX) fishery, although the numbers
are assumed to be small.
The coastal fisheries of 5Y and the New Brunswick are amongst the oldest
fisheries in the western Atlantic, dating back several centuries. Landings
data for these fisheries are presented from 1938 to 2002 in Figure
3.1.
During 1938-1954, a marked increase occurred in landings from the Gulf
of Maine coastal area while landings in the NB weir fishery ranged between
30,000 and 45,000t annually. Landings in the GOM fishery peaked at 94,200t
in 1950. Since then, annual landings have averaged 52,000t (1951-2002)
with lows of 25,000t occurring in the mid-1960s, mid 1970s and mid-1980s.
Landings since 1989 have been about equal to or have exceeded the long
term average. Conversely, NB weir landings have shown a marked decline
in recent years. Since 1994, the weir landings have been below the long-
term average (1951-2002) of 26,000t. and were only 11,800t in 2002. A
number of factors have contributed to the decline including a reduction
in the number of active weirs and changes in herring distribution.
In the early 1960s, total landings from the Gulf of Maine-Georges Bank
region markedly increased with the development of a predominately foreign
fleet herring fishery in the international waters of Georges Bank and
Southern New England (Figure 3.2). The Georges
Bank fishery began in 1961 when the former USSR landed 68,000 mt of herring.
Between 1961 and 1965 the fishery was dominated by the USSR where annual
catches ranged between 38,000 and 151,000 mt (Figure 3.2). The fishery
expanded rapidly after 1966 when Poland and the German Democratic Republic
entered the fishery. Over the next 9 years, vessels from 12 countries
harvested herring from Georges Bank, including Canada and the US (Anthony
and Waring, 1980). Annual catches during 1961-1977 are presented by country
in Table 3.1. Fishing gear varied by country and year. Drift gillnets
dominated during 1961-1963, followed by side and stern trawlers during
1963-1972, mid-water trawlers during 1971-1977 and purse seiners during
1969-1975. Fishing occurred throughout the year, but the majority of
catches were taken between May and October, when large numbers of herring
were on the Bank for summer feeding or spawning (September/ October).
The Georges Bank fishery dominated landings from 1962 to 1976, peaking
at 373,000t in 1968. This high level of exploitation could not be maintained
and by 1976 the Georges Bank spawning component had collapsed due to
over-fishing and a series of poor recruitment years. No directed fishery
for herring occurred on the Bank between 1979 and 1995, and it wasn’t
until 1996 that any substantial landings from the area occurred. Total
herring landings from Georges Bank exceeded 39,000t in 1998 and 2001,
but were less than 20,000t in 2002 (Table 3.2).
The Southern New England herring fishery has also increased substantially
since 1995. Traditionally, annual landings from Southern New England
have been a few thousand tonnes. However, during 1996-2000, the winter
fishery just south of Cape Cod exceeded 20,000t annually (Figure 3.2). Landings
of herring from the mid-Atlantic region have been minimal relative to
the other herring fisheries. Typically, annual landings in the mid-Atlantic
region have been a few hundred tonnes and have rarely exceeded 1,000t
(Table 3.2).
3.2. Recent Landings
The Maine Department of Marine Resources (MDMR) is the primary state
agency in the New England region involved in Atlantic herring research,
resource monitoring, and management. The two primary types of information
that are collected and processed at the Department’s Fisheries Research
Laboratory in Boothbay Harbor are: 1) catch and landings information
from the commercial herring fishery; and 2) age, size, and other biological
characteristics of the commercial catch throughout the range of the fishery.
The Boothbay Harbor Laboratory has played an important role in monitoring
the status of the Gulf of Maine herring resource and the US fishery for
over 30 years.
Prior to 1994, US landings were collected by a combination of canning
industry reports and reports by NMFS port agents. After 1994, harvesters
using Vessel Trip Reports (VTR) directly reported US landings data. With
implementation of the FMP in 1999, harvesters have been required to use
both VTR and Interactive Voice Reports (IVR). Federally licensed dealers
ware also required to submit monthly reports (NEFMC, 1999).
Harvesters report VTR data on a monthly basis. Because harvesters give
location data (coordinates or Loran) on a per trip basis, this reporting
system allows for summarizing catch information from specific areas.
VTR data are useful for stock assessment and effort evaluation, but because
they are reported on a monthly basis, the data are not useful for quota
monitoring (NEFMC, 2001).
Using the IVR call in system, harvesters report catches by management
area on a weekly schedule. Although trip level information and location
data are not reported, this system is useful for near real time quota
monitoring. IVR data are not generally useful for stock assessments,
or to address management questions that require information by area or
gear.
Dealer reports include detailed information on amounts landed, price
paid, and utilization of landings, usually on a per trip basis. The
dealer reports do not contain information on area of catch.
Both IVR and VTR data include landings to foreign vessels by domestic
harvesters. Dealer data only include landings made to domestic dealers.
NMFS and state observers collect data on landings to foreign processing
or fishing vessels. At the end of a fishing year, all reporting systems
are analyzed to detect and reconcile discrepancies.
Total landings peaked in the 1970’s (Figure 3.3),
due to fishing by foreign fleets. Since 1990, total landings of herring
from the stock complex have ranged between 77,000 and 150,000 mt (average;
107,000 mt).
Fixed gear was the predominate method of catching herring in the US
until the early 1980s. After1981, the fishery was dominated first by
purse seines, then by single mid-water trawls. Currently, most landings
are taken by single and pair mid-water trawls (Figure
3.4).
Historically most of the herring landings from the coastal complex
have been taken from Management Area 1A (Figure 3.5).
In recent years, there has been an increase in harvests from Georges
Bank and off of Rhode Island. This is in part due to the change from
purse seines to mid-water gear. Purse seines tend to be less effective
in deeper water.
3.3. Samples
Samples of herring collected from the commercial catch are processed
at the Maine Department of Marine Resources (DMR) laboratory in Boothbay
Harbor. Historically samples were obtained from canning plants, some
of which transported fish from other states, NMFS port agents, and fishery
biologists in various states. The Canadian Department of Fisheries and
Oceans would also provide samples to the State of Maine. Normally 4-8
samples are collected each month by statistical area harvested, with
more extensive sampling occurring during foreign fishing or processing
operations. The current sampling ratio is approximately 1 50 fish sample
per 500 mt.
Usually, between 150 and 200 length samples (7,500- 10,000 fish) are
processed each year (Figure 3.6). Samples of 50
fish are processed for length (mm total length), weight (grams), sex,
and, where applicable, sexual maturity and gonad stage, using standard
procedures and criteria. From each sample, the sagittal otoliths are
removed from two fish per centimeter group and embedded in plastic trays
for ageing. Periodic calibration of ageing procedure is done with NMFS
scientists.
A large reduction in weight at age (for age groups 3 and older) has
occurred, since the early 1980s (Table 3.2 & Figure
3.7). A similar reduction in total length is also evident (Figure
3.8)
3.4. BIOSTAT and Catch at Age
Biostat is a software program which uses catch and sample files to
produce catch at age data in both numbers of fish at age and total weight
of fish at age by Unit. A Unit is defined as a month, geographical area
(composed of one or more statistical areas) and gear type (fixed or mobile).
Currently geographical areas are defined for the purpose of catch-at-age
analysis as Eastern Gulf of Maine, Western Gulf of Maine, Southern New
England/ Mid-Atlantic, and Georges Bank. Gear type is defined as fixed
(stop seine and weir) or mobile (purse seine, pair mid-water trawl, single
mid-water trawl and bottom trawl). The sample parameters for a given
Unit are weighted by the total catch from that Unit. In the event that
sample data is unavailable for a particular Unit, sample data are borrowed
from the next adjacent Unit, with preference given to borrowing between
months as opposed to geographically. The catch-at-age matrix for each
Unit are summed across all Units for total catch at age for the year
for all US landings from the complex.
BIOSTAT first sums the catch (in metric tons) by a Unit. A length frequency
grouped by centimeters is then developed from the sample data in that
Unit. An age-length-key is then developed from the frequency of age
by centimeter length group (mm as total length) from all samples in that
Unit. The age frequencies are proportioned across ages for each length
group. Mean weights (grams) at age are calculated from all individual
weights within an age class from all samples in a particular Unit. The
mean weights at age are then multiplied by the sum of numbers at age,
which gives an expanded weight at age in that Unit. Catches in weight
(mt) at age are derived from total weight of the catch multiplied by
the weight at age proportion. Catch in numbers at age is calculated from
catch in weight at age divided by the mean weight at age times 1,000,000
(convert grams to metric tons) for each Unit.
Strong 1994, 1996, and 1998 year classes are evident in the catch-at-age
matrix (Table 3.3). Since the early 1990s, greater
numbers of older fish (7+) have occurred in the stock. This is probably
due to the demise of the inshore fixed gear fishery (which tended to
catch smaller fish) and an overall reduction in fishing mortality on
the stock complex during the last decade.
4.0. Research Surveys
Over the years both Canada and the United States have surveyed the
distribution and abundance of herring in the Gulf of Maine (Table
4.1). While both bottom trawl and larval surveys have been explored
as indices of abundance for herring, only the former provide a continuous
time series and both countries in the mid-1990s abandoned larval surveys
in the mid 1990s. Recently, the US and Canada have each moved toward
acoustic surveys to estimate herring biomass. The US now conducts annual
acoustic surveys to assess the abundance and distribution of herring
in the Gulf of Maine and on Georges Bank. Canada and the US also use
industry based acoustic surveys to provide supplemental estimates of
spawning stock biomass on specific inshore spawning grounds.
4.1. Indices of abundance
Indices of abundance, which are considered to reflect changes in the
population, are critical in the evaluation of stock status Both Canada
and the US have conducted fall larval surveys on Georges Bank and in
the Gulf of Maine. The US larval survey, which extends from 1971-1994,
was used in past Gulf of Maine/Georges Banks assessments (1991, 1993,
1996) as a tuning index along with indices from bottom trawl surveys.
The US index of the number of 4-7mm larvae per 10 m2 (#/larvae/10m2)
was developed as a composite of four individual annual surveys conducted
under various programs (Smith and Morse, 1993). Canada conducted fall
larval surveys from 1987-1995 and used the # larvae (<10mm)/m2 as
an abundance index on Georges Bank (Melvin et al. 1996).
Both surveys showed that rebuilding began in the mid-to late 1980s
and continued during the early 1990s (Figure 4.1).
4.2. Research Vessel Bottom Trawl Surveys
Several research vessel bottom trawl survey series have been conducted
within the geographical range of the Gulf of Maine-Georges Bank herring
complex. These surveys vary in temporal and spatial coverage from almost
the entire range to selected portions on Georges Bank and southern New
England. Trends in survey abundance indices are shown in Figure
4.2.
The Canadian spring bottom trawl survey (conducted primarily to assess
the abundance of ground fish, on Georges Bank and west as far as Nantucket
Shoal (in some years)), covers the northern extent of the winter/spring
distribution of herring in the GOM and on Georges Bank (Table
4.2).
The US autumn bottom survey covers the entire distribution of herring
off the northeast coast. The survey, which began in 1963, occurs when
the majority of adult herring are aggregated to spawn (Table
4.4). During the early years of the survey, catches of herring were
relatively low (Figure 4.3). Catches of herring increased during the
mid 1980s until 1992, declined slightly in 1993-94, and then sharply
increased in 1995. The 1995 increase is the result of large catches of
age 1 herring which nearly doubled the index. Catches thereafter declined
through 1998, but increased afterwards and reached a record high in 2002. Autumn
survey indices for ages 2 to 8 are presented in Figure
4.4.
The US winter bottom trawl survey was initiated in 1992 and covers
a large portion of the spatial distribution of over-wintering herring.
The annual survey, which begins in early February (Table 4.3),
extends from Cape Hattaras to Cape Cod and along the southern flank of
Georges Bank. The survey indices were variable during the early 1990s,
peaked in 1996, and then declined in 1997. Since 1998, the index has
steadily increased (Figure 4.5). Catch-at-age
indices for age groups 2 to 8 during 1992-2002 are presented in Figure
4.6.
The US Spring bottom trawl survey covers the entire US range of herring
during the late winter and early spring. This survey series , which began
in 1968 (Table 4.5), has used the same sampling
gear except for a net change to the Yankee 41 trawl during 1973-1981
and for a door change in 1985. The survey vessel has variously been
the R/V Albatross IV and the R/V Delaware II with appropriate fishing
power corrections incorporated into the index (Figure
4.7).
The spring survey herring abundance index was relatively flat during
1975-1983 when herring were scarce, then gradually increased during the
early 1990s, the index markedly increased and peaked at a record high
in 1999. The index has since declined to about the long-term (1983-2002)
mean in 2000 (Figure 4.7; Figure 4.8).
5.0. Growth
Annual growth, as represented by the mean length-at-age of herring
collected during the fall spawning season on Georges Bank, has undergone
some marked changes (P<0.01) since the early 1980s. Herring from
the 1983-1985 year-classes grew more rapidly than those spawned during
1987-1991 (Figure 5.1). At age 2, the year-class
mean lengths are distributed over a 2cm interval, however by age 4 there
is almost a continuous decrease in the mean length from 1983-1991. By
the time the fish reach age 5 and 6, a difference in mean length of 2
cm or more can be observed. The 1986 year-class seems to represent a
transition between the two trajectories. Assuming a constant weight length
relationship, a 6 year old from the early period would weight 245g compared
to196g for the same size fish collected in the late stages of the recovery,
a 20% difference in biomass.
6.0. Canada/US Age comparisons
Consistent and comparable aging is critical when data are being combined
from two independent sources. To investigate potential difference or
biases in aging, 213 otoliths, from the Gulf of Maine, were selected
for aging by readers from both countries. Three independent agers, one
from the NMFS Northeast Fisheries Science Center (US-1), one from the
Maine Department of Marine Resources (US-2), and one from the DFO St.
Andrews Biological Station (Can-1), read each otolith.
Age determinations between the two US readers were very consistent;
percent agreement between the two agers was 85% with a slight bias toward
under-aging (10%) vs over-aging (5%) by US-1 relative to US-2 (Table
6.1). However, significant differences (P<0.01) were detected
between the age determinations of the Canadian reader and both US readers
(Tables 6.2 and 6.3). Percent
agreement was 76% with US-1 and 78% with US-2. In both cases, there was
a tendency for the Canadian reader to under-age (16% and 18%) rather
than over-age (8% and 4%) relative to US age readers. Beyond age 6, the
disagreements exceeded 50%.
Differences between the Canadian and US age reading are of concern
and will require some time and effort to resolve. An aging workshop amongst
the readers from both countries will be convened during 2003 to address
this issue. In the interim age-length keys will be applied according
to the data origin. That is, data collected by the US will use US ages
and Canadian data Canadian ages. While this may affect the catch-at-age
matrix and age disaggregated indices of abundance, mixing of the data
sources would further complicate the issue.
7.0 Acoustic Surveys and Results
This section describes various acoustics surveys, their design, and
results. Only available as PDF file (7.5
KB). Tables and figures linked below.
| Table 7.1 |
Parameters for length weight equations for Atlantic herring from
NMFS autumn research vessel bottom trawl surveys in the Gulf of Maine-Georges
Bank Region |
| Table 7.2 |
Mean herring backscatter and SD of backscatter from acoustic
surveys on Georges Bank during 1999-2002 |
| Table 7.3 |
Intercepts from target strength equations from studies on herring
stocks in the North
Atlantic |
| Table 7.4 |
Geostatistical estimates of biomass, CV, CV inverse, weighted biomass
and weighted CV for Acoustic surveys on Georges Bank during
1999-2002 |
| Table 7.5 |
Point estimates of mean Sa and biomass from standard
statistical analysis for surveys on Georges
Bank during 1999-2002 |
| Figure 7.1 |
Cruise tracks for surveys on Jeffreys Ledge, Platts
Bank, Cashes Ledge and Fippennies Ledge during the 1998 Atlantic
Herring Hydroacoustic Survey. |
| Figure 7.2 |
Cruise track for the systematic parallel survey in the Gulf of
Maine during the 1998 Atlantic Herring Hydroacoustic Survey |
| Figure 7.3 |
Cruise track for one zigzag survey on the northern edge of Georges
Bank during the 1998 Atlantic Herring Hydroacoustic Survey. |
| Figure 7.4 |
Cruise track for surveys on Jeffreys Ledge, Platts Bank, Fippennies
Ledge, Cashes Ledge, Franklin Swell, and Georges Bank during the
1999 Atlantic Herring Hydroacoustic Survey. |
| Figure 7.5 |
Cruise track for the systematic parallel survey circumscribing
Georges Bank during the 1999 Atlantic Herring Hydroacoustic Survey |
| Figure 7.6 |
Cruise track for systematic parallel surveys conducted on Jeffreys
Ledge, Platts Bank, Fippennies Ledge, Cashes Ledge, and Georges Bank
during the 2000 Atlantic Herring Hydroacoustic Surveys |
| Figure 7.7 |
Cruise track for the systematic zigzag survey on Georges Bank during
2000 |
| Figure 7.8 |
Cruise track for the stratified random survey design on Georges
Bank during 2000 |
| Figure 7.9 |
Complete set of potential stratified random parallel transects
for survey in Atlantic herring on Georges Bank during the 2000 Atlantic
Herring Hydroacoustic Survey |
| Figure 7.10 |
Cruise tracks for systematic parallel surveys on Jeffreys Ledge,
Platts Bank, Fippennies Ledge, and Cashes Ledge during 2001 |
| Figure 7.11 |
Cruise track for the systematic parallel survey on Georges Bank
during the 2000 Atlantic Herring Hydroacoustic Survey |
| Figure 7.12 |
Cruise track for the random parallel survey on Georges Bank during
the 2001 Atlantic Herring Hydroacoustic Survey |
| Figure 7.13 |
Cruise track for systematic zigzag survey and experimental work
on Georges Bank during the 2001 Atlantic Herring Hydroacoustic Survey |
| Figure 7.14 |
Stratified random parallel transect design for surveying Atlantic
herring on Georges Bank during 2001 |
| Figure 7.15 |
Parallel design for surveying Atlantic herring on Jefferys Ledge
during 2002 |
| Figure 7.16 |
Parallel design for surveying Atlantic herring on Georges Bank
during 2002 |
| Figure 7.17 |
Herring backscatter on transects from a zigzag survey design
on Georges Bank during 1998 |
| Figure 7.18 |
Herring backscatter on transects from a zigzag survey design on
Georges Bank during 1999 |
| Figure 7.19 |
Herring backscatter on transects from a zigzag survey on Georges
Bank during 1999 |
| Figure 7.20 |
Herring backscatter on transects from a parallel design on
Georges Bank during 1999 |
| Figure 7.21 |
Herring backscatter on transects from a zigzag survey design
on Georges Bank during 2000 |
| Figure 7.22 |
Herring backscatter on transects from a Parallel survey design
on Georges Bank during 2000 |
| Figure 7.23 |
Herring backscatter on transects from a stratified random
survey on Georges Bank during 2000 |
| Figure 7.24 |
Atlantic herring backscatter on transects from a parallel
survey design on Georges Bank during 2001 |
| Figure 7.25 |
Atlantic herring backscatter on transects from a zigzag survey
design on Georges Bank during 2001 |
| Figure 7.26 |
Atlantic herring backscatter on transects from a stratified
random design on Georges Bank during 2001 |
| Figure 7.27 |
Atlantic herring backscatter on transects from a parallel
survey design on Georges Bank during 2002 |
| Figure 7.28 |
Variogram from the parallel survey design on Georges Bank during
2000 |
| Figure 7.29 |
Biomass from bootstrap analysis for zigzag part 1, zigzag part
2, and parallel survey designs on Georges Bank during 1999 |
| Figure 7.30 |
Biomass from bootstrap analysis for zigzag, parallel, and
stratified random survey designs on Georges Bank during 2000 |
| Figure 7.31 |
Biomass from bootstrap analysis for zigzag, parallel, and
stratified random survey designs on Georges Bank during 2001 |
| Figure 7.32 |
Biomass from bootstrap analysis for a parallel survey design on
Georges Bank during 2002 |
VPA Calibration and Diagnostics
8.0. Previous Assessments
Assessments of Atlantic herring have always been complicated due to
the migratory behavior and the intermixing of stocks. Over the years,
a variety of assumptions have been made about stock structure and seasonal
composition.
During the 1970s independent assessments of the Gulf of Maine and the
Georges Bank herring stocks were undertaken by ICNAF (Anthony and Waring,
1980, Stevenson et. al., 1997). Estimates of population size were based
on un-tuned VPAs or cohort analyses, had no fishery independent information
to estimate F, and relied on juvenile catch information to estimate recruitment.
These models estimate population biomass to be approximately 1.3 mt in
1967-1968, and 204,000t (4+) in 1976. In 1976, the population size of
the western GOM was estimated to have been 159,000t, less than 100,000t
in the early 1970s and only 65,000t in 1976 (ICNAF Redbook, 1976).
One approach to avoiding the problem of stock intermixing is to perform
a “pooled assessment”. Anthony (1977) combined herring catches from
south of Cape Cod, Georges Bank, the Gulf of Maine, and the Bay of Fundy
and as far north as Chedabucto Bay. Sissenwine and Waring (1979) did
the same thing when they pooled catch-at-age data for all herring fisheries
between Southwest Nova Scotia and Cape Hatteras in their analysis of
herring fisheries of the Northwest Atlantic.
After the decline in fishing effort by foreign fleets and the collapse
of Georges Bank herring stock around 1976, assessments undertaken during
the 1980s concentrated on the GOM stock. Three assessments of the GOM
stock were conducted during this period using the spring bottom trawl
survey indices (number/tow) to tune a VPA developed using pooled catch-at-age
data. These assessment were considered flawed in that the tuning indices
did not solely represent the GOM stock, but, also included fish from
GB and Nantucket Shoals . Spawning stock biomass estimates for the GOM
stock were relatively low in the late 1970s, 30,000t in 1982, then increased
rapidly throughout late 80s to exceed 150,000t.
Confusion over the definition of Georges Bank versus Gulf of Maine
fish continued in the 1980s. Anthony et al. (1981) attempted to exclude
Georges Bank fish in their assessment of herring stocks of the Gulf of
Maine, but they included herring from the southern New England winter
fishery in their analysis. At the 1989 SAW Fogarty et al. (1989) stated
that “Atlantic herring throughout the Gulf of Maine, Southern New England
and mid-Atlantic regions are considered to be part of a single stock. Accordingly,
we developed a single abundance index for this region”. Until 1989, it
was assumed that the US fall survey might provide an index of abundance
for the individual stocks. However, the fall survey data were determined
to be too variable, to be a reliable indicator of abundance for either
the individual stocks or the stock complex.
Uncertainties in distribution, stock inter-mixing, and assignment of
catches continued to plague the assessment in 1990. At the Eleventh SAW
(NEFC, 1990, p. 58) the Gulf of Maine herring stock was defined as:
“The Gulf of Maine stock was considered to include
all fish found in NAFO areas 5Y and 5Zw (i.e., excluding fish from
area 6, which were assumed to belong to either Georges Bank or Nantucket
Shoals stocks; and excluding fish from Sub-area 4, which were assumed
to belong to Atlantic Canadian stocks). However, an unknown amount
of mixing occurs during winter/spring between Gulf of Maine, Georges
Bank and Nantucket Shoals stocks in the Mid Atlantic and Southern New
England Areas”.
Prior to 1991, the Georges Bank/Nantucket Shoals and the coastal Gulf
of Maine stocks were therefore assessed separately (Anthony and Waring,
1980, Fogarty and Clark, 1983 and Fogarty et al, 1989).
In 1991, two major changes were made in the assessment of GOM/GB herring.
The first was the introduction of a correction factor (approximately
50% reduction in catch rates) to account for differences in fishing power
of the R/V Delaware II vs. R/V Albatross IV. Secondly, a change was made
from assessing the stocks separately to treating them all as a single
stock complex. Examination of the NEFSC spring survey data series revealed
that no geographical grouping of strata could be used to represent either
the Georges Bank or Gulf of Maine stock unit. Consequently, for the purposes
of assessing abundance, herring from the coastal Gulf of Maine, Nantucket
Shoals and Georges Bank were treated as a single highly migratory coastal
herring population that had distinct spawning areas.
“...the SARC consensus was that both the
catch at age matrix and the spring survey indices of abundance reflect
not only the “coastal” stock but also intermixing of fish from New
Brunswick weir catches and Georges Bank stocks. The SARC, therefore,
decided that the assessment should be based on an aggregate stock complex
including coastal, Georges Bank and New Brunswick weir caught fish” (NEFSC,
1992, p. 62).
Since 1991, Georges Bank/Nantucket Shoals and the coastal Gulf of Maine
stocks have been considered part of a migratory coastal herring complex
possessing distinct spawning components. Other important changes to the
assessment have included; the inclusion of New Brunswick fixed gear (weir/shutoff)
catches in the catch-at-age in the 1993 assessment; the use of a larval
abundance estimate as an index of SSB in the 1991,1993 and 1996 assessments;
and the introduction of the NEFSC winter survey index which in the 1998
assessment. The fall bottom survey was also reexamined in 1998, but was
not used as a tuning index.
The last assessment was conducted on the coastal stock complex in 1998
and estimated SSB through 1997. The VPA was tuned using the US spring
bottom trawl index from 1968-1997 (ages 2-8), and the winter survey index
during 1992-1997 (ages 2-8). The spring indices were based on herring
catches in survey strata 1-30, 36-40, and 61-76, while the winter survey
indices were based on herring catches in survey strata 1-3, 5-7, 9-11,
13-14,16, 61-63, 65-67, 69-71, and 73-75.
9.0 VPA
Input data (catch-at-age, winter and spring survey indices, mean weights,
etc.) for the VPA were updated through 2001 and a new calibration was
completed using the same formulation as in the previous (1998) assessment. The
VPA results indicate that spawning biomass increased greatly in the 1990s
and fishing mortality was very low in 1999-2001 (Figure
9.1 and Figure 9.2). Recruitment improved dramatically
in the 1990s with several large year classes (1994, 1998) and moderate
year classes (1993, 1995, 1996) being produced (Figure
9.3).
Retrospective Analysis of VPA
A retrospective analysis of the VPA for the herring complex was completed
using the FACT 1.05 software for the years 1995-2001. The formulation
was the same as used in the 1998 assessment except that catch-at-age
and research survey indices for winter and spring were added for 1998-2001. Results
were similar to those obtained in the last assessment, in that severe
retrospective patterns were apparent in spawning stock biomass, fishing
mortality, and recruitment. Both spawning stock biomass and recruitment
were overestimated in successive years during 1995-2001 (Figure 9.1 and
Figure 9.3), while fishing mortality was underestimated (Figure 9.2). Recent
landings of herring from the complex are low relative to stock size;
resulting in fishing mortality (F) being very low relative to natural
mortality (M) A succession of moderate to large year classes in the
1990s has apparently made it difficult to estimate recruitment accurately. In
addition, the increase in biomass apparent from survey indices in the
1990s is not estimated very well. Examination of trends in SSB from
the 1997 and 2001 VPAs, revealed that increases in biomass are very abrupt
with SSB reaching high values in only the last year of each run (Figure
9.4). Because of the retrospective pattern and the inability to
precisely estimate biomass in the 1990s, a forward projection modeling
was used to assess stock status.
10.0. Forward Projection Approach-Application
and Description
This section is only available as a PDF file.
Tables and figures are linked below.
| Table 10.1 |
Variance, number of observations, and degrees of freedom from spawner
recruit models for various North Atlantic stocks of herring |
| Table 10.2 |
Landings of Atlantic herring from the Gulf of Maine
- Georges Bank complex during 1959-2002 |
| Table 10.3 |
Research survey catch per tow (kg) for age 2 and age 3+ for US
winter, spring, and fall and Canadian spring during 1963-2002 |
| Table 10.4 |
Time series of survey catch for the US acoustic survey, the US
larval survey, and the Canadian larval survey during 1971-1995 |
| Table 10.5 |
Likelihood profile analysis for base case forward projection model |
| Figure 10.1 |
Overall proportion of mature herring at different maturity stages
during acoustic survey cruises during 1999-2002 |
| Figure 10.2 |
Maturity stages observed during consecutive herring acoustic surveys
(starting with zz01) on Georges Bank during 1999 |
| Figure 10.3 |
Maturity stages observed during consecutive herring acoustic surveys
(starting with pl00) on Georges Bank during 2000 |
| Figure 10.4 |
Maturity stages observed during consecutive herring acoustic surveys
(starting with zz01) on Georges Bank during 2001 |
| Figure 10.5 |
Maturity stages observed on a herring acoustic survey on Georges
Bank during 2002 |
| Figure 10.6 |
Log recruit numbers plotted against spawning biomass for ten North
Atlantic Herring Stocks |
| Figure 10.7 |
Distribution of variance estimates for log recruitment residuals
from nonparametric stock recruit models for ten North Atlantic herring
stocks |
| Figure 10.8 |
Surface-Bottom gradient from differencing the surface and bottom
temperatures from the Gulf of Maine during 1963-2000 |
| Figure 10.9 |
Sea surface temperature anomalies for the Gulf of Maine during
1963-2000 |
| Figure 10.10 |
Autumn survey timing (mean Julian date) during 1963-2001 |
| Figure 10.11 |
Autumn survey residuals and mean survey timing (Julian date) for
1963-2000 |
| Figure 10.12 |
Spring surface-bottom gradient from differencing the surface and
bottom temperatures from the Gulf of Maine during 1968 |
| Figure 10.13 |
Spring survey timing (mean Julian date) during 1968-2002 |
| Figure 10.14 |
Spring surveys age 2 weight/tow showing differences in residual
patterns for age 2 without and with a door covariate for 1968-2002 |
| Figure 10.15 |
Autumn surveys age 2 and age 3+ weight/tow showing differences
in residual patterns for age 2 without and with a door covariate
and age 3+ for 1963-2002 |
| Figure 10.16 |
Average biomass estimates from F/V Providian for inshore and nearshore
Gulf of Maine and Georges Bank during 1999 and 2000 |
| Figure 10.17 |
Observed vs predicted (log scale) and residuals vs time for the
spring age 2, spring age 3+, and winter age 2 US surveys |
| Figure 10.18 |
Observed vs predicted, and residuals vs time for the hydroacoustic,
US Larval survey, and the Canadian Larval survey |
| Figure 10.19 |
Observed and predicted relative abundance and residuals vs time.for
the Canadian age 2 survey, and Canadian age 3+ survey |
11.0 Forward Projection Analysis Results
A full table of output and results is provided in Appendix
II.
11.1 Estimates of Fishing Mortality
Fishing mortality was below 0.2 during the early 1960s followed by
a large increase in F to about 0.8 during the late 1960s (Figure
11.1). This coincides with a major increase in fishing effort during
this period. F increased again in the mid and late 1970s to above 1.0
but declined sharply in 1984 to F=0.2 (Figure 11.1). F remained steady
at about this rate during 1985-1989 and then fell further after 1990. The
fishing mortality in 2002 on the coastal complex was about 0.06 (Figure
11.1).
11.2 Estimates of Biomass
Total stock biomass in the Gulf of Maine-Georges Bank herring complex
was about 1.4 million mt in 1962, and steadily declined to a low of 87,000
mt in 1982 (Figure 11.2). Stock biomass increased gradually after 1983, reaching
1.0 million mt in 1994 and 1.8 million mt in 2000 (Figure
11.2).
Spawning stock biomass followed a trend nearly identical to total biomass,
declining from 1.2 million mt in 1962 to a low of 42,000 mt in 1982 (Figure
11.3). SSB increased steadily afterward this to 1.0 million mt in
1996 and 1.7 million mt in 2001.
11.3 Recruitment
Recruitment during the 1960’s was generally moderate with large 1968
and 1970 year-classes (Figure 11.4). All subsequent
year classes through 1986 were below average or poor (Figure 11.4). Recruitment
markedly improved during the 1990s with the very large 1994 and 1998
year-classes.
11.4 Stock-Recruitment
A Beverton-Holt stock-recruitment relationship was estimated within
the forward projection model. This relationship added some stability
to the model and provided a reasonable fit to the available time-series
of data (Figure 11.5). Most years fit the
model well with the exception of large residuals (implied) for the 1994
and 1998 year-classes (Figure 11.5).
11.5 Precision of FPA Estimates
The precision of terminal 2002 year estimates of spawning biomass and
fishing mortality were estimated using bootstrap procedures. Estimates
of spawning stock biomass in 2002 ranged from 0.8-2.7 million mt with
a median of 1.5 million mt and 80% CI of 1.2-1.8 million mt (Figure
11.6). Estimates of F in 2002 ranged from 0.02-0.12 with a median
value of F=0.066 and 80% CI of 0.054-0.084 (Figure
11.7).
11.6 Retrospective Analysis of FPA
A retrospective analysis was performed using the FPA model including
terminal catch years 1996-2002. No discernable patterning was evident
in the estimates of fishing mortality in the retrospective runs. Estimates
of fishing rates were relatively close in successive terminal years (1996-2002)
(Figure 11.8).
Similarly, no retrospective patterns were detected in spawning stock
biomass estimates (Figure 11.9). There is
a break between 1998 and 1999 associated with the time when estimates
of biomass from hydroacoustic surveys first become available (Figure
11.9). This discontinuity continues back until 1992 where it disappears.
Estimates of recruitment exhibit little retrospective patterning over
the 1996-2002 period (Figure 11.10). There
are some differences in the estimation of the size of the 1994 and 1998
year-classes in 1996 and 2000 respectively, but these do not occur in
a sequential pattern.
11.7 Losses to Natural Mortality
Landings greatly exceeded losses to natural mortality during the late
1960s through the mid 1970s (Figure 11.11). Since
1970, landings have been less than losses to M (Figure 11.11).
12.0 Biological Reference Points
12.1 YPR and SSB/R
Yield per recruit and SSB per recruit reference points for the Gulf
of Maine-Georges Bank herring complex were last estimated in the assessment
conducted in 1996 (NEFSC 1996). Reference points from that analysis
were F0.1=0.20, F20%=0.34, and Fmax=0.40. Yield
per recruit and SSB per recruit reference points were re-estimated with
more recent data (last 5 years) using the Thompson and Bell (1934) model
(Table 12.1). Herring were assumed to be fully
recruited at age 2 and fully mature at age 3. Estimated reference points
were F0.1=0.18, F40%=0.15 and Fmax=0.40
(Table 12.2; Figure
12.1).
12.2 Surplus Production
Estimates of surplus production parameters from the 1998 assessment
(NEFSC 1998) were derived using an ASPIC model that was conditioned with
the B1 ratio fixed at 1.0 to produce stable estimates of parameters. This
was the model accepted by the Overfishing Definition Review Panel (ODRP)
in 1998 (NEFMC 1998). Estimates of biological reference points from
1998 surplus production analysis were MSY=317,000 mt, Bmsy=1.066 million
mt and Fmsy=0.30.
Surplus production parameters were re-estimated using a Fox (1975)
model and also a Schaefer (1954) model. The Fox model is asymmetric
and was considered a better match with the Beverton-Holt stock-recruitment
model. Biological reference points from the Fox model were MSY=222,000
mt, Bmsy=896,000 mt and Fmsy=0.25 (Figure
12.2). Reference points from the Schaefer model were MSY= 243,000
mt, Bmsy=1.03 million mt, and Fmsy=0.24.
During the early 1960s through the late 1970s, landings and surplus
production were about equal (Figure 12.3). Starting
in 1982, landings declined leading to a gradual and then large increase
in the stock during the 1990s. Surplus production in the 1990s exhibited
several large peaks representing the recruitment of the very large 1994
and 1998 year-classes (Figure 12.3).
13.0 Projections
Given that total stock biomass of the Gulf of Maine-Georges Bank herring
complex has been above Bmsy since the mid 1990s, projections were conducted
to estimate 2+ stock size in 2004 and 2005 under several assumptions
of fishing mortality. The landings in 2003 were assumed to be 100,000
mt, approximately equal to that in 2002. Natural mortality was assumed
to be 0.2 and two levels of F were used in the projections; F=0.2, a
fishing rate approximately equal to the Fmsy reference point for the
complex and F=0.1. A delay-difference projection model was constructed
to simulate the dynamics of the herring complex. Bootstrap estimates
of stock biomass for 2001 and 2002 were input to the model. Recruitment
was modeled using the Beverton-Holt stock-recruitment relationship, parameters
from the FPA final model, and a lognormal error structure. Results were
summarized for the 750 bootstrap runs and median (50%) 2+ stock size
and F values were produced.
An F=0.2 in 2004 would produce a catch of 323,000 mt and a reduction
in stock size from 1.80 million mt in 2004 to about 1.64 million mt in
2005. An F of 0.1 in 2004 would produce a catch of 170,000m t but no
change in biomass (1.79 million mt in 2005).
14.0 Gulf of Maine Herring Complex
Adapt (VPA) analysis
14.1 Introduction
This section of the report deals with the ADAPT formulation used to
assess the status of the Gulf of Maine/Georges Bank herring stock complex.
Specific input parameters such as age composition, mean weight at age,
percent maturity and details on tuning indices are only discussed in
general as they are discussed in depth in other sections of this report.
14.2 Analytical Approach
The assessment process was initiated with the reproduction of the 1998
assessment formulation using data through 1997. Some difficulty was initially
encountered in exactly matching the F’s on the older ages and the use
of the age 11 as a plus group. However, when age 11 was considered as
a non-plus group the results were almost identical to the 1998 VPA results.
The ratio of population numbers in the preliminary VPA was compared with
population numbers from the 1998 assessment (Table
14.1). Most ratio values are close to 1 indicating little or no difference. Blank
cells in the table are due to zero values in the 1998 assessment while
large values (>1) are assumed to be due to precision errors from using
population numbers rounded to millions.
As an initial run, the data series were updated for 1998-2002 and the
VPA re-run using the 1998 assessment formulation. Indices of abundance
included the NMFS spring bottom trawl survey stratified mean number per
tow for strata 1-30, 36-40, and 61-76 from 1968 to 2002 and the winter
NMFS bottom trawl index for strata (1-3, 5-7, 9-11, 13-14, 16, 61, 63,
65-67, 69-71, 73-75 for 1992-2002).
The results were examined in relation to how the age 11 in the catch
at age was used and what effect various assumptions had on the results.
Treatment of age 11 as a plus group appeared to be inconsistent with
the observations from research surveys and the fishery. Specifically:
- Older ages formed a very small portion of the catch at age (less
than 0.5% for ages 9 and 10 since 1973 (Table14.2).
- There was no accumulation effect in the plus group.
- A high and variable mortality after age 7, especially in most recent
years, was seen in the VPA results and PR patterns.
- Very few fish older than age 8 were observed in the any of the survey
series.
Several modified versions of the initial formulations were investigated.
Estimates of SSB from the various treatments of age 11 are shown in Figure
14.1. All runs used the same formulations.
Treatments included:
- age 11 as a plus group using FIRST method with F11=F10, F10 = weighted
avg. 5-9. This method closely matches the 1998 assessment formulation.
- age 1-11 no plus group, oldest age using F11=F10
- age 1-10 no plus group, age 11 removed, oldest age using F10 = weighted
average ages 5-9.
However, since 11+ fish do not occur in either the fishery or in the
population (from trawl surveys) the treatment of these older fish should
not be a significant source of error for short-term projections. The
issue remaining is, will there be accumulation in the population at older
ages if F is maintained at moderate levels and the implications for stock
biomass, stock recruitment and reference points.
14.3 Summary of the 1998 Extended VPA
Input parameters and results for the initial VPA matching the 1998
assessment formulation and extending time series are summarized as follows.
1998 extended analysis (Appendix III):
- catch at age for 1967 to 2001 for ages 1-10 only (note that
the 2002 catch at age was not available when this analysis was first
done)
- no plus group assumed for reasons explained
- tune using winter survey 1992-2002 for ages 2-8 and spring survey
1968-2002 for ages 2-8; no special weighting with each value getting
equal weight
- m=0.2; F on oldest age 10 calculated based on F on age 9
- Year 2002: estimate ages 5,6,7 and assign ages 1-2
- Year 2001: remaining ages 2,3,7,8,9 calculated as a weighted F of
ages 4,5,6 in 2001
Details
of the 1998 extended ADAPT run are presented in Appendix
III. Using the above assumptions, total stock (age 1+) biomass
in 2003 was calculated to be 1,580 kt; 3+ biomass, 1,400 kt; and SSB,
1,350kt (Figure 14.11). The mean squared
residuals (MSR) by age were high for both surveys with many values >1.0
(Table 14.3, Figure 14.3) and the residual
plots by year and age showed patterns that were either all positive
or all negative by year and are large (Fig. 14.2, 14.3, 14.4). Age
by age plots of observed vs predicted abundance show a relatively poor
fit for the winter survey, but a much better pattern for the spring (Figs
14.5, 14.6) Diagnostic plots of survey
q’s by age also showed time trends and were not consistent over the
series (Fig. 14.7, 14.9).
Overall survey q’s by age were dome shaped (Fig.
14.8, 14.10) indicating lower catchability
for younger and older ages included in the formulation. There was an
indication of several strong recruiting year-classes in recent years,
a pattern of reduced fishing mortality since 1982, and a trend of increasing
biomass since about 1997 (Figure 14.11).
A severe retrospective pattern was detected beginning in 1999 but it
was not as pronounced in 2000 and 2001 (Fig
14.12).
14.4 Final VPA
Several factors affecting the input parameters and the diagnostics
were examined before adopting the final formulation. These included the
analysis of a truncated time series and the use of the mean square residual
as a selection criteria for the tuning indices.
14.5 Analysis using split survey series
In reviewing the survey abundance indices, several of the indices exhibited
inconsistencies between the pre and post collapse period. A breakpoint
was identified around 1984 which corresponded wit
