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INTRODUCTION
To evaluate the status of exploited fishery resources, we must
collect and analyze many different kinds of information. Basic landings
statistics (including the numbers and weight of each species landed) and
demographic data (such as length and age samples) characterize what is brought
ashore. At-sea sampling aboard commercial fishing vessels is used to establish
the numbers and length/age composition of animals culled overboard. Telephone
recall and roving samplers are used to estimate recreational catches. Effort
data collected with catches are combined into indices of stock abundance based
on catch-per-unit effort (CPUE) ratios. Data from these three types of
collection programs are generally referred to as fishery-dependent
information: these data are derived directly from the commercial and
recreational fisheries.
Fishery-dependent data are vital to our ability to monitor
stocks, and for some species are often the only reliable source of data.
However, use of fishery-dependent data alone may severely limit our ability
to evaluate and make predictions about the status of some stocks. For example,
in fisheries heavily dependent on the yearly incoming age group (the new recruits), fishery data alone cannot
be used to forecast catches because very small fish are generally not taken
with standard fishing gear. Likewise, CPUE may not be a reliable measure of
abundance for schooling species, or when the increase in fishing
technology cannot be factored into the relationship between catch and fishing
effort. Consequently, fishery scientists throughout the world are conducting research vessel sampling programs to gather fishery-independent information
(Clark 1981). The scope and use of research vessel surveys in assessing
Northeast fishery resources is reviewed here.
WHY CONDUCT RESEARCH VESSEL SURVEYS?
Fishery-independent surveys of Northeast fishery resources are
conducted for six important reasons:
(1) To monitor recruitment: Research
surveying is generally conducted with sampling gear equipped with smaller mesh
than is allowed in most fisheries. Small-mesh gear is used in order to
estimate the abundance of very small animals that will eventually become large
enough to be caught in standard fishing gear. To predict future landings and
stock sizes, we must estimate the survival of fish already large enough to be
retained by harvesting gear as well as the incoming recruitment to the fishery
each year. Depending on the species, research vessel surveys can allow
extrapolation of the strength of incoming age groups up to several years
before they are allowed to be landed. For example, American
plaice reach the minimum size for landing (14 in.) at about age 6.
Trawling surveys begin to sample small American plaice during their first year
of life, thus allowing five estimates of the relative numbers of small plaice
in the population before animals are large enough to land.
For some species, however, growth rates are much faster, and
thus the time interval between when fish are detected by the surveying
gear and when they are landed by the fisheries is much shorter. In the case of
the Atlantic sea
scallop, annual dredge surveys are conducted. The relative catch in
numbers of sea scallops per haul of the dredge is plotted for two size
categories of scallops: those with a shell height smaller than 70 mm (2-3/4 in.) are
designated prerecruits; and animals larger than or equal to 70 mm are
designated harvestable. The abundance of all sizes of sea scallops has
declined significantly since the peak in 1989.
(2) To monitor abundance and survival of
harvestable sizes: Research vessel samples generally span the full
size and age range of a population on the shelf. Although recruitment
prediction is one important element of fishery forecasts, it is equally
important to calculate the survival rate of the portion of the stock already
subjected to fishing. The catch-at-age data collected from the surveys are
one important source of information used to estimate survival rates from one
year to the next. A simple estimate of the survival of various age groups in
the population can be computed from the indices of abundance in two
consecutive years. If the catch-per-trawl-haul of age 4 cod
was 100 individuals in 1991 and the catch-per-haul of the same fish, now
age 5 in 1992, is 40 individuals, the estimated survival rate is 40/100 = 0.4
= 40 percent. In practice, fishery scientists usually combine catch-at-age
data from the surveys with similar data from the fishery catch to improve
estimates of fishing mortality and stock sizes. These combined estimates allow
calculation of the population that must have existed to yield the catch levels
observed during the recent history of the fishery.
Sampling the abundance of harvestable sizes from research
vessel surveys may be the only source of data available for species that have
never been fished in the past, or are only fished at very low levels. Thus,
dredging surveys conducted in the 1960s and 1970s were the only source of
information on the abundance of the ocean
quahog resource of the Middle Atlantic, Southern New England and Georges
Bank areas. Minimum population estimates were made by expanding the average
catch-per-square-nautical-mile from the surveys by the number of square
nautical miles of sea bottom inhabited by the stock. Similarly, current
knowledge of the stock biomass of spiny
dogfish and skates
is based only on surveys, since catch-at-age based studies have not been
undertaken.
(3) To monitor the geographic distribution
of species: Some species lead sedentary lives while others are highly
migratory. Research vessel surveys are a major source of data on the movement
patterns and geographic extent of stocks. Distribution maps can be drawn from
reports of fishermen, but these may give a biased picture of the stock,
emphasizing only where high density fishable concentrations exist.
Distribution data are important not only for fishery management, but also for
evaluating the population level effects of pollution and environmental change.
(4) To monitor ecosystem changes: With
few exceptions, surveys conducted by the Northeast
Fisheries Science Center are designed to be multipurpose. Bottom trawl
surveys are not directed at one species, but rather generate data on nearly
200 species of fish and invertebrates in northeastern U.S. continental shelf
waters. Many of these species are relatively rare, and have little or no
commercial or recreational value. However, when we evaluate the effect of
intensive harvesting on selected species, we can observe the response of the
entire animal community. The dramatic changes in the system reflect the
depletion of several important commercial fishery species, such as, haddock, yellowtail
flounder, pollock and plaice
and an increase in winter skate,
spiny dogfish,
and other commercial fishery catches. These data suggest ecosystem-level
responses to intensive harvesting, which may have important implications for
developing harvesting strategies for the community of species, rather than the
individual stocks. A multispecies surveying approach thus provides an
important research opportunity in the emerging field of ecosystem-based
management.
(5) To monitor biological rates of the
stocks: Apart from basic information on the abundance and distribution
of species, research vessel survey data are collected on a range of biological
rates for stocks. These processes include growth, sexual maturity, and
feeding. Changes in growth and maturity parameters directly influence
assessment calculations related to spawning stock biomass, yield per recruit
and percent of maximum spawning potential. Over the past three decades, these
parameters have changed dramatically for some species. Faster growth and
earlier onset of maturity have been observed for haddock
and cod. It is thus
important to monitor these rates continuously if stock status is to be
accurately determined. Likewise, diet data collected via examination of stomach
contents at sea will be increasingly important as scientists try to evaluate
how harvesting affects species linked by predator-prey relationships.
(6) To collect environmental data and
support other research: Research vessel surveys are generally
conducted 24 hours a day when the vessels are at sea. This presents a superb
opportunity to collect environmental information (temperature, salinity,
pollution levels, and so on) and to allow other researchers to piggyback on
surveys to collect a host of data not directly related to the stock
assessment. All research vessel surveys conducted by the Northeast Fisheries
Science Center collect and archive an extensive array of environmental
measurements and usually have a "shopping list" of samples to be obtained for
researchers at academic institutions, other government agencies, and the
private sector. On every survey there are scientific berths allocated to
cooperating scientists and students in order to foster this cooperative approach
to marine science.
WHAT TYPES OF SURVEYS ARE CONDUCTED
The various types of research vessel surveys conducted by the
Northeast Fisheries Science Center are described below:
(1) Spring and autumn bottom trawl survey:
The spring and autumn bottom trawl surveys conducted by the Northeast
Fisheries Science Center are the longest running continuous time series of
research vessel sampling in the world. The autumn survey began in
1963; the spring in 1968 (Azarovitz 1981). These surveys cover the ocean
environment from 5 to 200 fathoms deep, from Cape Hatteras, North Carolina to
well beyond the Canadian boarder. About 300 half-hour trawl sets are made at
sites (stations) randomly chosen prior to the beginning of each
survey. The objective of each tow is not to catch large numbers of fish, but to capture a
representative sample of the various species and relative numbers in a given
area. The distribution of trawling locations is allocated according to a
statistical method that divides the region into a number of smaller areas
(strata) with similar depth characteristics. The method is a
stratified-random sampling design, and is commonly used for a wide
variety of statistical estimates, including exit polling for elections. In the
history of the trawl surveys, only three research vessels -- NOAA's FSV Henry B. Bigelow, FRV
Albatross IV and the FRV
Delaware II -- have been used to conduct these surveys.
A small-mesh cod-end liner (1/2 inch mesh) is used to retain
prerecruits. All species in each tow are weighed and counted, and all or a
sub-sample is measured to determine the length composition of the catch. Hard
parts (scales, ear stones, fin rays) are removed from some of the fish of each
species taken in the trawl, and then cataloged. These hard parts are used to
determine the age of each fish selected. The age distribution of the whole
catch can then be estimated by expanding the sub-sample. Fish are also
examined to determine sex and state of sexual maturity. Stomach contents are
identified and any obvious disease-related conditions of the fish are
recorded. All data are brought back to the laboratory, where they are
entered into computer files. The accumulated trawl survey data
set (1963 to present) represents over 20,000 stations, with millions of
individual pieces of information concerning fishery resources of the region.
The entire data series is available to fishery scientists wishing to examine
trends in abundance, distribution, species associations, or numerous other
scientific questions.
(2) Sea scallop dredge survey: Each
summer, the Northeast Fisheries Science Center conducts a research survey
targeting sea
scallops. This survey is used to assess scallop abundance, distribution, size/age
composition and other factors. The survey encompasses the continental shelf
from Cape Hatteras through Georges Bank and the Gulf of Maine. The scallop
survey uses the same stratified random survey design as that used in trawl
surveys.
Scallop surveys began in 1975, and have been conducted every year since
1977. Since 1979, the standard gear has been an 8 ft-wide commercial scallop
dredge equipped with a 2 in. ring bag and a 1-1/2 in. mesh liner. The dredge
is towed for 15 minutes. The depths surveyed range from 15 to 60 fathoms. In
addition to sea scallops, the surveys catch significant numbers of flounders
(primarily yellowtail), hakes and goosefish. Results from the sea scallop survey have been used in assessing the
other species.
(3) Hydraulic clam dredge for surf clam and
ocean quahog: Since 1965, the Center has conducted hydraulic clam
dredge surveys aimed at the surfclam
and ocean quahog.
The surveys are not conducted every year for several reasons The exploitation rates of
these species are low: the time from first appearance in the survey gear until
commercial size is attained is long: and fisheries are not generally dependent
on the incoming year classes. The current schedule calls for the clam survey
to be conducted every third year and the last survey was conducted in 2005.
A 5 ft-wide hydraulic clam dredge is used on the surveys. The
dredge has a submersible electric pump that uses high pressure water jets to
loosen substrate and animals in the path of the dredge. The submersible pump
allows the dredge to fish in depths greater than those now commercially
exploited (based on commercial standard deck-mounted dredge pumps). The 2 in. aqua mesh-lined dredge retains small-sized clams, along with large quantities of
shell debris and live clams and associated invertebrates, all of which are
included in the database. The areas primarily surveyed are from Cape Hatteras
to Georges Bank. Additionally, some surveying has occurred in Massachusetts
Bay (to evaluate fishery potential for the Arctic surfclam, Mactromeris
polynyma) and off the Maine coast (to evaluate populations of ocean quahog).
(4) Summer Gulf of Maine trawl survey: Beginning
in 1991, the Center has conducted a special bottom trawl survey directed to
near shore areas in the Gulf of Maine. Traditionally, it has proved
impractical to survey Maine, New Hampshire, and northern Massachusetts coastal
waters with bottom trawls based on random station selections because of large
areas of hard, rocky bottom and fixed fishing gear such as lobster pots and
gill nets. Hence, large concentrations of inshore juvenile groundfish have not been adequately included in trawl catches. The survey methods
used in the summer Gulf of Maine survey are a modification of the stratified
random technique. In spring 1991, Center personnel met with fishermen and
others knowledgeable about locations of towable areas in the Gulf of Maine. From these meetings, a master list of towable areas was generated and based on this list sites are chosen randomly as the stations to be sampled.
This way, expensive and time-consuming gear damage is minimized, but the
statistical criteria necessary for the survey are met. Prior to the 1992
survey, NMFS again consulted with Maine fishermen and expanded the master list
of potential tows. The survey has generated important new information on the
inshore distribution and abundance of groundfish, and has added to the
biological data on the timing of spawning for a number of species.
Apart from the new Gulf of Maine survey, a monthly sampling
program has been established in conjunction with the Maine Department of
Marine Resources to improve the quality of groundfish population biology data.
(5) Winter trawl survey along the
continental shelf: (NOTE: Discontinued after 2007) Initiated in 1992, a winter trawl survey along the
Middle Atlantic, Southern New England and southern Georges Bank continental
shelves is specifically directed to improve the quality of flatfish
assessments. Standard groundfish surveys use rollers along the foot rope to
minimize gear damage in rough bottoms. Although the roller-rigged gear catches
flatfish, many of the animals pass under the foot rope, and thus abundance
measures from the survey tend to be highly variable. The new survey employs a
chain sweep (a "flat net") to minimize the escapement of flatfish
under the foot rope. Accordingly, abundance indices from the new survey are
likely to provide more accurate assessments than those derived from
roller-rigged gear. Specifically, the survey has been used to improve
assessments for summer
flounders. Assessments of other species caught along with this
flatfish have also been improved.
(6) Marine mammal sighting surveys: Shipboard
sighting surveys of marine mammals involve directing the vessel along a
predetermined transect and counting the number of each marine mammal species
sighted. The interpretation of the data is complicated by the range and
bearing of the sighting from the vessel track line, and the fact that the
probability of sighting a mammal falls off with its distance from the vessel.
These effects are well known when conducting sighting surveys, and are being
evaluated by Center scientists. An intensive survey to estimate the abundance
of harbor porpoise in the Gulf of Maine began in 1991. Other sighting
surveys have been conducted to evaluate the abundance and distribution of the
marine mammal community in coastal shelf waters and those associated with the
western boundary of the Gulf Stream. Marine mammal sightings are routinely
conducted as a piggy-back activity on bottom trawl and larval fish
survey cruises.
(7) Surveys of fish eggs and larvae: Surveys
of the distribution and abundance of "baby fish" (the free-floating
eggs and new hatchlings called larvae) are conducted several times per year to
evaluate the timing and distribution of spawning. Another important function
of these surveys is to estimate the quantity of baby fish spawned, and
thereby be able to work back to the size of the female population that must have been
present to produce the numbers of small fish counted. This
back-calculation of the spawning stock is an important tool used
in assessing fish populations worldwide, and in some cases may be the only
reliable information about the size of the spawning stock. For Northeast
fishes, egg and larval studies have been used to evaluate populations such as Atlantic
mackerel, yellowtail
flounder, sand lance, and bluefish.
Egg and larval surveys are conducted using a pair of very
fine-mesh nets towed in a frame resembling a bongo drum (hence the name
"bongo nets"). The nets are pulled with varying amounts of scope on
the towing line so the bongo net samples all layers of the water column from the surface to just off the bottom. Contents of the net are preserved at sea for
later intensive analysis in the laboratory. These fine-mesh plankton nets also
sample the community of free-floating plants and animals that support the base
of marine food chains. These animals are assessed to examine the
production and distribution of zooplankton (animals) and phytoplankton
(plants).
(8) Special experiments: A variety
of special experiments are conducted to augment the standard monitoring
surveys. For example, when vessels or surveying gear have to be changed, it is
necessary to estimate conversion factors accounting for differences in fishing
power. Several years ago the standard trawl net doors had to be changed, since
fishing gear suppliers could no longer manufacture doors to 1963
specifications. A polyvalent door was chosen for the new standard, and a
series of research vessel experiments was conducted to estimate the effect
that the door change alone had on catch rates. Similar experiments have been
conducted to relate catches between the FSV Henry B. Bigelow and the FRV
Albatross IV
Other special experiments using vessel surveys have been
directed at evaluating feeding interactions among species; relating
oceanographic processes to the survival of eggs and larvae; and evaluating
effects of area closures and openings on fish, benthic fauna, and sediment
geochemistry.
WHY DO RESEARCH VESSELS SAMPLE WHERE THEY DO?
The fundamental objective of the surveys is to provide
realistic estimates of relative abundance of populations over time. By only
fishing where fishing is exceptionally good (the "hot spots"), we
would potentially miss large fractions of the stock that may occur in lower
density areas. This would be akin to estimating the population of
Massachusetts or New Jersey by counting only people that live in the eastern
counties of these states, and extrapolating the rest based on numbers of
people-per-square-mile.
By selecting random stations within certain depth zones, any
fish that exists in that zone has an equal probability of being caught in the
survey. Thus, the method produces unbaised estimates of the relative stock size. One of the downsides to this procedure is that for
fish stocks highly clumped in their distributions, the error
bars around the estimates tend to be rather wide. Alternative survey
methods that produce more precise error bars include a grid of
fixed-station locations that are fished each year. However, this design may
provide biased abundance measures if changes in the abundance at these
locations do not reflect the actual changes in the stock as a whole. Some
countries use the stratified random technique, and others use fixed stations
(for example the English groundfish survey of the North Sea; ICES 1992). There
appears to be no sampling design that covers all cases. Rather it depends on
the particular situation of the species distribution and the number of
sampling sites that are included in the survey. Because of the large number of
species and stocks analyzed from the bottom trawl surveys and the different
bottom types and habitats in the Northeast, the stratified random sampling
plan appears to be the most efficient design.
For more information:
Azarovitz, TR. 1981. A brief historical review of the Woods
Hole Laboratory trawl survey time series. In: Doubleday, WG and Rivard, D,
eds. Bottom trawl surveys. Canadian Special Publication of Fisheries and
Aquatic Sciences 58, p. 62-67.
Clark, S. 1981. Use of trawl survey data in assessments. In:
Doubleday, WG and Rivard, D, eds. Bottom trawl surveys. Canadian Special
Publication of Fisheries and Aquatic Sciences 58, p. 82-92.
International Council for the Exploration of the Sea. 1992. Report of the workshop on the analysis of trawl survey data. ICES C.M.
1992:D:6.
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