RESEARCH VESSEL SURVEYS
Topics:
INTRODUCTION
In order to evaluate the status of exploited fishery resources, many
different kinds of information must be collected and and analyzed.
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 is combined into indices of stock abundance
based on catch-per-unit of effort (CPUE) ratios. Data from these
three types of collection programs are generally referred to as
fishery-dependent information - derived directly from the commercial
and recreational fisheries.
Fishery-dependent data are vital to our ability to monitor stocks, and
often are the only reliable source of data concerning some species.
However, using of fishery-dependent data alone may severely limit our
ability to evaluate some stocks, and to make predictions. For example,
in fisheries that are heavily dependent on the incoming age group to
the fishery each year (the new "recruits"), fishery data alone can not
be used to forecast catches, since very small fish are generally not
taken with standard fishing gear. Likewise, CPUE may not be a reliable
measure of abundance in all cases such as for schooling species, or
when the increase in fishing technology cannot be factored into the
relationship between catch and fishing effort. For these and other
reasons, fishery scientists throughout the world have initiated
research vessel sampling programs to gather fishery-independent
information(Clark1981). The scope and use of research vessel surveys in
assessing Northeast fishery resources is reviewed in this section.
WHY CONDUCT RESEARCH VESSEL SURVEYS?
Fishery-independent surveys of Northeast fishery resources are conducted for six important reasons:
(1) To monitor recruitment: Research sureying 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, estimates of the survival of fish already large enough
to be retained by harvesting gearmust be combined with estimates of 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 enter the surveying gear and when they
are landed by the fisheries is much shorter. One such case is that of
Atlantic Sea scallop. Annual dredge surveys are conducted
for this species in the Northeast. The relative catch in numbers of sea
scallops per haul of the dredge is plotted for two size categories of
scallops: those smaller than 70 mm (2-3/4 in.) shell height are
designated as prerecruits, and animals larger than or equal to 70 mm
are designated as 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 sampling generally results in catches that span the full size
and age range of the population in the ocean. Although recruitment
prediction is one important element of fishery forecasts, it is equally
important to calculate the survival rate of the pOnion of the stock
already subjected to fishing. The catch- at-age data collected from the
surveys are one imponant 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 S 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 popoulation that must have been existence to
give the levels of catches 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 latent 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 rather sedentary lives while others are highly migratory.
A major source of data concerning the movement patterns and geographic
extent of stocks comes from research vessel surveys. 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. The geographic distibution of Atlantic Cod is
based on trawl surveys conducted from 1987 to
1992. 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 taken in Northeastern Continental Shelf waters. Many of
these species are relatively rare, and have little or no commercial or
recreational value. However, by collecting data on such an array of
species, important patterns emerge when evaluating the response of
the entire animal community to intensive harvesting on selected
species.
The dramatic changes in the system reflect the depletion of
several important commercial fishery species ( Haddock,
, yellowtail flounder , Pollock , plaice ) and an increase in
winter skate, spiny dogfish, and other species 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 has thus provided an important research
opportunity in the emerging field of ecosystem-based management.
Species composition(inpercent by weight) of trawl survey
catches on Georges Bank in autumn of 1963 and autumn of 1990.
(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 parameters of the stocks.
These parameters include growth, sexual maturity, and feeding.
Changes in growth and maturity directly influence assessment
calculations related to spawvning 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 the stock status is to be accurately determined. Likewise,
diet data collected by examining stomach contents at sea will be
increasingly important as scientists try to evaluate how harvesting
affects species that are linked by predator-prey relationships.
(6) To collect environmental data, and to allow 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 duties to be accomplished for researchers in academic institutions, other government
agencies, and the private sector. On every survey there are
scientific berths allocated to cooperating scientists and students
so as 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 was initiated 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 just 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 employed is termed a "stratified-random
sampling design," one commonly used for a wide variety of statistical
estimation programs, including exit polling for elections. In the
history of the trawl surveys, only two research vessels --
NOAA's Albatross IV. and The Delaware II -- have been used to conduct these
surveys.
A small-mesh cod-end liner (1/2 in. 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. Hardparts(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 researched and any obvious disease-related, conditions of the fish
are recorded All data are brought back to the laboratory, where they
are subsequently 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 online
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 directed to the Sea scallop .
This survey is also used to assess 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 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 resources of surfclam and ocean quahog. The surveys are not conducted
every year, since 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. The last survey was conducted in
1992.
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 surveys to extend to deeper depths than are
now commercially exploited, based on standard deck-mounted dredge
pumps. The bar spacing of the dredge is such that very small-sized
clams are retained in the dredge, 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 nearshore areas
in the Gulf of Maine. Traditionally, it has proved impractical to
survey Maine, New Hampshire, and northern Masschusetts 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 juvenile
groundfish occurring inshore have not been adequately included in trawl
catches. The survey methods used in the summer Gulf of Maine survey are
a hybrid of the stratified random technique. In spring 1991, Center
personnel met with fishermen and others knowledgeable about where
towable areas in the Gulf of Maine were located. From these meetings,
a master list of towable areas was generated. Based on this list,
sites are chosen randomly as the stations to be sampled. This way,
expensive and time-consuming geardamage isminimized, but the
statistical criteria necessary for the survey are met. Prior to the
1992 survey, we 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: 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 a more precise assessment tool than that derived
from roller-rigged gear. Specifically, the survey will be used to
improve assessments for yellowtail and Summer flounders. Assessments of
other speciescaught along with these two important flatfish may also be
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 trackline, and the fact
that the probability of sighting a mammal falls off with the 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 was initiated 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 fished
spawned, and thereby 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 samples from the surface to just off the
bottom (all layers of the water column). 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 also assessed to examine the production and
distribution of zooplankton (animals) and phytoplankton (plants).
(8) Special experiments: . A variety of special one-time-only 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 Albatross IV. and the The Delaware II.
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 other associated
studies.
WHY DO RESEARCH VESSELS SOMETIMES GO WHERE THERE ARE FEW FISH?
This is a question that often comes up when explaining the theory
behind the surveying program. The fundamental objective of the
surveys is to provide realistic estimates of the trends in populations
over time. By only fishing where fishing is exceptionally good (the
"hot spots"), potentially large fractions of the stock that may occur in
low-density areas would not be adequately sampled. This would be akin
to estimating the population of Massachusetts or New Jersey by
counting only people that live in the eastem 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 have an equal probability of being caught in the
survey. Thus, the method produces estimates of the relative stock
size that are termed "unbiased." One of the downsides to this
procedure is for fish stocks that are 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 f1xed-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 fixedstations (for example the English
groundfish survey of the North Sea; ICES 1992). There appear to be no
best sampling design to cover 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 tobe most efficient design.
For more information
Azarovitz, T.R. 1981. A brief historical review of the Woods Hole
Laboratory trawl survey time series. In: Doubleday, W.G. 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 inassessments. In: Doubleday,
W.G. and Rivard, D., eds. Bottom trawl surveys. Canadian Special
Publication of FisheriesandAquatic 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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