•ICES CM 1997/Y:33Synthesis and CriticalEvaluation of ResearchSurveys(Y)

THE NORTHEAST FISHERIES SCIENCE CENTERBOTTOM TRAWL SURVEY PROGRAM

Thomas Azarovitz, Stephen Clark, Linda Despres, Charles Byrne

National Marine Fisheries ServiceNortheast Fisheries Science Center

Woods Hole, MA 02543

ABSTRACT

The National Marine Fisheries Service, Northeast FisheriesScience Center's Research Program includes multispecies bottomtrawl surveys; shellfish surveys for sea scallops, Atlanticsurfclams and ocean quahogs, and northern shrimp; longlinesurveys for apex-predators; and line transect surveys for marinemammal populations. This document provides an overview of themultispecies bottom trawl survey component and applications ofthe resulting data. The autumn bottom trawl survey has beenconducted since 1963; aspring survey was initiated in 1968, andsummer and winter surveys have been conducted intermittently.Since 1967, spring and autumn surveys have been conducted fromCape Hatteras to the Scotian Shelf. Standard vessels, gear, andsampling procedures have been used throughout the program.Considerable effort has been directed to maintaining theintegrity of the survey time series through standardizationstudies to adjust for fishing power differences between differentresearch vessels and trawl doors used in the time series. Thesesurveys provide quantitative assessment data for fifty to sixtyspecies stocks assessed each year. They provide indices ofabundance and recruitment; biological parameter estimates, i.e.,size and age composition and growth, mortality and maturationrates; distribution data; and data for specialized applicationssuch as virtual population analysis or VPA tuning and populationsize estimates through swept-area calculations or DeLurymodeling. The survey time series has also been highly valuablefor fisheries ecosystem research applications, e.g., implicationsof climate change, habitat evaluation and related studies, andmultispecies modeling.

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INTRODUCTION

During September and October of 1997 scientists aboard the R/V .ALBATROSS IV will be conducting the National Marine FisheriesService (NMFS), Northeast Fisheries Science Center's (NEFSC)annual autumn bottom trawl survey. When completed, it will bethe thirty-fifth consecutive survey in the series. Over theyears this and other surveys conducted by the NEFSC have becomevitally important for fisheries research and management in theNortheast. There are several reasons why this has occurred.Fishery independent surveys provide a wealth of stock assessmentdata that are not readily available from the commercial fishery;and in this region, fishery dependent data collection systemshave been impacted occasionally in the past by misreporting andother problems. Fishery independent data have also assumedincreased importance over time as fish and invertebrate stockshave varied in abundance. Finally, the availability of multi­decadal data sets has opened new dimensions for ecologicalresearch applications.

The longevity and value of the NEFSC survey time series are duein large part to the foresight of the Woods Hole scientific staffduring the early 1960s. Although the culmination of a teameffort, the basic format of the bottom trawl survey was due tothe vision and insight of three key individuals: Dr. Robert L.Edwards, Dr. Marvin GrossIein, and Richard Hennemuth.Logistically, it would have been easier to have designed a surveytargeting a few important species, but they chose broad scalemultispecies based surveys as being most appropriate. The valueof and importance of these surveys today is ample evidence thatthe correct decisions were made.

Since the first autumn survey cruise, the NEFSC has initiatedother resource surveys, conducted during all seasons, targetingecologically and/or economically important finfish and shellfish.In this paper we will confine our discussion to the ongoing NEFSCbottom trawl survey series. Today this program is based on threestandardized seasonal surveys: the autumn survey, beginning in1963; the spring survey, beginning in 1968; and, most recentlythe winter survey which began in 1992. It should be noted thatalthough a bot tom trawl is used and catchability is highest fordemersal species, useful data are also obtained for pelagic fish(e.g., Atlantic mackereI, Atlantic herring, butterfish) and someinvertebrates (e.g., American lobster, squids) as weIl.

The initial objectives of the program, described by Grosslein(1969) remain unchanged: to monitor population fluctuations, toassess fish production potential, to determine environmenta1factors controlling the distribution and abundance of fishspecies, and to provide the basic ecological data required tounderstand interrelationships between fish and their environment.

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Historical Notes#' . f "," -. ~

Fisheries research surveys have been eondueted by the predeeessoragencies to NMFS at the Woods Hole, Massachusetts, laboratorysince the federal research facility was established there by theU.S. Fish Commission in the late 1800s. Early cruises weregenerally exploratory in nature and were essentially designed tosupport life history and taxonomie studies. Following World War11, studies were initiated to determine abundance and spawningcapacity. Cruises were conducted mainly off southern New .England, on Georges Bank, and in the Gulf of Maine (Figure 1),targeting important species (e.g.~ Atlantic mackereI, Atlanticcod, haddock, redfish and Atlantie herring) . These eruisesprovided mueh useful information but were sporadie in nature andlack of standard methods precluded their use for monitoringpopulation trends.

The modern era of standardized population surveys was madepossible in the early 1960s with the arrival of the newly builtR/V ALBATROSS IV (eurrently operated by the National Oeeanic andAtmospheric Administration (NOAA). This ship was not just aresearch vessel, it was a research trawler. It had thecapability to earry a large scientific complement, to towrelatively large commereial type trawls, to operate during anyseason throughout the NEFSC's assigned area of coverage (roughlyfrom Cape Hatteras to the Nova Seotian Shelf), and to remain atsea for extended periods of time., It was the perfeet vessel tosupport and to be dedieated to a leng-term trawl survey.

SURVEY PROGRAM

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The stratum depth limits are <9meters(m), 9-18m, >18-27m, >27­55m, >55-110m, >110-185m, and >185-365m. The three sets ofstrata from <9m to 27m are often referred to as "inshore strata";these near shore coastal waters were not sampled until 1972 whenthe NMFS, Sandy Hook Laboratory (now the James J. HowardLaboratory) began its "inshore survey" on the charter R/VATLANTIC TWIN. Since 1974 the inshore strata have beenincorporated into the Woods Hole survey and sampled by either theALBATROSS IV or the R/V DELAWARE II. Stations are allocated tostrata roughly in proportion to their area and assigned tospecific locations at random. Larger strata are divided intosmaller subareas to assure a more even distribution of stations.

More information about the survey program sampling design can befound in Azarovitz (1981, 1994) while a discussion and evaluationof the design and statistical attributes can be found in NEFSC(1988) .

Vessels and Gear

A crucial aspect of this, or any other, survey time series isstandardization of the survey unit (Doubleday 1981, Carrothers1981). A survey unit consists of: officers and crew, vessel,and gear. To preserve the integrity of the survey time series,the NEFSC has: 1) developed an operational protocol for officersand crew to minimize variability due to individual differences;2) standardized the construction, maintenance and use of samplinggears; and 3) limited the number of ships employed insofar aspossible.

Vessels

Initially the ALBATROSS IV was used exclusively for all bottomtrawl surveys. However, beginning in the 1970s the DELAWARE IIwas used irregularly when ALBATROSS IV was unavailable due tomechanical problems, deactivation (1989-1991) or othercommitments; the DELAWARE II was also used in cases wheresimultaneous use of two research vessels was deemed desirable toimprove synoptic coverage. The ALBATROSS IV has been the onlyvessel used to conduct all bottom trawl surveys since 1992.

The use of more than one vessel during a survey time seriesintroduces the potential for bias due to possible differences infishing power. Accordingly, five paired tow experiments wereconducted by the NEFSC to assess the differences in fishing powerbetween the ALBATROSS IV and DELAWARE II (Anon. 1991). Data froma total of 510 usable pairs were analyzed. Results of theseexperiments indicate significant differences in relative fishingpower for total number and weight caught for all species combinedand for number and/or weight for several individual species. TheDELAWARE II caught significantly more than the ALBATROSS IV interms of total weight and total number for all but one experiment

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in which large numbers of anchovies and small squid were taken bythe ALBATROSS IV and not by the DELAWARE II. However, consistentsignificant. differences were' notiseen on an individual speciesbasis, either within or among cruises. Regardless, analyses wereconducted on pooled data for the 50 species for which there wassufficient data. Significant differences for 18 species (amongthem yellowtail flounder, American plaice, haddock, Atlantic cod,Atlantic herring, and Illex and Loligo squid) for both number andweight were found .. Nine additional species (including red hakeand sea scallop) were significantly different for either numberor weight. Only one species was caught in greater numbers andweight, and another in terms of number, by the ALBATROSS IV. Allother differences were in favor of the DELAWARE II.

Gear

The basic survey trawl in the autumn and spring series is the 36Yankee trawl or a "60-80" trawl. It is equipped with a 24. 4mwire footrope and 0.5m solid rubber rollers. It is constructedof nylon twine with no modern synthetics, and while small and ofan older design as compared to many trawls used in commercialfisheries today, it still remains a very adequate survey samplinggear. The spring survey series underwent a significant gearchange during the 1973 to 1981 period, whena larger (30.5mfootrope), higher opening 41 Yankee trawl was used in an effortto improve catchability for pelagic species. In 1982, use of thestandard 36 Yankee trawl was reinstituted and has continued tothe present. A 3/4 Yankee trawl was used from 1972 to 1975 forthe aforementioned inshore cruises. The winter survey trawl is amore modern Yankee trawl with a chain and "cookie" disk sweep andpolypropylene netting. Locally it is referred to as a "NewBedford flounder trawl." As its name implies, it was designed tocatch flatfish and therefore cannot be fishedlike the standardsurvey trawl on hard, rough bottom.· All trawls used in theprogram are equipped with a 1.25 cm. stretched mesh liner toretairi juveniles.

Since the inception of the survey, two additional changes insampling gear components have occurred. The first was a changein twine color. During the oil embargo of 1973, tan nylon nettwine became unavailable, requiring a change to white twine.Trawls constructed of white twine have since been considered tobe the standard. Although twine color is known to affect thebehavior of some species (Zijlstra 1969) no attempt was made tothe quantify the differences, if any, in the survey trawlefficiency due to the color change.

The second change that occurred involved trawl doors. At theinception of the bottom trawl survey time series, a patentedtrawl door type was selected and used for many years. In the

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early 1980s, specifications of these trawl doors began to change,and laboratory staff also learned that the manufacturer wasretooling and the standard door would soon become unavailable. Asearch for a domestic source (even by special order) of the trawldoors was unsuccessful. Accordingly, in 1983 the decision wasmade to change the standardized trawl door to one that was bothavailable and could be manufactured domestically, if needed. In1985, the standard survey unit was changed to include the newtrawl door.

A trawl door calibration study was initiated in 1984, to compareand standardize the relative fishing" power of the two types oftrawl doors (Anon. 1991). A paired tow experimental design wasemployed; 345 pairs of successful tows have been analyzed todate.

Significant differences were found in catch rates between the twodoor types for all species combined in terms of both number andweight. Sufficient data was collected for analyzing the resultsfor 42 species. Differences were found for several species interms of number and/or weight. The new doors caught more interms in both numbers and weight for ten species (among themAtlantic cod, haddock, pollock, red hake, winter flounder, andyellowtail flounder). Four other species (among them seascallop) were significantly different for either number orweight). Only one of those species was caught in greater numbersby the original trawl door type. All other species, and allspecies combined, were caught in greater quantities by the newtrawl door type.

Coefficients to convert DELAWARE II catches to those of theALBATROSS IV, and catches of the old trawl doors to those of thenew trawl doors were calculated (Anon. 1991). The use of thesecoefficients can significantly affect the interpretation of ..bottom trawl derived abundance indices. Analyses have shown that ..failure to account for vessel changes can have significanteffects upon estimates of abundance and fishing mortality(Fogarty 1997), and in turn, affect management advice.

Data Collection at Sea

Station Data

Field work for these surveys is conducted on a 24-hour basis witha six hours on, six hours off schedule. On arriving at a pre­selected station, a temperature profile is obtained using aconductivity, temperature, depth (CTD) apparatus. At certainstations, a plankton bongo frame fitted with 0.333 and 0.253 mmmesh nets is also lowered with the CTD and a double oblique towis made. Position, depth, time of day (GMT), operationsconducted (e.g., water bottle sampIe, bongo, neuston), and

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surface and bot tom temperature readings are recorded and storedin paper and electronic format. At the beginning and at the endof each trawl to~, ;certain p~~~me~ers are recorded: vessel speedover bottom, pos~t~on, depth, and tow duration in addition to thedate, time, amount of wire out, weather and sea state conditions.

Biological Data

Once the contents of the net have been deposited onto the sortingtable, the watch members divide the catch into species specificcontainers. Several species (e;g., dogfish, American lobstersand crabs) are further separated by sex. For each species/sex, atotal weight is obtained using motion compensated and calibratedelectronic scales and recorded to the nearest 0.1 kilogram.

An attempt is made to measure the entire catch at each station.If sampling is necessary, a representative length frequencysampIe is taken. For selected species, biological sampIes arecollected concurrently with the measuring operations for ageing,growth and predator-prey studies. As time and space allows,additional collections are also obtained for scientists fromother organizations. Data are recorded on standard water-proofpaper data sheets.

Data Processing

At sea, the trawl logs are reviewed by the individual recorders,Watch Chief and Chief .Scientist. Checks on species

° identification and code number assignment, expansions of totalnumber of fish sampled or subsampled, and completeness of alldata fields are examined closely. Upon return to port, logs aresent to a contractor for off-site data entry. Within two weeksof the completion of an entire survey (some surveys have two tofour parts), the original and key-entered datOa are returned inflat ASCII format. The data are then examined for obvious mis­entered information and corrected. From these pre-audited data,a pr~liminary "Fishermen's Report" which lists the location ofeach station and the total catch of 23 commercially importantspecies is generated and distributed to interested individuals.

Audit programs have been developed to examine the station,biological, and detail data on a field-by-field and inter-fieldcross checking basis. Inconsistencies are detected eitherthrough narrow minimum/maximum range allowances that have beenestablished in a number of support tables and through detailedreview by an auditor. Standard audit procedures have beendocumented and applied to all data fields. Length-weightequations also provide a basis for detecting errors. Mistakes ordiscrepancies made either at sea or during data entry arecorrected, and due to the relational nature of the data base,

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cascade to the ancillary data bases. The station, biological anddetail audits are submitted several times until all error flagsare resolved.

When auditing is completed, data are made available to thegeneral user community, usually about two months after completionof the field survey.

Currently the data are accessed by ORACLE, a UNIX basedrelational database system. The data are arranged into varioustables (station, catch, length frequency, age, and predator/prey)with each table linked on key fields (cruise code, stratum-tow,and station number). The biological data are additionally linkedby species and sex-of-catch fields. As age data becomeavailable, they are entered directly into the relational database and the master data are updated with this new.information.

During the bottom trawl survey time series, there have beenseveral mainframe computer transitions that have forced themaster data into different formats. From 1963-1981, data fieldsare in a coded format due to the 80 byte field restrictions thatwere in place at that time. From 1982 to present, the number ofstation data fields more than doubled and in 1992, the detailinformation with individual identification numbers which linksthe survey data to the age and predator-prey data bases wasinitiated.

APPLICATIONS

Uses of Survey Data in Stock Assessments

The NEFSC survey time series has been called "perhaps the singlemost important type of information available for the [Northeast]Region from the point of view of stock assessment" (NEFSC 1997a) . ..This is a consequence of many factors including the mixed speciesfisheries situation in the northeast, fishery trends, and theneed for fishery-independent data for research and monitoring.Massive increases' in fishing effort by distant-water fleetsduring the 1960s underscored these needs and led to the use ofsurvey data for evaluating the effect of "second-tier" totalallowable catch (TAC) regulations imposed under the InternationalCommission for the Northwest Atlantic Fisheries or ICNAF (Clarkand Brown 1977). Under the Magnuson Fishery Conservation andManagement Act (MFCMA) significant misreporting by species andarea occurred for several species during the late 1970s tocircumvent quotas again resulting in increased reliance on theNEFSC survey database. More recently, problems associated withchanges in commercial data collection systems in the Northeasthave again underscored the importance of fishery-independentmonitoring. In any case, the survey has proven to be highlyvaluable for assessments and related research applications. The

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following is an overview of assessment related uses to whichthese data have been put.

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Indices of Abundance

The desirability of research vessel survey data, as opposed tocommercial data for long-term indices of abundance, is obvious.Commercial catch-per-unit-effort (CPUE) indices can be biased bymany factors including changes in regulations~ fishing practices,and fishing power and technology. Such changes are verydifficult, if not impossible, to quantify over time. Also, themixed fishery situation in the northeast, in which target speciesmay change frequently, creates monitoring needs for many speciesfor which commercial measures of abundance and mortality are notreadily available.

The NEFSC providesstock assessments for approximately 50species-stocks, as reported in the annual "Status of the FisheryResources off the Northeastern United States" series. Forty orso of these employ survey data routiriely for monitoring trends inabundance (e.g., see NEFSC 1994), and for 35 species stocksreviewed in that issue the survey time series was the primarymonitoririg tool. With perhaps one or two exceptions, indextrends agreed closely with those evidenced by commercial fisherydata .. A more recent analysis (NEFSC 1996a) examined trends inabundance for demersal species and developed aggregate indices ofabundance for four species groups. Contradictory trends inabundance and biomass were noted in some cases which wereattributed to changes in relative abundance of individual species(light vs. heavy-bodied), age truncation from exploitation, andstrong year classes. It could be possible to refine such indicesthrough use of species-specific catchability coefficients (NEFSC1996a) .

Applications of seasonal survey time series have been reviewedpreviously (NEFSC 1988). For demersal species, the autumn surveyhas been preferred because of its longer duration and continueduse of the 36· Yankee trawl as noted above. (Survey data havealso been judged as useful for pelagics, e.g., Atlantic herringand Atlantic mackerel, in spite of lower catchability.) Springsurvey data have been used for some species due to improvedavailability or to corroborate autumn survey trends. Thisredundancy can be particularly important for short lived (e.g.,squids) or intensively managed species.

This study examined the agreement between survey indices(smoothed by time series modeling) and fishery-dependent measuresfor several demersal species-stocksand found agreement to bevery good (Fogarty et al. 1988; NEFSC 1988). Discrepanciesappeared to result primarily frombiases in reporting or analysisof commercial data. More recent analyses have supported the .useof time series models as a smoothing and variance-stabi1izing

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technique (NEFSC 1996a) .

Recruitment

One of the most important applications of survey data in generalhas been its use in predicting recruitment. Juveniles aresampled through use of codend liners often years beforerecruitment to commercial gear, thus providing predictivecapability. At the NEFSC, this formerly involved development ofempirical relationships between estimates of stock size at agecalculated from sequential population analysis (SPA) and surveycatch-at-age in numbers (examples provided by Clark 1981). Inmore recent years, recruitment estimates have been deriveddirectly from SPA calibrations employing ADAPT or other tuningprocedures (discussed further below) .

Both spring and autumn data have been important for estimatingrecruitment (NEFSC 1988). Young-of-year produced by springspawners may be partially recruited to the survey gear in theautumn of the year in which they were spawned and only fullyrecruited to the survey gear the following spring at Age 1.Additionally, the autumn survey data may be of limited value forprediction because of seasonal availability differences (e.g.,Atlantic mackereI) . Thus seasonal sampling provides severalindependent observations of year class strength before the yearclass actually recruits to the fishery. This is an importantconsideration for depressed groundfish resources now underintensive management in the northeast.

Fishing Mortality and Stock Size

Monitoring trends in instantaneous total mortality (2) isaccomplished in many stock assessments through analysis of pooledresearch vessel survey catch-at-age data. In NEFSC stock ~assessments, trends have generally agreed weIl with thoseobtained from analysis of commercial data. Such procedures areoften adequate for evaluating effects of fishing particularly atmoderate exploitation levels.

More sophisticated ("analytical") types of assessments provideestimates of fishing mortality and stock size through SPAtechniques which require estimates of instantaneous fishingmortality (F) for the terminal year of a catch-at-age matrix. Atthe NEFSC, it was formerly common practice to relate survey catchto estimates of stock size or fishing mortality calculated fromtrial starting F values through various iterative procedures(based on linear or nonlinear regression techniques) until theterminal F value stabilized (C1ark 1981). More recent1y,"tuning" procedures using both survey catch-per-tow-at-age andcommercial landings-per-unit-effort (LPUE) indices to estimate Ffor the terminal year have been preferred (again through

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continued iterations until correlations between the variablesselected are highest). Mohn and Cook (1993) have reviewed theevolution of various tuning methodsand underlying formulationsthrough ICES and other forums leading up to the Laurec-Shepherdand ADAPT methods. The ADAPT method (Parrack 1986; Gavaris 1988;Conser and Powers 1990) which employ survey data in variousformulations has been the tuning procedure of choice at the NEFSCand in regional stock assessment workshops (SAWS) in recentyears. ADAPT was used exclusively with both spring and autumnsurvey data to provide terminal F values and stock size estimatesin 1997 assessments of Atlantic cod, haddock and yellowtail atthe 24th SAW (NEFSC 1997a) .

Earlier methods used at NEFSC to estimate trends in stockabundance and biomass directly from survey data were reviewed byClark (1981). These included use of Baranov's (1918) catchequation to track abundance, incorporating commercial catch innumbers and recruitment estimates derived from survey data(Hennemuth 1969); the "survey population index ll used to modeltrends' in abundance for yellowtail flounder incorporating surveycatch-at-age values and survival rates derived from commercialCPUE (Brown and Hennemuth 1971); and calculation of trends intotal biomass from survey indices through application ofcatchability coefficients derived from analysis of commercialdata (Clark and Brown 1977). Swept-area biomass estimates havealso been developed for dogfish, skates, surfclams, squids, andother species. Research to develop capabilities to determine thedensity and abundance of pelagic species such as Atlantic herringand Atlantic mackerel from bioacoustic signals have recently beeninitiated at the NEFSC.

For invertebrate stocks in which age structure information is notavailable, the DeLury model of Collie and Sissenwine (1983) asmodified by Conser (1991, 1995) has been used with survey andcommercial data to estimate stock sizes and fishing mortalityrates for American lobsters (Conser and Idoine 1992; NEFSC1996b). It has also been used in assessmentsfor sea scallops(NEFSC 1992, 1995a, 1997b), surfclams (NEFSC 1995b, 1996c), andnorthern shrimp.

Biology and Distribution

since NEFSC surveys provide synoptic sampling for most speciesstocks over their life span and distributional range, resultantdata have been extremely valuable for estimating populationsize/age compositions and growth, maturation; and mortalityrates. The length of the survey time series also permitsexamination of changes in these parameters and determinations ofhow species react to changes in stock abundance and environmentalconditions (HeIser and Almeida 1997). Most NEFSC assessmentshave incorporated growth and maturation data from surveys inyield and spawning stock biomass per recruit models. O'Brien et

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al. (1993) provide maturity ogives for 19 finfish species basedon data collected in NEFSC bottom trawl surveys from 1985-1990:Spring and autumn survey data appear to have been of equal valuein determining growth and mortality rates, although spring surveydata have been judged as being more useful for determiningmaturation rates, since many important species in the Northeastare spring spawners (NEFSC 1988). The surveys have also been ofvalue in determining migratory pathways and seasonal distributionof adults and juveniles; and in recent years, data have beenwidely used to evaluate options for area/season closures andexperimental fisheries.

Other Uses

From time to time survey data have been useful in extrapolationsto compensate for missing commercial fishery data. Mayo et al. tt·(1981) developed a method to estimate commercial discard whichincorporates commercial mesh selectivity data, information onculling practices, and survey length compositions. These dataare used to estimate the length frequencies of discard in thepopulation. Regressions between commercial landings-at-age andsurvey catch-at-age are then used to generate coefficients which,when applied to the estimated survey discard length frequencies,provide estimates of discard for the fishery as a whole. Bottomtrawl survey data have also been used in conjunction withichthyoplankton survey data to provide estimates of spawningbiomass for several species including yellowtail flounder,Atlantic mackereI, and silver hake (e.g. Berrien 1981, 1984,1990). Berrien and Sissenwine (1988), Pennington (1988), andother papers consider relative applicability and use of bot tomtrawl and ichthyoplankton survey data for direct estimation ofspawning biomass (Smith 1988). Murawski and Finn (1988) u~ed thesurvey database to examine the degree of temporal and spatial co­occurrence of demersal finfish species on Georges Bank to providea basis for minimizing bycatch.. •

Ecosystems Research Applications

The NEFSC survey time series has been invaluable for fisheriesecosystems research applications. A particularly valuable aspectof the trawl survey program has been the opportunity provided toobtain synoptic food habits data from a wide variety of species.These data have been utilized to examine the diet compositions ofthe species inhabiting the northeast continental ~helf andbiological interactions between species. The long time seriesalso provides data for analyses to examine changes in diets dueto changes in stock abundance or environmenta1 conditions.

Fish distribution and environmental data from the survey timeseries have been used to investigate possible environmentalinfluences on abundance and distributions of fish stocks and, by

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extension, potential implications of climate change (Mountain andMurawski 1992; Murawski 1993) .. The surveys have also providedthe basis for representative· sampling of a variety of ecosystemcomponents for habitat studies and other research. Mountain andHolzwarth (1990) developed a surface and bottom temperature andtemperature anomaly time series for different regions of thenortheast shelf based on areal averages from the survey datawhich have been widely used. More recent analyses have .identified habitat associations·based on these data which haveimportant implications for sampling programs and stock assessmentmethods (NEFSC 1996a) .

•The surveys have been particularly important in providing datafor quantitative ecological studies. Papers by Overholtz andTyler (1985) and Gabriel (1992) identified assemblages on theNortheast continental shelf and examined effects of perturbationsfrom exploitation in terms of changes in biomass and speciescomposition and diversity; Clark and Brown (1977) and Mayo et al.(1992) monitored trends for various species groups based on thesesurveys. Murawski and Idoine (1992) examined trends inmultispecies size composition and examined potentialapplicability of size-based indices based on NEFSC survey datafor Georges Bank. These and other studies have documentedecosystem changes over the past 30 years, notably the largeincrease in abundance of pelagic species (Atlantic herring andAtlantic mackereI) and elasmobranchs (dogfish and skates)coinciderit to declines in abundance of groundfish and flounderstocks. Together with food habits data and other information,these data provide a priceless time series for multispeciesmodeling.

EVALUATIONS OF SURVEY PERFORMANCE

.. Periodically, the NEFSC survey time series has been examined interms of statistical properties and potential methods forimprovement. Earlier papers by Grosslein (1971), Pennington(1985, 1986), Pennington and Brown (1981); and Collie andSissenwine (1983), among others, considered aspects of surveyprecision including between-year variability. A longer-termproject was initiated by NEFSC staff in 1983 to define anddocument gear and procedural changes and their effects and toevaluate survey design, precision and efficiency, and analyticalmethods. This culminated in a 1986 study by NEFSC staff whichreviewed the uses of survey data and focused more directly onprecision and efficiency and options for improvement (Almeida etal. 1986; Fogarty et al. 1988; NEFSC 1988). The more significantfindings were as folIows:

1. Precision tended to be higher for demersal species ascompared to pelagics. However, precision for flounderstended to be poor compared to other demersals. The modal

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80% confidence interval for demersals (flounders excluded)was 40% of theomean. It was concluded that for generalmanagement applications, levels of precision achievablethrough use of appropriate estimators (Delta distribution)were reasonable for demersal species; this was less true forpelagics and flounders.

2. Time series models proved to be very effective in filteringout random variability, and ~rovided considerable insightinto reliability of the indices tested.

3. The impact of reductions in vessel time on survey coverageis not linear and can be quite significant, e.g., a 30%reduction in sea days would necessitate a 45% reduction innumber of stations that can be occupied. The disparityreflects the relative increase ·in steaming time (relative to 4ttime on station) needed to maintain coverage of the surveyregion.

4. Analyses to determine the effects of changes in samplingintensity on precision indicated that for modest losses invessel time (ca. 15% of the current number of sea days)reductions in precision were minor, but with greaterreductions precision dropped off rapidly, e.g., a reductionof 36% increased the standard deviation of survey indices byover 50%. On the other hand, an increase in vessel time ofcomparable magnitude reduced the standard deviation by only20%. No consistent differences in precision were detectablebetween species stocks or species groups with changes insampling intensity.

The above findings, in part, resulted in development of a winterbot tom trawl survey with chain sweep ground gear for flatfish andindicated the need for different approaches to improve theprecision of assessments for pelagics (e.g., Atlantic herring and ..Atlantic mackereI) . They also supported the basic samplingdesign, intensity and allocation of resources by season forfishery-independent research vessel surveys that has evolved atNEFSC since the early 1960s.

FUTURE DIRECTIONS

Enhancing field and laboratory aspects of the survey program is aconstantly sought objective or goal. However with today'sclimate of downsizing, enhancements are often precluded bylogistical needs to maintain the time series. Routine operationsrequire a great amount of attention and maintenance of the database. Constant communication and training are required to becertain established'procedures are adhered to and followed.Also, one must proceed cautiously with any enhancement, sinceenhancement is synonYffious with change, and change always carries

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the risk of compromising the integrity of the time series.Nonetheless the NEFse is still committed to actively pursuingprogram enhancements; ,Two of~the more important efforts nowunderway involve using the ship's new scientific computer system(SCS) and development of an onboard data entry system.

The SCS system is being used to monitor vessel and trawlperformance parameters.The network system now in place on boththe ALBATROSS IV and DELAWARE II is capable of monitoring andrecording 153 real and derived sensors including speed, depth,position, wire out, warp tension, and engine rpm. A suite ofmeteorological and oceanographic measurements are also available.The software necessary to utilize fully the information andincorporate it into the survey protocol is still in development.However, as an example of its utility, the SCS has alreadyyielded important information about errors in the ship's dopplerspeed log. By using the ses to record data simultaneously from .several speed and position sensors (GPS, LORAN, and differentdoppler systems), it was found that the sensor used to recordtowing speed since the early 1980s was in error, i.e., at thedesignated speed of 3.5 knots (kts), the correct speed wasactually 3.85 kts. Fortunately analysis of historic data showedthe error was consistent which allowed for proceduraladjustments.

The NEFSC has also invested considerable time and effort indevelopment of onboard data entry systems. Such systems couldsignificantly reduce errors, speed up data processing, andexpedite making data available to scientists and managers. Todate several systems have been tried with limited success. Oneof the larger obstacles hindering completiori of the project isthe need for weather-rugged hardware, since much of the work isdone during all seasons on an open deck. Another problem is thelarge amount of data collected at each station. On every stationlengths and weights and often other data are recorded for allspecies caught. In the northern regions of the survey area theaverage is about 15 species per tow but in the southern areas thenumber of species can exceed 50 on a given station; The largeamount of data combined with age and food habit sampling, hasoverwhelmed several key-at-sea systems tested to date. However,progress has been made recently and we hope to have a functioningsystem available in the near future.

eONCLUSIONS

The NEFSC bot tom trawl survey series has provided priceless datafor assessment and management, and many research applications.Recently it has played a major role in providing the informationnecessary to design and implement management plans now in placeaimed at rebuilding the stocks. We expect the survey to remain avital research and monitoring tool as the time series progressesinto its fourth decade and beyond.

15

ACKNOWLEDGMENTS

We would like to thank the hundreds of scientists, volunteers,ship's officers and crew members who have staffed our researchvessels for their hard work and good spirits when often workingunder less-than-ideal conditions.

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21

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Figure 2. Area of the Northwest Atlantic showing NortheastFisheries Science Center offshore (>27meters) bottom trawl surveystrata. Inshore strata «27m) between Cape Hatteras and Cape Codnot shown.