Definition and analysis of multispecies otter-trawl fisheries off the northeast coast of the United States

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1 J. Cons. int. Explor. Mer. :. S Definition and analysis of multispecies ottertrawl fisheries off the northeast coast of the United States S. A. Murawski, A. M. Lange, M. P. Sissenwine, and R. K. Mayo National Marine Fisheries Service Northeast Fisheries Center Woods Hole Laboratory Woods Hole. Massachusetts, USA Otter trawls fished off the northeast coast of the United States catch a variety of demersal and semipelagic species depending on the area, depth, and time of year they are used. Because of the relatively nonselective nature of the gear, and biological cooccurrence, fisheries are rarely characterized by an individual species. Fishery management by singlespecies quota control has thus proved particularly difficult. An alternative approach to fishery definition can be based on determining the spatial/ temporal boundaries of relatively unique and homogeneous species compositions, from commercial landings data. Accordingly, operational fisheries were defined by grouping statistical area/depth zone/month categories (n = ) exhibiting similar species compositions in aggregated commercial vessel trip reports for to. Based on the results of numerical cluster analyses, mixedspecies fisheries (subfisheries) were identified and organized into nine major fisheries reflecting geographic/ecologic zones. A review of historical ( to ) trends in nominal catch and fishing effort, catch per effort, relative vessel size distribution, and species composition for selected major fisheries is presented. Definition of operational fisheries based on the spatial/temporal variability of species compositions is consistent with most forms of fishery management including total catch and effort regulation, meshsize restrictions, and seasonal or areal closures. This concept allows managers to implement strategies for optimizing yields from multispecies groups based on various economic and biological criteria (e.g. mesh size resulting in the greatest physical yield from the system), while recognizing that technological and/or biological interactions will result in a mixedspecies catch. Introduction Singlespecies catchquota regulation has been the primary method of fishery management off the northeast coast of the USA, both under the auspices of the International Commission for the Northwest Atlantic Fisheries (now the Northwest Atlantic Fisheries Organization) and the US Regional Fishery Management Councils. However, a large proportion of total landings from the area are derived from otter trawls, which are relatively nonselective to species. Rather, the composition of trawl catches is determined by the distribution of fish populations in relation to locations where the gear is deployed, and by the physical characteristics of the gear itself. Because of the multispecies nature of the ottertrawl fisheries, application of a series of singlespecies quotas to them has proved particularly difficult. Linear programming (LP) techniques have been used to determine optimal catches in multispecies ottertrawl fisheries from a series of historic bycatch ratios for each target species (Fukuda, ; Brown, Brennan, and Palmer, ). These procedures allow calculation of the overall yield given that a series of singlespecies quotas (linear constraints) cannot be exceeded. The utility of LP solutions in assessing various hypothetical management scenarios is a function of the availability of previous data on the impacts of regimes under consideration relative to species mix, size composition, and quantity of catch. An alternative approach is to define fishery management units in time and space such that catches with a specific type of fishing gear exhibit a relatively unique and homogeneous species composition. Total catch (all species combined) and/or fishing effort could then be regulated with reasonable expectation of the species composition of the resulting catch, and fishing mortality on each constituent. Other forms of fisheries management (e.g. mesh regulation, closed areas, or seasons) are also compatible with fishery management units based on species composition. Fishery units may be defined as ecological communities (Gabriel and Tyler, ; Tyler, Gabriel, and Overholtz, ). However, knowledge of the influ Downloaded from at Penn State University (Paterno Lib) on March,

2 W Figure. Statistical areas used to designate location of capture of fish and shellfish landings off the northeast coast of the United States. ences of fishing patterns and technologies on particular groups is important since the size and species composition of landings may be dramatically different from the natural community due to gear selectivity and discarding practices. In this study we develop a method for fishery definition by examining aspects of the presence and persistence of multispecies groups, defined in space and time, that are exploited by otter trawlers. The approach is empirical in that we define groups based on patterns of species composition from catch, effort, and biological sampling data available from the study area during to. Further, we present an analysis of trends in total catch, species composition, and fishing effort for selected fisheries defined in this study for the period to, and discuss aspects of fisheries management under this scheme. Data sources and methodologies A potential criterion for the delineation of operational units within a regional ottertrawl fishery is the presence of a relatively unique species composition of landings from one or more geographic areas, at specific times. If recurring species groups can be mapped and are contiguous in space and time they may then be treated as a unit. Accordingly, the feasibility of defining fisheries based on similarity of species composition in commercial landings was evaluated using available data from ottertrawl vessels operating off the northeast coast of the United States during to. A modern series of catch and effort data have been collected by United States National Marine Fisheries Service (NMFS) port agents in the Northeast Region from individual vessel trips since. Each trip record includes date, catch in weight by species, statistical area (Fig. ) and depth zone fished, days fished, and other data not considered in this study. Vessel trip records are available for most trips landing at New England ports. They are also available for some trips landing at mid Atlantic ports, but these data are either only available for recent years (i.e., New Jersey since ) or are not readily available for computer processing. Therefore, only trips landing in New England were considered. The analysis of statistical areas off the midatlantic coast is based on trips from that area, but landing at New England ports. Thus, the proportion of the total landings represented by trip records is smaller and more sporadic in time and space in the midatlantic area than in New England. Species composition data for our analyses were generated by summing landings from individual vessel trips falling into various space/time categories. Spatial designations were by statistical area (Fig. ), and depth zone (Table ). Changes in the species composition of ottertrawl catches with depth are well known (Gabriel and Tyler, ; Bortone et al., ). Many of the species of interest inhabiting the region display seasonal migratory patterns in response to cyclic changes in environmental parameters. Thus, it was appropriate to examine the species composition in each statistical area/ depth zone more frequently than on an annual basis. As the date of landing is recorded for each trip, virtually any relevant time scale could be taken into the analysis. Initially, data were grouped by month, giving sets of species compositions for each area/depth category. Because of the large quantity of data to be analysed, a subset of years was chosen for an examination of spatial and seasonal trends in landings composition. The per cent species composition of landings derived from each statistical area, depth zone, and calendar month was calculated using aggregated data for the period Downloaded from at Penn State University (Paterno Lib) on March,

3 Table. Depth zones and vessel size class designations used in the definition and analysis of multispecies ottertrawl fisheries. Depth zone Vessel class Fathoms > mixed unknown Subclass Range Metres > Gross register tonnage to. These particular data were considered in the analysis because, presumably, the most recent available information reflects current fishing practices and includes data on species that are presently abundant and important, but that may have been previously scarce e.g. haddock during the early s (Clark and Brown, ). Depth zones and (Table ) were not included in our initial definitional analysis, but landings from these depths were later prorated to appropriate fishery units based on species compositions for evaluation of longterm trends in catch, effort, and catch per effort. A total of possible area/depth/month combinations ( areas x months x depths) yielded cells actually containing catch information. Most of the combinations lacking catch data were in the mid Atlantic region, where fishing activity of commercial otter trawlers landing in New England is least. Additionally, several of the areas were characterized by depth ranges beyond those normally associated with otter trawling (i.e., areas,,, etc., Fig. ). Initial groupings of area/depth/month cases exhibiting similar species compositions were assigned using a clustering algorithm and computer program given in Dixon (). The task of the computer was to describe the relationship between area/depth/month cases based strictly on the relative proportions of species in the landings. The quantity of landings was not an objective criterion for grouping at this stage. A total of different species was recorded from at least one of the area/depth/month cases during to. However, many of the species occurred rarely and contributed little to the total landed weight. The computer program used in the clustering process (BMDPM) did not have the capacity to analyse all species () and cases () simultaneously. Thus, the number of species considered was reduced to, based on rankings of landed weight and frequency of occurrence (Table ). The per cent of the total landings in weight contributed by each of the species was coded as a distinct variable. Thus, a vector of percentages described each case. These percentages generally did not sum to % for each case since a portion of the landings was contributed by species not included in the analysis. However, in most cases, over % of landed weight was accounted for. It is unlikely that any two cases have exactly the same per cent compositions for all species. In order to group (cluster) cases with similar overall species compositions, initially, each case is considered to be in a cluster of its own. At each step of the analysis the two clusters with the shortest distances between them (most similar) are combined (amalgamated) and treated as one cluster. The process of combining clusters could Table. Common and scientific names of species or groups of fish and shellfish used to define fisheries based on similarities in landings composition. Common Butterfish Cod, Atlantic Cusk Dogfish, smooth Dogfish, spiny Flounder, summer Flounder, windowpane Flounder, winter Flounder, witch Flounder, yellowtail Goosefish Haddock Hake, red Hake, silver Hake, white Herring, Atlantic Industrial Mackerel, Atlantic Ocean pout Plaice, American Pollock Redfish Scup Sea bass, black Shrimp, northern Squid, longfinned Squid, shortfinned Wolffish, Atlantic Species aggregated. Species names Scientific Peprihis triacanthus Gadus morhua Brosme brosme Muslelus canis Squalus acanthias Paralichthys dematus Scophlhalmus aquosus Pseudopleuronectes americanus Glyplocephalus cynoglossus Limanda ferruginea Lophius americanus Melanogrammus aeglefinus Urophycis chitss Merluccius bilinearis Urophycis tenuis Clupea harengus Several species Scomber scombrus Macrozoarces americanus Hippoglossoides platessoides Poilachius virens Sebastes fascialus Stenolomus chrysops Cenlroprislis striata Pandalus borealis Loligo pealei Illex illecebrosus Anarhichas lupus Downloaded from at Penn State University (Paterno Lib) on March,

4 continue until all cases have been combined into one cluster with the degree of homogeneity reduced as the number of clusters is reduced. A measure of the distance (or degree of dissimilarity) between two cases or clusters/ and / is defined as: where; X',, = A",, V. distance between clusters/ and /, standardized per cent of total landings contributed by species / in the /th case (/ =,,,...,), standardized per cent of total landings contributed by species / in the /th case (/ =,,,...,). Input data were first standardized to minimize the influence of variables with large standard deviations on distance measures (Dixon, ), viz.: where; x, S.D,. X', n = S.D., standardized per cent of total landings contributed by species / in the /th case (n =,,,..., ), per cent of total landings contributed by species/ in the/?th case (n =,,,..., ), mean per cent composition of the /th species over all cases, standard deviation of the per cent composition of the /th species over all cases. Results of the stepwise clustering procedure were printed as a dendrogram, illustrating the distances at which clusters are combined (Fig. ; Goodman, ). ] Following the cluster analysis we reviewed results and in some instances rearranged cases and combined clusters in order to affect significant groupings that were intuitively correct and relatively continuous in time and space. Cluster analysis was considered a preliminary, objective method of using a computer to initially order large numbers of cases with a virtually infinite number of combinations. Since the method is totally objective, it Cluster analysis experiments utilizing percentage or proportion data are often conducted employing an arcsin transformation (Goodman, ) to stabilize withincase variances and eliminate functional dependence of variances on mean percentages. A parallel cluster experiment was thus performed using arcsin transformed, nonstandardized data. Comparison of the two dendrograms (Sokal and Rohlf, ) indicated no meaningful differences in conclusions concerning the relationships among individual cases. does not consider information which is not easily quantifiable (such as space/time continuity, stock structure within species, and the significance of small quantity aberrant catches). Definition of fisheries The definition of distinct groups from cluster analysis is dependent on arbitrary assumptions of distances necessary for separation. In the extremes, separation distances may be so large or small that all cases are considered as one group, or each as a distinct cluster. We chose a level of definition sufficient to generate a moderate number of groups ( to ), recognizing that considerable variation in the distance separating clusters was likely. For example, the individual area/depth/ month cases that exhibited a high percentage of summer flounder catches separated as a group that was quite distinct (a relatively large distance) from other groups, whereas, several groups with high percentages of redfish were separated by relatively small distances. The area/depth/month cases were initially separated into groups based on results of the cluster analysis using standardized percentage species composition data (Fig. ). The number of cases in each group ranged from one to. Several problems exist, however, if groups identified as having similar species composition from the cluster analysis are to be accepted as logical operating units for assessment and management purposes. A number of groups contain individual cases that are noncontiguous in space and/or time. It would be operationally difficult to define fishery units that have widely separated areal components, or that occurred at divergent times (i.e. winter, summer). There are several potential reasons for these discontinuities, including the geographical separation of distinct stocks of particular species (i.e. silver hake in the Gulf of Maine and the midatlantic), and aberrant catches from specific areas/depths/months. The latter case occurred particularly frequently in the midatlantic region from where ottertrawl landings at New England ports were relatively sporadic. Accordingly, we combined several of the clusters and shifted individual cases among groups with the specific objectives of reducing the total number of fishery units, and eliminating area/time discontinuities that did not reflect significant fishing patterns. A total of fishery units (subfisheries) were thus generated and organized into nine major fishery units (Tables to ). Major fishery units generally reflect broad geographic/ ecologic zones, while subfisheries are defined by the dominance of one or more species or groups (Tables, to ). Percentage species compositions generally do not sum to % since a number of species were not considered explicitly in our analysis (Table ). In the process of combining clusters and changing group membership of individual area/depth/month Downloaded from at Penn State University (Paterno Lib) on March,

5 Or Clustered area/ depth/ month cases WW, ^ I o o Figure. Cluster analysis dendrogram illustrating the relationship between arbitrary groups of area/depth/month cases. Group designations are based on similarity of species composition of landings. Vertical scale is the distance (see text) at which groups were separated. Width of rectangles corresponding to numbered groups is proportional to the number of cases in each group. Branching within groups (relationships of cases within clusters) is not shown. Total number of cases is. cases to establish the subfishery units some cases with relatively dissimilar species compositions were combined in the interest of space/time continuity. Accordingly, it is relevant to assess the degree to which this intuitive process affected withingroup variability in species composition. If the logical fishery groupings of cases exhibit, on average, substantially different withingroup variation from the original clusters, then the utility of using cluster analysis as an intermediate step in the process may be open to question. A measure of the degree of withingroup variation in species composition for each of the clusters and fishery units can be calculated as: where: " /" \ I s (X',x', k y rjj D; = average dispersion of individual area/depth/ month cases around the mean species composition vector in cluster (or fishery)/, A",. = average standardized per cent composition of species / for all area/depth/month cases comprising cluster (or fishery) /, X' ik = standardized per cent species composition of species / in case k (where k =,,,...,«; n is number of cases in the cluster (or fishery)). Frequency distributions of D/s for the original clusters and the subfishery units are given in Figure. The two sets of average distances exhibit distinct modality at the same frequency interval; data for the clusters appear slightly skewed to the left. Two of the clusters were defined by single area/depth/month cases and thus there was no dispersion around the average species composition for these clusters. If these two clusters are not considered, withingroup dispersions for the clusters and subfisheries are quite similar Downloaded from at Penn State University (Paterno Lib) on March, Journal du Conscit

6 Table. Weighted average per cent species composition of ottertrawl landings from subfisheries off the northeast coast of the Species Butterfish Cod, Atlantic Cusk Dogfish Flounder, summer Flounder, windowpane Flounder, winter Flounder, witch Flounder, yellowtail Goosefish Haddock Hake, red Hake, silver ' Hake, white ' Herring, Atlantic Industrial Mackerel, Atlantic Ocean pout Plaice, American Pollock ' Redfish Scup Sea bass, black () Shrimp, northern () () Squid, longfinned () () Squid, shortfinned Wolffish, Atlantic () () Offshore lobster fishery (otter trawl). c D CT a ~E 'era u A/ = X = S.D.= ^ A/ = X = S.D.= (Fig. ). Overall average distances change only slightly (, ) and are not significantly different when % confidence regions are evaluated. A similar analysis was extended to the nine major fishery groups; average withingroup dispersion was (S.D. = ). These data imply the major fishery groups exhibit withingroup dispersions similar to those of the subfisheries and clusters. However, major fisheries were not weighted by landings. If average dispersions for the nine major fishery groups are weighted by the landings from each (Lange et al., ), average withingroup dispersion increases to. This implies that fisheries with greatest landings (and containing most of the cases) are characterized by the greatest withingroup dispersion. Subfisheries defined in this study were generally Downloaded from at Penn State University (Paterno Lib) on March, n Average withingroup dispersion Figure. Frequency distributions of average withingroup dispersions around mean species compositions for initial groups of statistical area/depth/month cases identified by cluster analysis, and final groups (subfisheries). Measures of dispersion are standardized sums of squares distances (see text) of individual cases from mean withingroup species composition.

7 USA. to. Subfishery o o oo oo oo oo oo oo oo oo oo oo oo oo oo oo oo consistent with initial groupings identified from numerical clustering techniques. The advantages of the numerical methods for group definition are their ability to organize information in large data sets, such as ours, and to provide an initial framework for intuitive reclassification based on stock structure, fishing practices, etc. The nine major fisheries exhibited proportionately more withingroup heterogeneity in species composition than the subfisheries or the original clusters when relative landings within the units are considered. However, this behaviour is to be expected considering the more subjective nature of the major fishery unit classification scheme. Major fishery. Broadly defined as the nearshore Gulf of Maine unit, major fishery comprises seven subfisheries ( to ). Fishing activity occurs in statistical areas to, primarily in depth zones to. Most of the area/depth cases assigned to major fishery are in the months April through July. However, fishing occurs throughout the year (Table ). Landings during to consisted primarily of cod ( %), pollock ( %), American plaice ( %), and haddock ( %), although significant quantities of other groundfish were o o oo oo oo oo o o o o oo oo oo oo oo oo oo () taken. Subfisheries variously generated predominantly cod, pollock, American plaice, and witch flounder (Table ). Major fishery. This fishery can generally be described as occurring in the deep waters of the Gulf of Maine and is composed of nine subfisheries ( through, ). Catches are primarily derived in statistical areas to, and depth zones and. Fishing is conducted throughout the calendar year (Table ). During to landings were primarily redfish ( %) with silver hake ( %), haddock ( %), cod ( %), and pollock ( %) contributing most of the remainder. Subfisheries variously contributed predominantly redfish (,,,,, ), haddock (, ), and silver hake (). Major fishery. Major fishery is composed of one subfishery () and is located along the northeastern edge of Georges Bank. Fishing is most widespread from April through October (Table ). The fishery is prosecuted in statistical areas to, and in depth zones to. During to landings consisted primarily of cod ( %), and haddock ( %), winter flounder ( %), yellowtail flounder ( %), and pollock Downloaded from at Penn State University (Paterno Lib) on March,

8 Table. Classification of subfisheries within nine major fisheries, and the number of vessel trips on which the classification was based. Major fishery Subfishery Ottertrawl landings, depth zones to. Number of trips to ( %), as well as small quantities of several other species (Table ). Major fishery. This fishery encompasses the shallow water areas of Georges Bank, and extends seasonally into the western Gulf of Maine and southern New England waters. Major fishery includes five subfisheries ( to ) and occurs throughout the year. Fishing activity is prosecuted exclusively in depth zones through. During to the predominant species landed were cod ( %), yellowtail flounder ( %), winter flounder ( %), and haddock. Subfisheries,, and generated primarily cod, while subfishery yielded mostly yellowtail flounder, and fishery generated mostly winter flounder. Major fishery. This fishery consists of one subfishery (), and can be described as the offshore trawl fishery, primarily for American lobster Homarus americanus, prosecuted off the southern and eastern flanks of Georges Bank. The fishery is conducted in statistical areas and and in depth zones to. During to landings occurred from January through July, but primarily from March through May. Although not considered explicitly, the primary species landed was lobster ( %), with lesser proportions of silver hake ( %) and redfish ( %). Major fishery. Major fishery includes one subfishery () that encompasses the spring (May and June) fishery in Nantucket and Vineyard Sounds (statistical area ) generating a high proportion of Loligo (% in to ). Other major species taken were scup ( %) and winter flounder ( %). The fishery is prosecuted in depth zone exclusively. Major fishery. Major fishery is generally characterized as the mixedspecies industrial/southern New England smallmesh fishery. It is seasonally prosecuted in statistical areas,,,,, and, and in depth zones to. During to the major species (groups) landed were industrial species ( %), silver hake ( %), butterfish ( %), and yellowtail flounder (%). Three subfisheries within major fishery generated primarily industrial species (subfishery ), butterfish (subfishery ), and Atlantic herring (subfishery ). Major fishery. Major fishery consists of one subfishery () that is conducted in various southern New England and midatlantic statistical areas, and in depth zones to. The fishery operates primarily in spring and autumn months, in response to the seasonal inshore/offshore migratory patterns of major constituent species including scup ( %), butterfish ( %), and summer flounder ( %). Major fishery. Major fishery unit includes one subfishery () that represents the southern New EnglandmidAtlantic summer flounder fishery. The fishery operates primarily during the winter and spring when summer flounder are found in relatively deep offshore waters. The lack of significant data for most midatlantic statistical areas, particularly for the summer months, precludes a definitive analysis of seasonal patterns of this fishery. Summer flounder accounted for most of the landings ( %) followed by tilefish, Lopholatilus chamaeleoniiceps, ( %), and butterfish ( %) during to. The degree to which fisheries defined in this study represent distinct operational units in the regional ottertrawl fishery can only be evaluated using time series data on landings, species composition, fishing effort, and vessel size distribution. Accordingly, we present an analysis of time series data on these parameters for two During to the industrial category was primarily composed of red hake, silver hake, and ocean pout, although numerous other species contributed a portion of the total. Downloaded from at Penn State University (Paterno Lib) on March,

9 Table. Weighted average per cent species composition of ottertrawl landings from nine major fisheries off the northeast coast of the USA. to. Species Major fishery Butterfish Cod, Atlantic Cusk Dogfish Flounder, summer Flounder, windowpane Flounder, winter Flounder, witch Flounder, yellowtail Goosefish Haddock Hake, red Hake, silver Hake, white Herring, Atlantic Industrial Mackerel, Atlantic Ocean pout Plaice, American Pollock Redfish Scup Sea bass, black Shrimp, northern Squid, longfinned Squid, shortfinned Wolffish, Atlantic Z ' oo oi oo 'l oo oo oo oo of the nine major fisheries previously defined: major fisheries (the deepwater Gulf of Maine), and (central Georges Banksouthern New England groundfish). Analysis of fishery trends, to Between and % of the annual landings and effort (days fished) information contained in the vessel trip record data base could be assigned to the nine major fisheries for each of the years between and. Most of the landings and effort not accounted for by the defined fisheries were taken in mixed or unknown depths (zones and, Table ). For an analysis of trends in catch and effort over the time series it was thus appropriate to prorate catches from the mixed and unknown depth categories to the major fisheries. Individual vessels are assigned vessel class and subclass designations based on gross register tonnage (Table ). Thus, it is feasible to analyse trends in catch and effort for vessels of relatively homogeneous size. We chose to combine the various subclasses within the three vessel class categories; three separate analyses are presented for each major fishery. To detect possible changes in the average size of vessels within the three vessel classes operating in particular major fisheries, an oo oo oo oo oo oo oo oo oo oo oi oo oo oo oo oo oo oo oo oo annual index of vessel size was calculated as the average subclass designation weighted by fishing effort, viz.: where; VI;, = relative size index of vessel class; (J =,, ), in major fishery k (k =,,,..., ), total fishing effort in vessel subclass / of vessel class/, and major fishery k (n = number of vessel subclasses in vessel classy). Landings and effort from depth zones and and all statistical areas were prorated on an annual basis to appropriate major fishery by vessel class category. Prorations are based on methods presented in detail by Lange et al. (). A detailed analysis of catch, nominal effort, catch per effort, species composition, and vessel size indices for the period to is presented for major fisheries and. Analyses for all major fisheries are given in Lange et al. (). Downloaded from at Penn State University (Paterno Lib) on March,

10 Table. Distribution of statistical area/depth zone/month cases grouped by major fisheries off the northeast coast of the statistical area are indicated. Major fishery X Depth zone Total cases Statistical at Penn State University (Paterno Lib) on March, Downloaded from

11 United States. The numbers of months within the year during which each major fishery operates in particular depth zones in each Downloaded from at Penn State University (Paterno Lib) on March,

12 Table. Number and per cent (in parentheses) of statistical area/depth zone/month cases allocated to major fisheries, by month. Total number of cases is. Month Major fishery ' Jan ( ) Feb ( ) Mar ( ) Apr () May (HI) Jun () Jul () Aug ( ) Sep ( ) Oct ( ) Nov ( ) Dec ( ) m O x sz a ro O x LU >^ o Q. O / N I/) x ~ D I/) C /) Vessel \ i i ( o) ( ) ( ) ( ) ( ) ( ) ( ) ( ( ) ( ) ( ) ( ) ( ) ( ) ( ) () () () () () ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) (H ) ( ) ( ) ( ) ( ) ( ) () () ( ) ( ) ( o) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ( ( ( ( ( ( ( ( ( ) ) ) ) ) ) ) ) ) ) ( ) ( ) ( ) ( ) () () () class y \ / ^ i r Major fishery ( ) ( This fishery (the deepwater Gulf of Maine) accounted for between and % of the annual USA ottertrawl landings from the northeast region during to. Landings ranged from t in to a peak of tin. Total catch and fishing effort by vessel class (Fig. ) generally declined through the late s and mids, but increased markedly in the late s. Mediumsized vessels (vessel class ) contributed between and % of the total effort annually. Since catch per effort for all vessel classes has generally declined. However, landings and total effort increased to nearrecord levels in each category. Vessel size indices remained relatively stable for the three vessel classes, indicating no progressive trends over the study period (Fig. ). Trends in species composition of landings for four of the most important species are given in Figure. Haddock comprised most of the landings during the midand late s. However, with the decline in population size of haddock, redfish became the predominant species caught during the s. The relatively high levels of catch per effort for large vessels (class ) dur ) Downloaded from at Penn State University (Paterno Lib) on March, I I i Year Figure. Total catch (all species), fishing effort, catch per effort, and relative vessel size (see text) for three vessel classes operating in major fishery, to.

13 \ ^ ^ \ ' ' / ^ c o o CL eo I/) i/ CL in ent u c Q / \ ' \ \ \ \ HaddockX V / ; / / ' / \ ' \ ' \ / \ / \ ; \. \ /Redfish \ /""""N Silver hake Sx y Cod i i i i i i i i i i i i t i i A A Year Figure. Per cent species composition of landings from major fishery, to. ing to are indicative of the deepwater, offshore nature of the redfish fishery. Cod and silver hake each accounted for between and % of the annual landings during to. Haddock landings increased progressively during to, combined with a simultaneous decline in the percentage of redfish and silver hake in the catch. Major fishery The central Georges Banksouthern New England groundfish fishery generally accounted for the largest proportion ( to %) of USA ottertrawl landings from the northeast region annually during to. Mediumsized vessels ( to GRT) contributed the largest proportion of landings and effort, but average catch per effort for this class was generally lower than that of the others (Fig. ). Total landings declined from t in to tin and subsequently rose to t in. Mediumsized (class ) vessels exhibited a progressive increase in average size during the period, other vessel classes remained relatively stable (Fig. ). Yellowtail flounder accounted for most of the landings during to. However, several other trends in relative species composition of the landings are notable (Fig. ). Haddock landings declined in importance between the mids and the mids, and subsequently increased slightly from to. Cod exhibited a steady increase in proportion of landings, from about % in to nearly % in o u n O x "D LLJ a CL N ct, X CL/ Vessel class : Year \ \ \ \ \ Figure. Total catch (all species), fishing effort, catch per effort, and relative vessel size for three vessel classes operating in major fishery, to. Downloaded from at Penn State University (Paterno Lib) on March,

14 I in o CL Yellowtail Haddock Year Figure. Per cent species composition of landings from major fishery, to. and. Winter flounder contributed about % of the landings throughout the period. Relevance to fisheries management Because of the subjective assignment of statistical area/ depth zone/month categories to subfisheries and their arbitrary classification into major fishery, particular groupings presented herein should not be construed as being definitive for management purposes. Rather, it is our intent to present a methodology and some criteria with which to define the spatial/temporal boundaries of multispecies groups. For management purposes it is desirable to define fisheries that are relatively homogeneous with respect to species composition of landings (as are our subfisheries). However, the number of such fisheries is limited by practical management concerns such as the accuracy of statistical reporting and the enforcement of compliance with regulations. The grouping of subfisheries into major fisheries represents an attempt to balance the goal of defining units that are biologically distinct and homogeneous with the practical limitations of such a management scheme. Identification of the spatial and temporal bounds of fisheries was based on data averaged over a relatively short timespan ( to ). It is clear from analyses of selected major fisheries (Figs. to ) that species composition of landings and other indices varied widely over the full time series ( to ). However, this does not necessarily imply that management units need be periodically redefined. Research vessel sampling data from the same time period (e.g. Clark and Brown, ) indicate that significant changes in species utilization in major fisheries analysed generally reflected shifts in the relative abundance of these species/stocks. Thus, depending on management objectives for particular fishery units, specific regulatory devices may have to be reevaluated in light of changing economic and/or biological conditions. A typical scenario may, for example, be that meshsize restrictions are modified periodically to reflect significant changes in the relative abundance of important species. Whether or not operational fisheries defined in to are persistent over a longer time period is conjectural, due to fluctuations in the abundance of various species. However, fisheries were separated not only on the basis of varying proportions of common species landed in several fisheries, but also by the presence of species virtually unique to single fisheries. Thus, defined fisheries may reflect relatively unique ecological communities, although this was not considered explicitly in our analyses. The identity of subfisheries over time is probably more sensitive to changes in relative abundance of species than that of major fisheries due to the hierarchical nature of the definition scheme (i.e. the major fisheries are more dissimilar in species composition than are subfisheries within each). Given that various vessel size classes operating in a defined fishery generate a relatively homogeneous species composition, total catch and/or effort could then be regulated with reasonable expectation of species composition of the catch, in the short term. For specific Downloaded from at Penn State University (Paterno Lib) on March,

15 levels of fishing effort or total catch in a nondirected multispecies fishery, F(instantaneous fishing mortality) for each species will be different since catchability coefficients are species specific. If a vector of catchability coefficients for species or stocks comprising a fishery unit can be empirically established, impacts of various catch or effort levels on stock size, yield per recruit, etc. of each species can then be evaluated. Such an evaluation would allow managers to assess the implications of various harvest strategies as they affect particular species/stocks and total yields from the multispecies systems. The definition of distinct operational fisheries in time and space may allow for improved precision in estimates of the impact of fishing on species and size groups that interact with the gear, but are not landed. Mayo et al. () present a quantitative method for assessing the effects of gear selectivity and discarding practices in multispecies fisheries based on coincident researchvessel sampling. Further, they present an analysis of discards for major and subfisheries we have defined. Such data and analyses provide an objective basis for evaluating fishing mortality on submarketable sizes and species not ultimately retained. References Bortone. S. A., Shipp, R. L., Mayer, G. F., & Oglesby, J. L.. Taxometric analysis of a demersal fish fauna. In The deep sea ecology and exploration, pp.. Ambio, Spec. Rep.,. Royal Swedish Academy of Sciences. pp. Brown, B. E.,Brennan, J. A., & Palmer, J. E.. Linear programming simulations of the effects of bycatch on the management of mixedspecies fisheries off the northeastern coast of the United States. Fishery Bull. Nat. Mar. Fish. Sen. U.S., :. Clark, S. H., & Brown, B. E.. Changes in biomass of finfishes and squids from the Gulf of Maine to Cape Hatteras, as determined from research vessel survey data. Fishery Bull. Nat. Mar. Fish. Serv. U.S., :. Dixon, W. J. (Ed).. BMDP Biomedical computer programs Pseries. Univ. Calif. Press, Berkeley. pp. Fukuda, Y.. A note on yield allocation in multispecies fisheries. Res. Bull. int. Commn NW Atlant. Fish., :. Gabriel, W. L., & Tyler, A. V.. Preliminary analysis of Pacific coast demersal fish assemblages. Mar. Fish. Rev. Nat. Mar. Fish. Serv. U.S., ():. Goodman, M. M.. Distance analysis in biology. Syst. Zool., :. Lange, A. M. T., Murawski, S. A., Sissenwine, M. P., Mayo, R. K., & Brown, B. E.. Fishery trends off the northeastern coast of the United States,. Nat. Mar. Fish. Serv. U.S. Woods Hole Lab. Ref.. pp. (mimeo). Mayo, R. K., Lange, A. M., Murawski, S. A., Sissenwine, M. P., & Brown, B. E.. A procedure for estimating rates of escapement and discard, based on research vessel bottom trawl survey catches. ICES CM /G:, pp. (mimeo). Sokal, R. R., & Rohlf, F. J.. The comparison of dendrograms by objective methods. Taxon, :. Tyler, A. V., Gabriel, W. L., & Overholtz, W. J.. Adaptive management based on structure of fish assemblages of northern continental shelves, pp.. In Multispecies approaches to fisheries management. Ed. by M. C. Mercer. Spec. Symp. Can. J. Fish. Aquat. Sci.. pp. Downloaded from at Penn State University (Paterno Lib) on March,