Abstract

The trend towards use of commercial vessels to enhance survey data requires assessment of the advantages and limitations of various options for their use. One application is to augment information on size-frequency distributions obtained in multispecies trawl surveys where stratum boundaries and sampling density are not optimal for all species. Analysis focused on ten recreationally and commercially important species: bluefish, butterfish, Loligo squid, weakfish, summer flounder, winter flounder, silver hake (whiting), black sea bass, striped bass, and scup (porgy). The commercial vessel took 59 tows in the sampled domain south of Long Island, New York and the survey vessel 18. Black sea bass, Loligo squid, and summer flounder demonstrated an onshore-offshore gradient such that smaller fish were caught disproportionately inshore and larger fish offshore. Butterfish, silver hake, and weakfish were characterized by a southwest-northeast gradient such that larger fish were caught disproportionately northeast of the southwestern-most sector. All sizes of scup, striped bass, and bluefish were caught predominately inshore. Winter flounder were caught predominately offshore. The commercial vessel was characterized by an increased frequency of large catches for most species. Consequently, patchiness was assayed to be higher by the commercial vessel in nearly all cases. The size-frequency distribution obtained by the survey vessel for six of the ten species, bluefish, butterfish, Loligo squid, summer flounder, weakfish, and silver hake, could not be obtained by chance from the size-frequency distribution obtained by the commercial vessel. The difference in sample density did not significantly influence the size-frequency distribution. Of the six species characterized by significant differences in size-frequency distribution between boats, all but one was patchy at the population level and all had one or more size classes so characterized. Although the variance-to-mean ratio was typically higher for the commercial vessel, five of the six cases that were otherwise were among the species for which the size-frequency distribution differed between the two vessels. Thus, the origin of the significant differences observed between vessels would appear to lie in the spatial pattern of the species as it interacts with the tendency for one vessel to obtain large catches more frequently for some size classes. One consequence of differential distribution and catchability is that more large fish were present in the commercial vessel catches than in the survey vessel catches in cases where the two vessels obtained different size-frequency distributions. Application of commercial vessels to the evaluation of size frequency hinges on understanding how to interpret differences among boats, gear, and sampling design. Here we show that key ingredients to this understanding are the degree of nonlinearity in catchability across a range of size classes, the interaction of varying spatial arrangements among size classes and the sampling design, and the interaction of varying spatial arrangements with differential catchability.

Highlights

  • Sophisticated approaches to fisheries management have increased emphasis on the reliability and adequacy of the principal underlying data, landings, CPUE, and survey indices (e.g. Ricker, 1975; NEFSC, 1988; Schnute, 1985)

  • The survey vessel averaged a larger number of individuals of a certain species and size class per tow than the commercial vessel (Table 3)

  • In the vast majority of cases, the ratio of catch means fell below 0.2, indicating that the commercial vessel caught more than five times as many fish as the survey vessel

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Summary

Introduction

Sophisticated approaches to fisheries management have increased emphasis on the reliability and adequacy of the principal underlying data, landings, CPUE, and survey indices (e.g. Ricker, 1975; NEFSC, 1988; Schnute, 1985). In the Mid-Atlantic Bight, continental shelf surveys of fish stocks are conducted in the fall, winter, and spring by the National Marine Fisheries Service using trawl gear (NEFSC, 1988; Brodziak and Hendrickson, 1999). These surveys have a stratified random design (e.g. NEFSC, 1988; Dawe and Hendrickson, 1998). One option to resolving issues related to coverage and gear is to augment the survey using commercial vessels (e.g. Otto, 1986; NEFSC, 2000a,b) These vessels typically have a more limited range, are available for a more limited time, and have varying catch efficiencies. Multispecies programs are likely to exacerbate these difficulties because optimal sampling design cannot be achieved for many species in either the base survey or the survey augmentation program

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