The use of the swept area method for assessing the seabob shrimp Xiphopenaeus kroyeri (Heller, 1862) biomass and removal rates based on artisanal fishery-derived data in southern Brazil: using depletion models to reduce uncertainty
The seabob shrimp (Xiphopenaeus kroyeri) represents an important fishing resource for artisanal fishermen in coastal areas of southern Brazil. Stock assessments of this species ha ve generally relied on biomass dynamics models as applied to CPUE time-series, which (a) are only available for a small offshore fraction of the exploited population and (b) does not comprise patterns of the shallowest artisanal fishing grounds. This work explores the use of extensive catch and effort data derived from a small-scale trawl fishery to obtain swept-area estimates of abundance and removal rates in a limited coastal area of southern Brazil (Tijucas Bay, Santa Catarina State, 27°15'S-48°33'W). Data were obtained from 7,198 fishing trips monitored at the fishing communities between June 2004 and August 2005. Because three parameters of the swept-area equation (i.e. trawl velocity, catch efficiency and wing spread) were unknown, they were defined through a stochastic procedure and calibrated by estimates produced by a Leslie depletion model applied to concurrent catches obtained in one fishing ground. A 21.7% removal rate was estimated for the period June 2004-January 2005; this increased to nearly 34% between February and July 2005. This removal scenario predicted that a five-month fishery would suffice to remo ve 90% of the biomass available in the Tijucas Bay, nearing the 87% CPUE reduction observed in the same period. Whereas abundance and harvest rate estimates were likely affected by inadequate knowledge of the swept-area equation parameters, the similarity of these estimates with relative abundance indexes supports the convenience of the proposed method and justifies future efforts to improve its accuracy.