Catch rates and size composition of blue sharks (Prionace glauca) caught by the Brazilian pelagic longline fleet in the southwestern Atlantic Ocean

Abstract

Distribution and relative abundance of blue sharks ( Prionace glauca ) in the southwestern Atlantic Ocean was modeled based on catch-per-unit-effort (CPUE) per 1000 hooks and length frequencies of blue sharks caught by the Brazilian pelagic tuna longline fleet. As a measure of relative abundance, CPUE of blue sharks caught in 58 238 fishing sets by the Brazilian pelagic tuna longline fleet (national and chartered), from 1978 to 2009, was standardized by a Generalized Linear Model (GLM) using three different approaches: i) a negative binomial error structure (log link); ii) the traditional delta lognormal model; and iii) the Tweedie distribution, recently proposed to adjust models with a high proportion of zeros. A cluster analysis using the K-means method was used to identify target species and incorporate it as a factor into the GLM. Cluster analysis grouped the data into six different fishing clusters according to the percentage of target species. Target factor (cluster) was the most important factor explaining the variance in all three CPUE models. The Tweedie model showed a relatively better fit compared to the other models. Blue shark nominal and standardized CPUE showed a relatively stable trend from 1978 to 1995. From 1995 onwards, however, there was an increasing trend in the standardized CPUE, up to a maximum value in 2008. In general, nominal CPUE and standardized CPUE tracked well up until 2000, after which standardized CPUE’s values were at a noticeably lower level than nominal CPUE. Length frequency data were analyzed for 11 932 blue sharks measured as part of the Brazilian onboard observer program operating on the pelagic tuna longline fleet between 2006 and 2008, with sizes ranging from 91 to 224 cm fork length. Overall, blue shark size data showed clear spatial and seasonal distributions for males and females in the southwestern Atlantic Ocean, with juveniles predominantly concentrated in the most southerly latitudes.