Reproductive activity of the tropical arrow squid Doryteuthis plei around São Sebastião Island (SE Brazil) based on a 10-year fisheries monitoring

Monthly samples of Doryteuthis plei were obtained from the small-scale hand-jigging fishery around São Sebastião Island (24° S), Brazil, across seven fishing seasons (November–April) during the period 2002–2012, with the aim of identifying the exploited population patterns with emphasis on the reproductive activity. In order to explore the process of development of reproductive organs, the size-at-maturity, and the spatial–temporal factors explaining maturation, an analysis of the sex-ratio, maturity, gonado-somatic index (GSI), concentration of spermatophores, biometric relationships, and two GAMLSS (Generalized Additive Model for Location Scale and Shape) was applied. D. plei recruited to local fisheries at around 161mm mantle length (ML), males being, on average, 1.6 times larger than females. Females appeared more frequently during the first three months of the fishing season while males tended to be more frequent at the end of the season, confirming the existence of female post-spawning mortality. The presence of high proportions of mature squid was found throughout the fishing seasons for both sexes. The size-at-maturity was estimated at 143.3mm ML in females, and 210mm ML, in males. Both statistical models indicated robust non-linear correlation between gonad weight, biometric variables, and spatial–temporal factors (p<0.001). Ovary weight mostly correlated with accessory glands weight, while testis weight correlated with ML. In both sexes, depth was found to be a significant variable explaining maturation, with higher gonads weight between 15m and 20m, and especially in the island's South and Southeastern areas. Statistical models also evidenced a significant peak of gonad weight in February and March, for females, and in March, for males. Inter-annual variation showed a peak of gonad weight in the fishing season 2011–2012. Overall, findings based on long-term monitoring should assist resource management (e.g. for seasonal, size, and spawning area protection) which seems potentially valuable for long-term sustainability of local fishing communities.