Population structure, distribution and relative abundance of the blue shark (Prionace glauca) in the Caribbean Sea and adjacent waters of the North Atlantic

Abstract

Catch and fishing effort data from the Venezuelan Pelagic Longline Observer Program (VPLOP) in the Caribbean Sea and adjacent waters of the Atlantic were used to investigate the population structure, distribution and relative abundance of the blue shark, Prionace glauca. A total of 959 blue sharks were sampled by trained observers during the period of January 1994 to December 2007. Fork length (FL) in males ranged from 79 to 355cm and from 97 to 349cm in females. Sexual segregation at the spatial and temporal scale was observed, males dominated January–June and during October–December across all areas; while females dominated during July–September with a higher magnitude in the Atlantic side (Guyana-Amazon area). Sex ratio for mature (>180cm FL) blue shark favored females in the Caribbean Sea from July to December, and in the Guyana-Amazon area in July–September; while immature blue sharks were dominated by males across all areas. Thirty-four pregnant females were caught in the Caribbean Sea and three in the Guyana-Amazon area. Pregnant females displayed a seasonal trend which increased from September through January, where >50% of the pregnant females occurred during November–January; of the pregnant females caught in the Caribbean Sea two had embryos close to the size-at-birth [>35cm total length (TL)]. The spatial distribution of blue sharks in the Caribbean Sea was marked by high concentrations around the offshore islands of Venezuela and the Netherland Antilles year round; while in the Guyana-Amazon area the high concentrations were more seasonally localized off northeastern coast of South America but diminished progressively and by the end of the year was absent in the area. Relative abundance of blue shark was estimated from the Venezuelan pelagic longline (VPL) fleet using a General Linear Model Approach with a delta lognormal model distribution to account for the high proportion of zero observations. The variability within vessel groups was partitioned into each vessel variance and the auto-correlation within a vessel was evaluated by using a variance–covariance matrix model. Diagnostic checks and deviance table analysis were performed to identify the set of model factors and interactions that adequately explained the observed variability. The results indicated that variability of catch rates within vessels is as large as the variability among vessel groups, where smaller and medium size vessels show higher catch rates of blue sharks in the VPL fleet. Standardized catch rates indicated in general a lower relative abundance in the most recent years followed by a slight recovery that requires caution due to the predicted high variance. Fishing activities can have uneven effects in the blue shark population due to its spatial and temporal segregation in the study area; potential changes in the fishing operations of the VPL fleet can have negative effects in the population structure of blue sharks in the Caribbean Sea and adjacent waters if habitat for blue sharks are increasingly used to target other valuable species (e.g., large tunas and swordfish).