Distribution patterns of sizes and sex-ratios of blue shark in the Indian Ocean

The blue shark is the most captured shark in pelagic longline fisheries targeting tunas and swordfish. As part of an ongoing cooperative research between several institutes and national scientists, information on blue shark catch-at-size was collected, compiled and analyzed for the Indian Ocean. This included information from fishery observers, logbooks, scientific projects and scientific surveys from several fishing nations, specifically EU.Portugal, EU.France, Japan, Taiwan, South Africa and the USSR (data from historical surveys). Datasets included information on catch location and date, and specimen size and sex. A total of 77,396 blue shark records collected between 1966 and 2014 were compiled, with the sizes ranging from 41 to 369 cm FL (fork length). Considerable variability was observed in the size distribution by region and season, with larger sizes tending to occur in equatorial and tropical regions and smaller sizes in southern latitudes in more temperate waters. Some fleets/surveys showed bimodal size distributions, which may be related with the fact that those fleets/surveys operate in several locations throughout the Indian Ocean. Differences in the sex ratios, both spatially and seasonally, were also detected. The distributional patterns presented in this study provide a better understanding of different aspects of the blue shark distribution patterns in the Indian Ocean that can help to promote more informed management and conservation measures.