Habitat modeling for the blue shark (Prionace glauca) by sex and size classes in the Indian Ocean
This paper is a regional focus in the Indian Ocean (IO) of a global analysis of blue shark (Prionace glauca) habitat by size and sex classes (small juveniles, large juvenile males and females, adult males and females, Druon et al., in prep.). The habitat modeling, calibrated using fishing interaction data (i.e., fishery observer data) and electronic tracking data, uses two feeding proxies, i.e., the satellite-derived productivity fronts in mesotrophic areas and the mesopelagic micronekton in oligotrophic areas, and two abiotic variables, i.e., temperature and sea surface height anomaly. The temperature niche includes sea surface temperature (SST) and temperature 100 m below the mixed layer depth (Tmld+100) to ensure that both the horizontal and vertical extent of this thermoregulated species‘ habitat are covered. Here we show that the overall feeding niche displays highly diverse biotic and abiotic conditions although the blue shark population tends to progress from mesotrophic and relatively cold surface waters for the juvenile stages (North and South of IO) to more oligotrophic and warm surface waters for the adults (central IO). However, warm temperatures or low productivity limit the habitat of mostly the juveniles in the Central and/or North IO mainly in Apr-Jun and Jul-Sep. Large females tend to have more habitat overlap with small juveniles than large males, notably driven by temperature preferences. Large females also display an intermediate range of SST avoidance resulting in an important lack of habitat overlap with large males mostly in Jan-Mar and Apr-Jun in the South IO around 30°S. In Oct-Dec however, fisheries observer data show a higher habitat overlap between large males and females in this intermediate SST range, which may correspond to mating. These results on blue shark habitat provide key elements useful to stock assessment models and potential leads for conservation and management measures of this near-threatened species.