Demography and ecology of blue shark (Prionace glauca) in the central North Atlantic

In the North Atlantic, blue shark has become an important by-catch species for the pelagic swordfish fisheries, and even the target species of these fisheries when swordfish abundance is low. Still, their complex life cycle and population structure remain poorly understood, limiting current management and conservation efforts for this species. This study provides information on the population structure and seasonal abundance of blue shark around the Azores Archipelago, in the central North Atlantic, based on detailed analyses of fishery independent and observer data for the area. A total of 23,119 blue sharks were sampled during 388 research longline fishing sets conducted between 1993 and 2004. Standardised catch rates varied greatly over the year, reflecting the changing presence of different sex and life stages in the area, and were strongly influenced by environmental factors, namely sea surface temperature, sea surface height anomalies and lunar cycle. In general, the catches were strongly dominated by juveniles, with higher catch rates during winter months. Young-of-the-year (YOY) juveniles of both sexes were present throughout the year. While older juvenile and sub-adult females dominated during winter, juvenile males dominated during the rest of the year, indicating that different juvenile life stages of both sexes alternate in using the area. The presence of mature females in advanced stages of pregnancy during spring, and the appearance of the smallest size-classes of YOY in early summer also suggest that the area is used as a pupping ground. Mature males appear mainly during summer, probably for feeding. The periodic presence of all these life stages emphasises the central role of the area, as some of these population components are seasonally associated with one of either sides of the North Atlantic. (C) 2014 Elsevier B.V. All rights reserved.