Size and season matters: diel vertical movement of blue sharks in the southern Adriatic Sea

Abstract

Satellite tracking technology has been used to gain information on blue sharks (Prionace glauca, L. 1758) across the major oceans but is lacking in the Mediterranean Sea. Here, blue sharks are ranked as critically endangered due to a high impact of fishing. Blue sharks are often bycaught in longline fisheries. In this study, pop-up archival tags optimized to transmit short-term data for survivorship studies (sPAT, Wildlife Computers) were used to elucidate the vertical movement and provide management strategies for blue sharks bycaught in the swordfish longline fishery in the southern Adriatic Sea. Of 20 tagged individuals, archival data were subsequently obtained from eleven (55 %). Although some individuals were restricted by the bathymetry, two patterns of vertical diel movement were revealed: 1) small and medium sized blue sharks (128- 158 cm TL) display the most regular diel vertical movements, larger blue sharks (189- 206 cm TL) the least regular, more spike-like; 2) most regular diel vertical movements occur during September. Thus, deploying surface longlines in September during daylight hours may reduce blue shark bycatch as well as introducing intervals of reduced surface longline fishing or using mid-water longlines in October.