Blue sharks (Prionace glauca) by-catch in the Indonesian industrial tuna longline fishery in the eastern Indian Ocean

Abstract

The Indonesian tuna longline fisheries in Indonesia have been targetting four main tuna, including yellowfin tuna, bigeye tuna, albacore, and southern bluefin tuna. In reality, there are many by-catch species, including blue sharks. This study aims to provide integrated information on blue shark species as by-catch species in Indonesian industrial tuna longline fisheries, including nominal and standardized CPUE and biomass of blue shark exploited by Indonesian longliners in the eastern Indian Ocean. This research is expected to become a reference point in shark management in the Indian Ocean. The scientific observer collected 2951 set-by-set longline fishing data based on Research Institute For Tuna Fisheries (RITF) from January 2006 to December 2018, on which the present analysis was made. The result indicated that there were eighteen shark species caught by Indonesian longliner were dominated by a blue shark (Prionace glauca) (60%) followed by crocodile shark (Pseudcarcharias kamoharai) (23.84%), bigeye thresher shark (Alopias superciliosus) (3%) and other species (<3%). Nominal CPUE was fluctuating and needed to be standardized in GLM analysis. The results of CPUE standardization can then be used as a reference in calculating the biomass blue sharks exploited by Indonesian Longliners. By considering several aspects, including the average of hook/set/fleet, an average of size (length and weight), the total number of set/year, reported the number of vessels, it is obtained that the estimated catch of blue sharks ranges from 366.3-2,616 tons per year with an average of about 1,282 tons.