Bycatch of hammerhead sharks caught by the French pelagic longline (2007-2021) and purse seine (2005-2021) fisheries in the Indian Ocean
Hammerhead sharks (Sphyrnidae family) are sensitive species present in the Indian Ocean that are classified as globally “Critically Endangered” for the great hammerhead (Sphyrna mokarran – SPK) and scalloped hammerhead (Sphyrna lewini – SPL), and “Vulnerable” for the smooth hammerhead (Sphyrna zygaena SPZ) by the IUCN. Hammerhead sharks are occasionally bycaught by the French tuna purse seine fishery and swordfish-targeting longline fishery operating in the western Indian Ocean. Using data collected through onboard observation programs on purse seiners (2005-2021) and longliners (2007-2021), we present here an overview of available data related to hammerhead sharks: distribution, abundance indicators, status at capture, discard rate, status at release, and size distribution. We found that hammerhead sharks occur very rarely in purse seine bycatch. On the other hand, hammerhead sharks occur in 5% of the longline sets and nominal CPUEs are relatively low: below 0.077 and 0.233 individuals per 1000 hooks for SPL and SPZ respectively. The respective nominal CPUE may indicate a decreasing trend for both species. SPK is never caught in neither of the two fisheries and SPL is 3 times more frequent than SPZ in longline bycatch. SPL’s survival at haulback (70%) is on average higher than SPZ’s (30%) when caught by longliners. Hammerhead sharks are almost always released (97%) and the percentage of individuals released alive is highly variable and tend to have decreased throughout the period for both SPL and SPZ. The mean sizes caught by longliners are 171 and 160 cm FL for SPL and SPZ respectively.