Mitigation of Silky Shark Bycatch in Tropical Tuna Purse Seine Fisheries

ISSF (2016) Mitigation of Silky Shark Bycatch in Tropical Tuna Purse Seine Fisheries. In: IOTC - 12th Working Party on Ecosystems and Bycatch. IOTC-2016-WPEB12-39, Seychelles

Pelagic sharks are not targeted by tropical tuna purse seine fisheries, but they are caught
incidentally, especially around floating objects like FADs. The shark bycatch-to-tuna catch
ratio in purse seine fisheries is quite small, on average, less than 0.5% in weight. Over 90% of
that bycatch is composed of silky sharks, Carcharhinus falciformis. Because of their low
reproductive rates and other life history characteristics, silky sharks are a vulnerable species.
Other gear types such as longlines or gillnets have a larger impact on silky sharks than purse
seine fisheries do. The contribution of purse seining to the total catch of this species varies by
Ocean: From 4% in the Indian and eastern Pacific Oceans, to about 25% in the western and
Central Pacific Oceani.
Within the purse seine fishery, all set types catch silky sharks, with the highest catch rates
being on natural logs (which represent a relatively small fraction of the total number of sets)
followed by man-made FADs. Catches on floating object sets (both natural and man-made)
tend to be 2 to 6 times higher than they are on free swimming schools.
The global magnitude of catch of the purse seine fishery is quite large, so reducing the
mortality caused by these fisheries can contribute towards global conservation efforts. This
document summarizes mitigation techniques that can be used in this fishery.