The Use of Zinc and Graphite to Reduce Shark Bycatch in Canadian Pelagic Longline Fisheries

Blue sharks (Prionace glauca) represent a significant portion of the catch in the Canadian pelagic longline fishery targeting swordfish and tunas. The capture of sharks represents a significant financial loss to industry, therefore, reducing the capture of sharks is a shared interest between conservationists and fishermen. Sharks have the ability to detect minute electric currents and thus, the use of substances that produce electrical currents upon immersion in salt water are being tested to determine their ability to deter sharks from fishing gear. The combination of zinc and graphite (Zn/Gr) has been tested in experimental trials and has shown to deter sharks. We tested the use of these substances to reduce shark bycatch in a commercial fishery in collaboration with a longline captain off Nova Scotia, Canada.  In September 2013, we deployed a total of 9 longline sets (~6300 hooks) with three different treatments: standard hooks, hooks with Zn/Gr and hooks with plastic controls. Sharks, primarily blue sharks, represented 29.6% of the total individuals caught while swordfish (Xiphias gladius), comprised 26.2%. The combination of zinc and graphite did not significantly reduce the bycatch of sharks in this fishery nor did it significantly affect the catch of the target species.