Seabirds, sea turtles, sharks and rays, and marine mammals are the four species groups that are the focus of the BMIS. These species are incidentally caught in oceanic longline, purse-seine and gillnet tuna and billfish fisheries. Sharks are always treated as bycatch for the purposes of the BMIS, although they may be targeted or treated as an associated catch in some tuna fisheries.
ISSF provide a useful overview of the major commercial tuna species here.
Cetaceans are the principal marine mammals affected by tuna fisheries. They may be incidentally captured in purse seines or become entangled or hooked on longlines (e.g., false killer whales in the Hawaii-based deep-set longline fishery). Entanglement is a significant problem in medium-scale, semi-industrial and artisanal tuna drift gillnet fisheries. Successful mitigation techniques include the 'backdown procedure' and 'Medina panel' (purse seine), 'weak hooks' (longline), safe handling and release methods, spatial & temporal measures, and, to a limited extent, sub-surface gillnets and pingers (acoustic alarms).
Sea turtle bycatch in tuna fisheries occurs in several ways, through hooking on longlines, entanglement in longlines, gillnets and purse seine FAD netting (both above and below the waterline), and they are sometimes encircled in purse seine nets. Successful bycatch mitigation techniques for longlines include replacing squid with fish bait, using circle hooks with a moderate (<100) offset, deep setting, reducing daylight soak duration, limiting retrieval during daylight hours, and dynamic spatial and temporal measures (e.g. TurtleWatch). For gillnets, net illumination is effective. Improved purse seine FAD designs, i.e., biodegradable and non-entangling, are being tested for their potential to reduce bycatch. Utilisation of safe handling and release techniques successfully improves post-release survival, for all gear types.
In pelagic longline tuna fisheries, seabirds may become hooked and drown when they take a baited hook, either during line setting or hauling. Mitigation of seabird bycatch aims to deter and prevent birds from taking bait, principally through the combined use of three techniques, including streamer (tori) lines, fast sinking, weighted branch lines and night setting. Alternatively, ACAP recommend the use of an assessed* hook shielding device (e.g., HookPod) or underwater bait setting device. Seabird bycatch in purse seine tuna fisheries is rare, while poorly described for gillnet tuna fisheries.
Incidental shark and ray capture occurs in longline (hooking), purse seine (captured in nets or entangled in the netting beneath FADs) and gillnet (entanglement) tuna fisheries. The most effective mitigation measures for longline fisheries include deep-setting, reduced soak time, avoiding wire leaders and hook and bait changes [2], while for purse seine fisheries the most promising include limiting FAD use and modifying FAD designs and practices [2]. There has been some success with sub-surface gillnets [3] but further validation of this technique is required. For all gear types, safe handling and release procedures have been developed which significantly reduce post-release mortality of sharks.
