Bycatch Species Groups

    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 and purse-seine 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 five major commercial tuna species here.

    More About Species Groups

    Marine Mammals

    Marine mammals may be accidentally captured in purse seines. They may also become entangled in longlines or hooked on them, e.g., false killer whales in the Hawaii-based deep-set longline fishery. Successful mitigation techniques include the 'backdown procedure' and 'Medina panel' (PS), 'weak hooks' (LL), safe handling and release methods, and spatial & temporal management.

    Sea Turtles

    Sea turtle bycatch occurs in several ways, through hooking or entanglement on longlines, entanglement on FADs, and they are sometimes netted in in purse seines. Successful bycatch mitigation techniques include replacing squid with fish bait, using circle hooks, dynamic spatial and temporal measures (e.g. TurtleWatch), and improved FAD design.


    In pelagic longline fisheries, seabirds may become hooked and drown when they take a baited hook. Mitigation of seabird bycatch aims to deter and prevent birds from taking bait, principally through the combined use of multiple techniques, including streamer (tori) lines, fast sinking, weighted branch lines, night setting and, most recently, hook shielding devices (i.e., HookPod).

    Sharks and Rays

    Most shark and ray bycatch is due to hooking on longlines, but significant bycatch is attributed to capture in purse seine nets (mostly when fishing on FADs) and also to entanglement in netting on FADs. Research into mitigation of shark bycatch is looking at a range of options (for longlines, read Clarke et al 2017). Among these are: the effect of circle hooks on shark catch rates; ‘Eco' or 'Bio' FADs to reduce entanglement and marine debris; electrical and magnetic hook deterrents; selectivity of artificial bait; and ways to lure or release sharks from seine nets. Safe handling and release procedures have been developed which significantly reduce post-release mortality of sharks.