Bycatch Species Groups

    Bycatch Reduction by Species Group

     

    The BMIS focuses on four bycatch groups in oceanic longline, purse-seine and gillnet tuna and billfish fisheries: seabirds, sea turtles, sharks and rays, and marine mammals. While sharks are treated as bycatch here, they may be targeted or treated as an associated catch in some tuna fisheries. The species group summaries, below, outline:

    • nature of bycatch interactions
    • techniques to reduce bycatch, by gear
    • links to species identification resources
    • links to biological & distribution data
    • links to safe handling advice.

    ISSF provide a useful overview of the major commercial tuna species here.

     

    More About Species Groups

    Marine Mammals

    Dolphins

    Cetaceans are the marine mammals most affected by tuna fisheries. They may be incidentally captured in purse seines or become entangled or hooked on longlines (e.g., depredation by 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 Turtles

    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, can reduce both bycatch and marine pollution. Utilisation of safe handling and release techniques successfully improves post-release survival, for all gear types.

    Seabirds

    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.

    Sharks and Rays

    Sharks

    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.