Integrated weight longlines with paired streamer lines - Best practice to prevent seabird bycatch in demersal longline fisheries

To evaluate a new technology - integrated weight longlines (IW) - as a viable seabird mitigation technology for demersal longline fisheries, we compared three experimental mitigation treatments, IW line alone, IW with paired streamer lines (IWPS) and unweighted longlines (UW) with PS (UWPS), to a control of no deterrent (UW alone). Trials took place on two vessels targeting Pacific cod (Gadus macrocephalus) over a five-month period in the Bering Sea, Alaska, USA. We used multiple criteria for evaluations - catch rates of all taxa, seabird abundance and attack rate, and gear sink rate and performance - making this study the largest and most comprehensive experiment of its kind.All mitigation technologies dramatically decreased seabird bycatch rates while having little to no effect on fish catch rates. Mitigation was more effective for surface foraging seabirds (Fulmarus glacialis and Larus spp.) than for diving seabirds (short-tailed shearwaters, Puffinus tenuirostris), reducing mortality rates by 91-100% and 80-97%, respectively. Based on multiple criteria, IWPS performed best, sinking gear within the protection of streamer lines eliminating surface forager catch completely and reducing shearwater catch by 97%, relative to the control. UWPS, status quo in the Alaska fishery, and IW alone performed similarly reducing surface forager catch rates by 98% and 91%, respectively, and shearwater rates by 87% and 80%, respectively. Seabird abundance and attack rate were poor proxies of seabird mortality, especially for IW gear. IW lines reduced the distance astern that birds have access to sinking baits by near half and its handling qualities proved superior to UW. We conclude that IW longlines and paired streamer lines are the core mitigation techniques and when deployed together, constitute the best management practice for seabird conservation in demersal longline fisheries using autoline systems.