Seabird Bycatch

Bycatch of seabirds in longline fisheries occurs when birds attack baited hooks and become hooked and drown as the line sinks. In trawl fisheries, birds foraging on discards or offal may be injured or killed when they collide with net-monitoring (‘third wire’) cables and trawl warps (cables used to tow the net). The birds are then dragged underwater and drown when their wings become entangled around the warp, or when they become entangled in the nets. Bycatch in gillnet fisheries is due mostly to entanglement while diving for prey, and is a global threat to penguins and other species. The volume and reliability of bycatch information are severely limited for many areas and fisheries across the world, particularly artisanal fisheries and those using gillnets. However, it is clear that the scale of bycatch is huge, with hundreds of thousands of seabirds killed globally each year in longline, trawl and gillnet fisheries.

​

Assessing conservation implications on species’ populations requires quantitative estimates of bycatch rates or risks for each species in different fisheries based on the spatio-temporal overlap between fisheries’ effort and bird distributions, in addition to contemporary data on the size and trends of affected populations. Analyses need to consider not only bycatch by multiple fleets across ocean basins, but the impacts relative to other threats. It is not necessarily the most frequently captured species that suffer the most severe population-level consequences. For example, the Amsterdam Albatross Diomedea amsterdamensis has a small and increasing global population, but models show that bycatch of only six individuals per year would eventually drive the species to extinction. Impacts of bycatch can also vary regionally, and be biased towards males or females, potentially reflecting differential access to bait mediated by sexual size dimorphism, or sex-specific differences in foraging distributions. This exacerbates the impact on breeding numbers by reducing the effective population sizes and future breeding success.

​

Bycatch in longlining and trawl fisheries can be mitigated effectively with the implementation of operational and technical measures. Depending on the characteristics of the fishery, location, season and associated at-risk seabird species single measures can be effective, such as discard and offal management or bird scaring lines on trawl vessels and hook-shielding devices in pelagic longline vessels. However, measures used in combination are most effective, such as night setting, bird-scaring lines and weighted branch lines for longline vessels, and bird scaring lines and offal management for trawl vessels. Mitigation measures are less developed for gillnet fisheries and development of appropriate methods for this gear type poses a significant conservation challenge.

​

Wider and more effective implementation and, for some gear types, further development of best-practice mitigation measures in national (particularly gillnet, trawl and artisanal) and international (particularly pelagic longline) fisheries, together with much better information on bycatch rates and levels of compliance remain important future actions.