Gillnets are responsible for the accidental entanglement of large numbers of some seabird and seaduck species. Despite a ban on their use in the high seas, commercial gillnet fisheries continue to operate in territorial and coastal waters around the world.
Gillnets—static curtains of netting designed to entangle fish by their gills—are typically used to target large species, such as salmon, tuna and cod. The nets are suspended vertically in the water column, usually across known migration routes or adjacent to target species refuges (Jennings et al. 2001). In coastal waters nets are anchored in place; on the high seas, however, drifting gillnets (or driftnets) are favoured. Although selective in terms of the size-class of the target species, gillnets are responsible for the incidental capture of large numbers of birds, sharks and marine mammals (e.g., Hall 1998, Tasker et al. 2000, Johnson et al. 2005, Rogan and Mackey 2007). Amongst birds, pursuit-diving species, such as divers (loons), grebes, seaducks, auks and cormorants, are the most vulnerable to entanglement (Piatt and Nettleship 1987, Žydelis et al. 2009). While many data gaps remain, hampering assessment, a review of existing data has estimated that at least 400,000 birds may be being killed each year (Zydelis et al. 2013).
Commercial gillnetting expanded rapidly during the 1960s following the development of nets made from synthetic material. Man-made fibres, such as monofilament nylon, are cheaper, stronger and, most importantly, virtually invisible once in water. The new nets resulted in improved catches, but also caused greater mortality of non-target species, especially seabirds. For example, the introduction of synthetic nets in the Iberian Atlantic quickly propelled Common Guillemot Uria aalge towards extirpation as a locally breeding species (Munilla et al. 2007). Globally, high seas gillnet fisheries have had a substantial impact on numerous seabird populations (e.g. Tasker et al. 2000, Uhlmann et al. 2005). At their height, the driftnet fisheries of the North Pacific were resulting in the deaths of 500,000 seabirds every year (Northridge 1991).
Since 1991, the use of gillnets has been prohibited within international waters (U.N. Resolution 46/215). Gillnet fisheries are, however, still permitted to operate within 200 nautical miles (370 km) of the coast—within a State’s Exclusive Economic Zone (EEZ). For example, a large salmon driftnet fishery continues to operate in the Russian Federation’s EEZ. The Russian fleet accounts for about half of the total catch, with the remainder taken by Japanese vessels. The seabird mortality associated with the fishery is considerable. Between 1993 and 1999 about 482,500 seabirds, predominately Procellariids and Alcids, perished in nets set by Japanese boats alone (Spiridonov and Nikolaeva 2004).
In Europe, coastal gillnet fisheries are widespread in the Baltic Sea and North Sea. Žydelis et al. (2009) reviewed 30 studies on seabird bycatch in order to calculate the scale of gillnet mortality within the region. They estimate a cumulative bycatch of at least 90,000 seabirds annually, but suggest the real death toll could be as high as 200,000 birds per year. The study found Long-tailed Duck Clangula hyemalis to be the most frequently entangled species—with tens of thousands likely to die in Europe’s gillnets each year.
It is imperative that mitigation methods are introduced that prevent or reduce seabird bycatch (Melvin et al. 2001, Bull 2007). Mitigation strategies, such as better targeted fishing effort and the use of more visible nets, need not seriously reduce catches. Melvin et al. (1999) trialled these measures at a salmon fishery in the Puget Sound, USA and found that seabird bycatch, primarily of Common Guillemot and Rhinoceros Auklet Cerorhinca monocerata, could be reduced by up to 75% without significantly impeding the efficiency of the fishery. However, as yet, there is not a suite of effective bycatch mitigation measures that can be effectively advocated for gillnets as there is for longline fisheries. Significant research is required to develop technical modifications to gillnets that reduce bycatch and maintain fishermen’s catches, an area where BirdLife’s Global Seabird Programme is now beginning to focus significant effort. In the meantime, closures of commercial driftnet fisheries where bycatch is an acute problem may be necessary.
Related Case Studies in other sections
Compiled 2008, updated 2010, 2013
BirdLife International (2013) Gillnets are catching significant numbers of some seabird populations. Presented as part of the BirdLife State of the world's birds website. Available from: http://birdlife.org/datazone/sowb/casestudy/215. Checked: 27/04/2015