The Incidental Catch of Seabirds in Gillnet Fisheries: A Global Review
Bringing together a wide range of data, both published and unpublished, this paper takes a look at incidental catch (or “bycatch”) of seabirds in gillnets, from a global perspective. Though the bycatch of seabirds has been studied around the world regionally, there has been virtually no literature reflecting the global picture. In addition to giving potential estimates of the total amount of seabird bycatch annually, the authors also make suggestions on how those unnecessary deaths can be mitigated.
Getting a global perspective on any animal issue is a difficult task, and the problem of seabird bycatch, where seabirds are caught unintentionally in gillnets that are used to catch fish, is no exception. Study authors Zydelis, Small, and French note the magnitude of the issue: “the problem of seabird bycatch in gillnet fisheries has long been known in the Pacific, Atlantic oceans and Baltic Sea, and gillnets have been the cause of some of the highest recorded mortalities of seabirds worldwide. In the North Pacific, drifting gillnets were estimated to be killing c. 500,000 birds per year, prior to a UN moratorium in 1992.” This is indeed a staggering number, but despite the scale of things, the authors note that “surprisingly, the global magnitude and significance […] remains largely unknown.” Because of the “large and diverse” amount of small-scale fishing taking place with gillnets, as well as a lack of data from larger fisheries, there is a dearth of global numbers, inspiring the authors to undertake this study. Their objectives at the outset were to “identify seabird species susceptible to and impacted by gillnet fishing; summarise seabird bycatch in gillnet fisheries globally by region and identify likely data gaps; assess factors determining bird captures in gillnets; review bycatch mitigation measures in use or under development; and identify areas where conservation actions are most needed.”
Why are these nets such a problem for seabirds in the first place? Gillnets are a type of vertical net that hangs downwards in the water column, attached to weights on one end and a boat on the other. Depending on the size of the mesh used, they more or less act as a type of wall in the ocean that blocks the pathway of larger organisms, simultaneously creating a risk of entanglement for “non-target species” (i.e. seabirds). In the past, gillnets would have been made from “hemp, cotton, or multi-filament nylon,” materials which were “highly visible” to seabirds. However, with the increasing popularity of monofilament line (transparent fishing line that is cheaper and more durable), the potential for bycatch has also increased.
Zydelis, Small, and French faced numerous challenges in bringing together global data, which they characterize as “highly fragmented,” and note that “even from regions where numerous reports are available, e.g. the Baltic Sea, information often originates from short-term studies and opportunistic observations.” All in all they reviewed 343 world seabird species, identifying 148 species that are potentially susceptible due to diving / foraging behavior, and of those found 81 species that have been recorded as caught in nets. Their estimates are quite staggering. “By reviewing available data on seabird bycatch in gillnet fisheries,” they say “we derived an annual minimum mortality estimate of 400,000 birds.” Keeping in mind that these numbers are gathered with a noted lack of comprehensive data, the key word in their estimate seems to be “minimum.” The actual numbers could be much higher.
Perhaps unsurprisingly, the authors urge more research into bycatch globally, saying bluntly that “better knowledge is needed from every region where seabird bycatch is known or could be anticipated.” They add that “several regions can be identified as being especially information deficient” including areas around Japan and Korea in the Northwest Pacific. They offer two potential solutions in the form of “spatial and temporal regulation of fishing effort,” as well as “increasing visibility of nets.” Though both of those solutions could indeed mitigate some of the staggering numbers, it is hard to know just how much they would help when the problem is so large in scope and the data is already likely to be grossly underestimated.
Based on bird feeding ecology we identified 148 seabird species as susceptible to bycatch in gillnets, of which 81 have been recorded caught. The highest densities of susceptible species occur in temperate and sub-polar regions of both hemispheres, with lower densities in tropical regions. Gillnet fisheries are widespread and particularly prevalent in coastal areas. A review of reported bycatch estimates suggests that at least 400,000 birds die in gillnets each year. The highest bycatch has been reported in the Northwest Pacific, Iceland and the Baltic Sea. Species suffering potentially significant impacts of gillnet mortality include common guillemot (Uria aalge), thick-billed guillemot (Uria lomvia), red-throated loon (Gavia stellata), Humboldt penguin (Spheniscus humboldti), Magellanic penguin (Spheniscus magellanicus), yellow-eyed penguin (Megadyptes antipodes), little penguin (Eudyptula minor), greater scaup (Aythya marila) and long-tailed duck (Clangula hyemalis). Although reports of seabird bycatch in gillnets are relatively numerous, the magnitude of this phenomenon is poorly known for all regions. Further, population modelling to assess effects of gillnet bycatch mortality on seabird populations has rarely been feasible and there is a need for further data to advance development of bycatch mitigation measures.