'Seabird' is a general term used to describe collectively any bird species which spends a substantial part of its life foraging and breeding in the marine environment. Birds considered to be seabirds include penguins, albatrosses, petrels, shearwaters, cormorants, gannets, boobies, gulls and terns. Some species of pelicans, ducks and grebes also may forage at sea at times but are not generally considered to be seabirds
Here we mainly consider those seabirds that are known to be at risk of extinction through the influences of human activities, principally commercial fishing. The species most at risk from this threat are the albatrosses, petrels and shearwaters of the Order Procellariiformes. This Order is collectively known as ‘tubenoses' because all members possess tubular nostrils.
Biology, diet and breeding
Seabirds spend much of their lives roaming the oceans in search of prey. Most species tend to forage on their own, though large feeding flocks will gather at rich or passing food sources. Squid, fish and krill are common sources of food. Many species will follow fishing vessels, aggressively competing with each other for discarded fish and baits.
Many seabirds, such as albatrosses and petrels, have long life spans with some individuals known to live for more than 60 years. They have low rates of natural mortality. Albatrosses and petrels achieve sexual maturity at 5-12 years of age and breed in colonies on remote islands, with the pair bond being reinforced by elaborate courtship displays. Most species attempt to breed annually, although the larger albatrosses breed successfully only every second year (biennially).
Parental duties are shared by both sexes. Petrels and shearwaters may variously nest in simple scrapes on the ground, on cliff ledges, in a burrow or in a rocky crevice. Albatrosses (and giant petrels) breed on the surface and, where nest material is available, build large bowl-shaped nests of grass and mud. A single, large (relative to body size), white egg is laid which both parents incubate for 35-85 days in alternating shifts. After hatching, chicks are brooded for a short period until they are able to regulate their own body temperature. After this period the chick is generally left alone, with parents returning to provide food at intervals. Both parents feed the chick until it fledges at 110-304 days, depending on the species.
Seabirds occur widely across the world's oceans. Nineteen of the World's 22 albatross species occur in the Southern Hemisphere. Many species, such as Grey-headed Albatrosses, are extremely dispersive, spending most of their time over the surface waters of the high seas. In contrast, others, like adult Shy Albatrosses, tend to be more sedentary, regularly foraging over coastal waters throughout their adult lives.
Past threats to seabirds include being killed for meat, eggs and feathers. Modification of breeding habitats and introduced predators, such as cats, also impact considerably on populations. More recently, the global expansion of longline fisheries has posed one of the greatest overall threat to seabirds, although trawling and gillnetting also has led to the death of many albatrosses and petrels.
Longline fishing is a method used to target finfish and shark species. A longline consists of a main line with numerous baited hooks attached to branch lines. The line can be set relatively close to the sea surface (pelagic) or on the seabed (demersal). The number and type of hooks and the length of the branch lines depend on the target species. Pelagic longlines can be up to 100 km long and carry few thousand hooks. However, demersal longlines, although shorter in length, can hold more than 20 000 hooks. Each year thousands of seabirds are accidentally killed when birds ingest baited hooks during the setting or hauling of the line. Birds hooked are subsequently pulled under the water by the weight of the line and drown. The level of mortality that occurs in longline fisheries is not sustainable for many populations of seabirds.
However, this threat can be greatly minimised by modifying fishing practice and adopting mitigation measures. These include the use of bird-scaring lines, weighted lines to reduce the amount of time baits are available to birds at or near the surface, setting lines at night and seasonal closures of fisheries to avoid fishing when birds are more susceptible to being caught, such as around colonies during the breeding season. Adoption of these measures has now virtually eliminated seabird bycatch in some fisheries.
More recently, mortality in trawl fisheries has also been shown to be a serious problem for albatrosses and petrels, and mitigation measures are being adopted to reduce this threat. Seabird interactions with trawl vessels fall into two broad categories: those focused on the trawl warps (the thick cables that link the net to the vessel) with which birds collide when flying or which strike birds when they are feeding on the water behind a vessel, and those focused around trawl nets in which birds become entangled when trying to access fish in the net. For reducing seabird strikes on trawl warps, the use of bird-scaring lines has proven to be the most effective mitigation device. However, the retention or strategic management of fish waste (offal and discards), which discourages birds from attending fishing vessels, is recommended as the most effective primary measure for bycatch reduction in trawl fisheries.
Gillnets kill many of the smaller petrels and shearwaters by drowning birds when they become entangled. There have been few studies of mortality of seabirds by gillnet fisheries scarce, and development of mitigation measures for this gear type is still in its infancy. Mitigation measures have been tested in only a few gillnet fisheries, and this work needs to be expanded to other areas where interactions with seabirds occur.
Last updated 14 September 2015