The threats to seabirds were; being killed for meat, eggs and feathers. Today, modification of breeding habitats, oil spills and introduced feral animals are among the threats that can impact considerably on seabird populations. More recently, the global expansion of longline fisheries has begun to pose the greatest overall threat to seabirds.
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 branchlines. The line can be set parallel to the surface or on the sea-bed. The number and type of hooks and the length of the branchlines, depend on the target species. Longlines can be up to 100 km long and have up to 10.000 hooks. Each year, thousands of seabirds are accidentally killed on longline hooks when birds ingest baited hooks during the setting or hauling of the longline. 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 seabird by-catch mitigation measures. These include the use of bird-scaring lines and streamers, weighted lines to reduce the amount of time baits are available to birds, setting lines at night, setting lines beneath the waters surface, and seasonal closures of fisheries to avoid fishing when birds are more susceptible to being caught, such as around nesting colonies during the breeding season. Adoption of these measures has now virtually eliminated seabird by-catch in some fisheries.