Set nets of various types are a particular hazard to diving seabirds and have been implicated in the decline of seabird populations in some parts of the world ^7,8,9,17,29. In northern Norway, for example, the breeding populations of guillemots at two sites are estimated to have declined by 95% from the early 1960's to 1989, a figure which could be explained entirely on gill net mortalities based on observed catch rates. The numbers of birds killed in nets depends on their abundance, diving habits and distribution within the fishery area⁷. Species which have been caught in these nets include shearwaters, red-throated divers, Leach's petrel, gannet, shag, guillemot, razorbill, and great northern diver. Ducks such as the common scoter and long-tailed duck are also known to have become entangled and die in set nets⁷.

Inshore gill nets can have a relatively high incidental by-catch around diving seabird colonies or where there are high densities gathered on the water surface, making it inadvisable to set nets in such areas. Large numbers of razorbills are known to have drowned in gill nets at the mouth of the Tagus estuary in Portugal, for example, where this species congregates on occasions⁴³. Nets set for bass have caught large numbers of diving birds (mostly razorbills and divers) and in one incident in the UK an estimated 900 auks were caught over 8 days in nets set below seabird colonies¹⁷. Herring nets and bottom-set cod nets have also killed large numbers of diving seabirds (an estimated 25,000 in the southeast Kattegat between 1982 and 1988), most of which were found in the bottom-set cod nets⁴⁵, and catches of shags in trammel nets may be a threat to populations of this species in Spain⁴³. The threat will depend on which species are present at the time nets are put out, weather, tidal fluctuations and fishing effort. Gill and tangle net fisheries in Cardigan Bay, for example, often occur at or near the cormorant colony but to date there has been no major entanglement problem¹⁰.

High incidental catches of guillemots, razorbills and divers have been reported in drift nets from Danish fisheries, and significant catches of auks in the salmon driftnet fisheries in Ireland and Denmark⁴³. Anti-predator nets around aquaculture facilities are also known to entangle seabirds^59,82. Ghost fishing by lost nets and fragments of nets is also known to entangle birds but the scale of mortality associated with this is unknown⁴⁵. Similarly, the effect of non-net fisheries, such as long lining and pots, and in mobile nets is not well known in UK waters although catches are reported from elsewhere.

The direct and indirect effects of molluscan shellfisheries and aquaculture on birds are mentioned elsewhere.

An indirect effect of some finfish fisheries has been an increased food source for some seabirds resulting from the discarding of by-catch and offal. The discards are taken by species such as fulmar, gannet, great skua, common gull, great black-backed gull and herring gull and appear to have contributed to the rapid growth of some seabird populations. It is now considered to be such an important component of the diet of scavenging seabirds in the North Sea that changes in the amount of discards may affect the relative and absolute abundance of various species. Using fisheries data from the late 1980's and early 1990's, the number of seabirds potentially supported by the fishery waste from North Sea fisheries has been estimated to be around 5.9 million and an area based analysis suggests that discards may easily support all scavenging seabirds in southern and southeastern sub-regions of the North Sea⁵⁵.

Summary of potential effects of fishing on sea birds listed in the Birds

References