Research being presented at the World Conference on Marine Biodiversity in Aberdeen today will report this global estimate of the impact of the commercial fishing technique.
The technique uses a long line, sometimes up to several miles in length, with thousands of baited hooks on branch lines at regular intervals often to catch swordfish, tuna, and halibut.
But adult and juvenile birds become snared on the hooks and are dragged underwater to a premature death.
The study, recently published in the journal Endangered Species Research, was carried out by scientists from the RSPB and BirdLife International.
Presenting the research is Dr Orea Anderson, policy officer for their Global Seabird Programme and lead author of the study.
She said yesterday: “It is little wonder so many of the affected seabird species are threatened with extinction. Their slow rate of reproduction is simply incapable of compensating for losses on the scale this study has demonstrated.”
She said a major factor is the emergence of fleets, with previously unaccounted for bycatch problems, adding to the global tally. While some fisheries have reduced their impacts on seabirds, problems in others were just being observed
According to the research. the Spanish longline fleet, on the Gran Sol grounds off SW Ireland, is one such fleet, with preliminary data suggesting it may be responsible for killing large numbers of seabirds, potentially upwards of 50,000 annually, mostly shearwaters and fulmars.
The Japanese tuna fleet came second in scale -- more than 20,000 killed each year, but with the largest impact on albatrosses.
However, seabird deaths around South Georgia have declined by 99% since regulations were enforced. South Africa achieved a drop of 85% bycatch in its foreign-licensed fleet in 2008, when a cap was placed on the number of seabird deaths permitted.
More recently, in April 2011, Brazil passed a law requiring the use of stringent seabird bycatch measures in their domestic tuna longline fleets.