By Alex Kirby BBC News Online environment correspondent

The world's largest seabirds, the albatross family, should soon benefit from more protection around the globe.

Hook in the throat of an albatross (Image: RSPB Images)

A fourth country, South Africa, has now ratified an international treaty, the Agreement on the Conservation of Albatrosses and Petrels (Acap).

The biggest threat to the birds is longline fishing, which kills many thousands every year.

Only one more signatory must ratify the agreement for it to enter into force.

BirdLife International, an alliance of non-governmental groups working in more than 100 countries, welcomed the South African decision.

Dragged down

Leon-David Viljoen, co-ordinator of BirdLife's Save the Albatross campaign, said: "Acap has made great strides since its adoption just over two years ago.

"The strength of the treaty is that it is legally binding on signatory states, requiring them to take specific measures to reduce seabird by-catch from longlining and improve the conservation status of albatrosses and petrels."

The toll of one trip by one vessel (Image: Peter Ryan)

BirdLife says longline fishing for Patagonian toothfish, tuna and other commercially valuable species is the single greatest global threat to seabirds, killing more than 300,000 annually.

Of that total, it estimates 100,000 are albatrosses, with the rest a mixture of petrels, fulmars, shearwaters and other species.

The fishing lines can be up to 130 kilometres (80 miles) long. Seabirds scavenge behind the boats, are caught as they try to take the bait from the hooks, and drown when they are dragged underwater.

Other measures Acap signatories have to implement include research and monitoring, eradication at breeding sites of introduced species such as rats and feral cats, reduction of disturbance and habitat loss, and reduction of marine pollution.

Plummeting stocks

Ten countries have signed the agreement, and apart from South Africa those which have ratified it are Australia, Ecuador and New Zealand. The UK is expected to ratify Acap next month.

Wandering albatross (Image:Tony Palliser/BirdLife International)

There are thought to be about four million albatrosses, of 28 distinct species. The rate of attrition from longline fishing is about 2.5% annually.

But BirdLife says it is the rate of loss for individual species that is important, because it varies widely.

It says: "For example, British Antarctic Survey (Bas) scientists have been studying wandering albatross, grey-headed albatross and black-browed albatross colonies on South Georgia since the 1960s and 1970s.

Remedies at hand

"They have discovered a chronic year-on-year decline, not only in the survival of adult birds but also in the number of young birds surviving to enter the population.

"These breeding populations have decreased by approximately one-third since then. This is a huge amount for species whose whole life history strategy is based on a high survival rate in a stable marine environment."

Dr Deon Nel, scientific co-ordinator of the Save the Albatross campaign, said: "Simple effective by-catch mitigation measures such as bird-scaring streamers and line-setting at night exist.

"But they must be much more widely adopted if these magnificent birds are to be saved from extinction."