LICENCES to kill over 2500 protected birds have been issued by Scottish ministers, despite doubts over whether they can be scientifically justified.

Over the past five years the Scottish Executive has allowed angling groups across the country to shoot 1626 goosanders, 625 cormorants and 281 mergansers in order to prevent them from eating salmon.

But scientists say that killing the birds may not make any difference to fish numbers. "At the moment there is no scientific evidence which shows that shooting fish-eating birds is an effective way of enhancing salmon populations or increasing catches, " said Mick Marquiss, an expert from the government's Centre for Ecology and Hydrology in Banchory.

Studies he has done on goosanders showed that they did eat large numbers of young salmon in the north of Scotland.

It was possible that killing the birds might allow more fish to go to sea, he reasoned. "However, sea losses are so great that killing ducks cannot have a major effect on fish catches and such an effect has never been demonstrated."

Details of the annual culls of fish-eating birds have been released for the first time in response to a request by the Sunday Herald under the Environmental Information Regulations. They show licences to kill 400-600 birds a year are issued to 15 or more fishing groups.

The River Tweed Commissioners has won approval to shoot the most - 103 in 2004 and 90 in 2005. Other major shooters were the Esk District Salmon Fishery Board (68 and 53), the Nith District Salmon Fishery Board (47 and 55) and the Deveron District Salmon Fishery Board (42 and 40).

Many goosanders, cormorants and mergansers have also been shot on the Spey, the Tay and the Dee. Licences to kill 53 eider ducks have been granted since 2000, including 12 in the past two years to Loch Etive Farmed Shellfish.

The Royal Society for the Protection of Birds is "deeply concerned" at the number of licences being issued. "Legally, such licences may only be granted to prevent serious damage to fisheries, and where no alternative solutions are available, " said Rob Tomlinson from RSPB Scotland.

"Yet, as far as we are aware, scientific evidence of any impact is scarce and, in most cases, has not been quantified.

We would encourage greater openness in the justification of such licensing."

The Scottish population of goosanders is thought to be around 2000 breeding pairs, with maybe 1800 pairs of mergansers. Far more licences to shoot the birds are granted in Scotland than in England.

But Ronald Campbell, a biologist with the Tweed River Foundation, the research arm of the Tweed Commissioners, claimed the bird populations were "rapidly expanding". He also defended the science in support of the shooting, and criticised the RSPB.

"The RSPB has never, to my knowledge, produced any scientific evidence that the totals shot have any impact on goosander numbers, " he said.

"Each bird is capable of eating six or seven smolts [young salmon] a day. Every smolt that doesn't get to sea is a missed opportunity for an angler."

He accepted that the birds might not cause biological damage to the overall population of salmon in the Tweed, but claimed more than 500 jobs depended on fishing in the river.

According to the Scottish Executive, licences were granted when there was evidence of serious damage to fisheries.

"Licences are only issued after careful consideration, " said a spokeswoman.