IT seems that Dr Martin Jaffa (Letters, July 17) is engaging in a cynical manipulation of statistics to further his thesis that salmon farms cause no significant damage to wild salmon and sea trout stocks.

He maintains that sea lice from salmon farms only kill one or two per cent of the wild salmon population in the west Highlands and Islands and that is trivial compared with the overall 95 per cent losses at sea.

What he conveniently fails to acknowledge is that sea lice from salmon farms kill the juvenile wild salmon (smolts) as they migrate from rivers on their way out to sea. In order to reduce the population from five per cent returning to three per cent returning (as adults following their marine migrations), the lice must be killing 40 per cent of the smolts. The remaining 60 per cent are then reduced by 95 per cent by other factors, giving a three per cent return rate. There are indeed other factors at play in marine survival but the great majority of these, unlike sea lice releases from salmon farms, are outwith the control of managers.

In round terms, the kill rate of juvenile west Highland wild salmon by lice is about 40 per cent. To put this in context, on the River Erriff in Ireland, which has been extensively monitored for more than 30 years, Inland Fisheries Ireland (a government body) has demonstrated that returns of wild salmon can be reduced by more than 50 per cent in years in which lice levels on salmon farms are high.

As regards the causes of the calamitous decline in wild sea trout numbers in the west Highlands and Islands, the views of Dr Jaffa, a diehard lobbyist, in support of the aquaculture industry to which he is connected, have no credibility. There is overwhelming acceptance by eminent fishery scientists that salmon farming is responsible for the significant, sea-lice-induced, reduction in marine survival.

Andrew Graham-Stewart,

Director, Salmon and Trout Conservation Scotland,

Siskin, Bonar Bridge, Sutherland.