Ecology and conservation of arctic charr, Salvelinus alpinus (L.), in Loch Doon, an acidifying loch in Southwest Scotland

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Several populations of arctic charr Salvelinus alpinus in southern Scotland have become extinct and Loch Doon is the only site left for the species in the southwest. The catchment of the loch lies mainly on granite and much has recently been afforested. Most of the inflowing streams are acid and the loch itself appears to have acidified over the last two decades. Few acid-sensitive invertebrates occur in the streams or in the loch. The fish populations of the most acid streams are low in density. Growth of arctic charr in the loch resembles that in some other Scottish lochs but appears to have increased in recent years. The charr feed on benthic invertebrates for much of the year but on zooplankton in the summer. Spawning occurs in early October, leaving the fish in poor condition which improves during the spring to reach a maximum in late summer, just prior to spawning. Spawning takes place over gravel and stones in shallow water all round the loch. Angling data indicate that the average size of both trout and arctic charr in Loch Doon has increased significantly over the last 15 years. Chemical and biological evidence suggests that Loch Doon is acidifying and that management is necessary if the stock of charr there is to be saved.