Long-term and short-term fluctuations in the numbers and catches of Arctic charr, Salvelinus alpinus (L.), in Windermere (northwest England)

Abstract

All English populations of Arctic charr are found in the Lake District (northwest England). There are at least four races of charr in Windermere, the largest natural lake in England, with the north and south basins of the lake each containing two distinct races that spawn in autumn and spring respectively. Three methods have been used to estimate levels of the charr stocks ; namely gill-netting each November (1939-1991) for autumn spawners in the north basin, anglers' catches from both basins (1966-1991), echo-survey data from both basins (July 1989 - December 1991). An increase in gill-net catches from 1945 to 1965 was associated with a cull of larger pike, Esox lucius L. ; since 1965, catches have fluctuated around a high mean level. There was good, if not excellent, agreement between the anglers' catches and the gill-net catches. The ratio of anglers' catches in the north and south basins showed that catches have been relatively low in the south basin since 1984. Although there has also been an increase in brown trout, Salmo trutta L., taken by the charr anglers since 1984 in the south basin, this increase was not responsible for the lower charr catches. When estimates of fish densities with an echo sounder were restricted to fish > 20 cm and depths down to 20 m, this being roughly the stock available to charr anglers, there was good agreement between monthly estimates and anglers' catches, especially in the north basin. The echo-sounder data confirmed the higher numbers in the north basin, and lower numbers in winter in both basins. The three different methods therefore provided comparable data on the fluctuations in levels of charr stocks in Windermere. Possible reasons for these fluctuations are discussed briefly.