Long-term studies on the Windermere populations of perch (Perca fluviatilis), pike (Esox lucius) and Arctic charr (Salvelinus alpinus)

Abstract

SUMMARY. 1, Windermere is a glacial valley lake with two basins separated by shallows and a total area of 14.8 km2. Perch, pike and charr are the dominant fish species and have been studied on a continuous basis for over 45 years.

2. The initial slow-growing stocks of numerous small perch and moderate numbers of large pike were subjected to a heavy initial fishing pressure that was subsequently relaxed for perch but not for larger (>550 mm) pike. The number of charr in the lake has increased 6–8-fold during the study, probably due to the sustained reduction in the abundance of the larger pike that heavily predate the autumn spawning race of charr when it enters the shallow littoral areas to spawn.

3. The biomass of perch did not recover from the initial fishery until the late 1950s when strong year-classes occurred in warm summers. However, it was now composed of larger and faster growing individuals. In 1976, disease killed over 98% of all adult perch in the lake and the subsequent recovery has again been slow.

4. Existing models have explained the variation in perch year-class strength for the years 1959–74 in terms of adult perch biomass, pike cohort strength and temperature in the year of hatch. A new. more generally applicable model, incorporating a stock-recruitment expression, gives a better fit to the earlier data, particularly from the North Basin, where there have been periods of extremely low adult biomass.

5. The importance of continuing to extend these long-term data sets and the scope for future modelling work are discussed.