Experimental manipulations of the fish populations in Windermere

Abstract

A large scale experiment has been carried out on the perch and pike populations of Windermere in the English Lake District for nearly 40 years, and the objectives have changed over this period. Initially the aim was to manage the trout population, but the work has produced data of interest in predator-prey relationships and pike management. Perch were first heavily fished and then monitored for size, and numbers by trapping. Large pike have been culled by gill netting. The effects of pike gill netting have been to increase the growth rate and decrease the average age in the population. These changes in the pike population structure, together with climatic variations, have modified the perch population. The perch did not recover from the heavy fishing for many years and the population numbers remained low due to pike predation supressing recruitment. It was not until a large cohort, mainly controlled by the weather, was produced that the population increased. Cannibalism by these perch later combined with the pike predation to suppress future cohorts. There have also been as yet unexplained changes in the perch mortality rates. The experiments have led to a greater understanding of the relationship between pike and perch populations, but the use of this knowledge for management depends on what the objectives are.