Introduction. Evidence on the biological history of Windermere indicates a change in the associations of fish from one dominated by trout and char to another dominated by perch and pike.

Scheme of work. The experiment has involved rapid and drastic reduction in the population of perch since 1940 and pike since 1944, and has been accompanied by research on the bionomics of the various species of fish and on the invertebrate fauna inhabiting Windermere.

Perch trapping. The method is described. Trials in 1939–40 were followed by intensive operations with up to 770 traps in 1941 to 1947, The annual migration of perch has been determined in detail. Complete statistics of the fishery have been collected, so that observations are available on more than four million perch. In consequence the varying factors of the environment and of individual behaviour are so masked that conclusions can be drawn from smooth curves plotting the data directly. From 1940 to 1944 inclusive the curves show a simple exponential relation as the population was reduced. Thereafter the fishery reached a condition which approximates to that of a sustained yield, which was one-eighth to one-tenth of the original yield in different parts of the lake. The sex ratio has been much disturbed because far more males have been caught than females. Marking experiments and other evidence show little lateral movement of perch along the shore-line. Pike and eels were taken in perch traps in considerable numbers.

Pike netting. The method is described. Trials in 1943–44 were followed by intensive operations in 1944–47, during which period the catches have fallen and presumably the population has been drastically reduced, giving curves similar to those of perch trapping.

Char. Regular observations of the fish coming to breed suggests an increase in population following the reduction of perch and pike.

The opinions of anglers have given some corroborative evidence of the change of fish populations.

Discussion. The probable effects on the rates of reproduction, survival and growth of the five principal species of fish concerned are discussed.