Changes in the Population of Pike (Esox lucius) in Windermere from 1944 to 1981

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Abstract
From Oct., 1944-March, 1982, 13,195 pike, E. lucius L., were caught in Windermere in gill nets set in winter. Estimates of numbers of adult pike were: 1944, 1200 north basin (NB), 1100 south basin (SB), maximum 1961, 1800 NB, 2600 SB, minimum 1981, 300 NB, 400 SB. Estimates of year-class strengths at age 2 yr were: maximum 1959, 1250 NB, 1550 SB; minimum 1977, 130 NB, 130 SB. Regressions of year-class strength on an index of temperature in the 1st summer after hatching were year-classes 1944-1959 very significant, 1960-1968 significant, 1969-1978 not significant and for 1944-1978 very significant. Correlations between biomass of parent stock and year-class strengths were not significant from 1944-1959 nor from 1960-1968, but correlation was significant from 1969-1978 in the south basin but not in the north. Spawning may have been adversely affected in the late 1970s by low numbers of parent stock. Three periods with different characteristics were identified. In the 1940s and 1950s, temperature was the dominant factor determining year-class strengths and growth. In the early 1960s, food supply became important, food for pike more than 1-yr-old was scarce, adult pike were abundant and growth was poor. In the late 1960s and 1970s, food for young-of-the-year pike was scarce because of increased growth of perch; after 1976, because of the virtual elimination of the adult perch population by disease, pike year-class strengths were poor, and the few fish which survived grew very fast. Cannibalism became the dominant factor. As fishing policy was consistent from 1948, changes in the population were caused by natural circumstances. Temperature, food supply and cannibalism were identified as important factors in causing changes in numbers, biomass and growth.