Estimates of the Numbers, Biomass and Year-Class Strengths of Perch (Perca fluviatilis L.) in Windermere from 1967 to 1977 and Some Comparisons with Earlier Years

Abstract

Estimates of the numbers and biomass (to 1977) and of year-class strengths (to 1975) of perch (P. fluviatilis L.) in Windermere [England, UK] are presented. They continue the series of Le Cren, Kipling and McCormack (1977) and were calculated by the same methods. Numbers calculated by these methods and by summing separate estimates for each year-class did not differ significantly. Mortality rates for combined year-classes were higher from 1969 in the north and from 1968 in the south basin than in the previous 20 yr. Mortality rates calculated for each year-class from 1955-1972 also showed an increase in later years. A dramatic increase in mortality occurred in 1976, when a disease, whose primary cause has not yet been identified, killed at least 98% of adult perch. A highly significant inverse relationship (P < 0.001) was found between mortality from egg to age II yr and summer temperature in the 1st yr of life. Biomass from 1967-1975 remained fairly high in both basins and was at a similar level to that of 1941 although far fewer fish were present in the later period. Year-class strengths were higher after 1966 than in the previous 5 yr due to warm summers (the correlation between year-class strengths and temperature was highly significant (P < 0.001)). Possible reasons for the observed changes in population numbers, biomass and year-class strengths over the period from 1941-1976 are discussed. The severity of the mortality in 1976 was confirmed by samples taken in 1977, when only 2 perch over II yr old were captured.