Growth and Production of the 1955 to 1972 Cohorts of Perch, Perca fluviatilis L., in Windermere

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Abstract
Mean lengths for age of cohorts of perch from 1955-1972 were calculated from directly measured fish and back-calculated lengths. Back-calculated lengths were corrected for allometry by using exponent values of 0.814 for immatures and males, and 0.723 for mature females. Back-calculated lengths were not usually significantly different from those measured directly. The von Bertalanffy growth model fitted the data well and values for the model are presented for male (1955-1972) and female (1955-1971) cohorts. The general trend in growth since 1955 (and before that period) has been an increase in mean lengths with age and an increase in K [rate at which growth curve approaches asymptote] and L.infin. [length at which growth rate is 0] values for both sexes. There was a linear relationship between growth and mortality of cohorts. The correlation between K and Z (annual instantaneous mortality rate) was highly significant (P < 0.001) for cohorts from 1964-1972. Estimates are given of cohort production from age 2 until an age when the number in the population (N) < 1000. There were sharp differences in production, consumption and gross (or ecological) efficiency between slow and fast growing cohorts (1955 and 1968). Some approximations were made of the juvenile production, and its contribution to total production was much greater for fast growing cohorts (1968 male = 79%) than slow growing cohorts (1955 male = 59%). Annual production, gonad losses and biomass are given for the years 1961-1972. Absolute fecundity (F) per unit length has increased over the period of investigation. Ecological life tables and age specific fecundity rates were determined for female cohorts. Factors affecting the net reproduction rate are discussed and a model was developed to predict these effects.