Reproduction effort versus the environment; case histories of Windermere perch, Perca fluviatilis L., and pike, Esox lucius L.

The theoretical limits of net reproduction rates for perch, Perca fluviatilis L. and pike, Esox lucius L, have been estimated from the upper and lower limits of fecundity, growth, mortality, age of maturity and biomass of parental stock, observed in Windermere over 40 years. Differences in egg numbers due to changes in fecundity were for perch × 1.5, for pike × 2. Assuming the same mortality, a fast growing cohort would produce more eggs during its lifespan than a slow growing, for perch × 2, for pike × 9. Changes in mortality from high to low resulted in more eggs, for perch × 7, and for pike (where high mortality includes natural and fishing mortality) × 10. Change in age at first maturity from 3 to 2 years was unimportant. Biomass of parental stock varied by × 6 for perch and × 3 for pike, and number of eggs laid varied by x2 for perch and × 5 for pike. The number of recruits at age 2 years varied by × 300 for perch and × 7 for pike. It was concluded that temperature in the first summer of life and predation were of major importance in regulating the number of recruits. Ricker recruitment curves and a simple model showed that perch, but not pike, could produce maximum recruitment from minimum parental stock. For perch, but not pike, compensatory density dependent mortality increased with increased parental stock. It was concluded that responses by the perch and pike populations to a changing environment can not be of sufficient magnitude to prevent wide fluctuations in recruitment.