Productivity and fitness in a fjord cod population: an ecological andevolutionary approach

摘要

Two modeling approaches were used to evaluate the combined responses in a fish population to environmental fluctuations and exploitation. Short-term effects of growth and survival of year classes of cod and the effects of fishing were evaluated by using a modified version of a dynamic ecosystem model (DYNECOMAS II), originally developed for extensive mariculture purposes in Masfjorden, western Norway. Simulations indicated that the availability of zooplankton and density-dependent predation and cannibalism were key factors for the carrying capacity for cod in the fjord. On a short-term scale, there was a strong-density-dependent regulation of cod productivity, individual growth and survival, while fisheries had little impact. Long-term effects on the fitness of cod in relation to fishery and environmental variability were evaluated by means of a life history model. The intrinsic rate of increase r was chosen as a measure of fitness. The impact on cod fitness of maturation age, variable survival at egg, larval and 0-group stage (reflecting variable recruitment to age 1) and the degree of density-dependent growth were studied for different entry ages to the fishery for both moderate and high fishing intensities. A modified Euler-Lotka equation was parameterized using available empirical data on growth, age-dependent fishing mortality and fecundity for cod from Masfjorden and DYNECOMAS II model-predicted natural mortality for juveniles (predation mortality). The solution (r) was found by iteration for combinations of maturing age, fishing mortality and survival probability until 1 year of age. The predictions obtained were that (a) the lower the survival probability until 1 year of age, the lower the fitness; (b) the higher the maturing age the lower the fitness; (c) an early entry to even a moderate fishery resulted in r<0 for low survival probability to age 1; (d) if a high degree of density-dependent growth was associated with an early entry to an intensive fishery (F=1) from age 1 year and onwards, the surviving individuals from an average year class may not produce enough replace spawners regardless of maturing age, since r<0.