PRODUCTIVITY AND FITNESS IN A FJORD COD POPULATION - AN ECOLOGICAL AND EVOLUTIONARY APPROACH

Two modeling approaches were used to evaluate the combined responses i n a fish population to environmental fluctuations and exploitation. Sh ort-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 dy namic ecosystem model (DYNECOMAS II), originally developed for extensi ve mariculture purposes in Masfjorden, western Norway. Simulations ind icated that the availability of zooplankton and density-dependent pred ation and cannibalism were key factors for the carrying capacity for c od in the fjord. On a short-term scale, there was a strong-density-dep endent regulation of cod productivity, individual growth and survival, while fisheries had little impact. Long-term effects on the fitness o f cod in relation to fishery and environmental variability were evalua ted 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 matu ration age, variable survival at egg, larval and 0-group stage (reflec ting variable recruitment to age 1) and the degree of density-dependen t growth were studied for different entry ages to the fishery for both moderate and high fishing intensities. A modified Euler-Lotka equatio n was parameterized using available empirical data on growth, age-depe ndent fishing mortality and fecundity for cod from Masfjorden and DYNE COMAS II model-predicted natural mortality for juveniles (predation mo rtality). 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 onwa rds, the surviving individuals from an average year class may not prod uce enough replace spawners regardless of maturing age, since r<0. (C) 1998 Elsevier Science B.V. All rights reserved.