Individual Variation in Acquisition/Allocation Reaction Norms

Abstract

Fisheries impose upon populations selective forces that are probably proportional to fishing mortality, even when that is not size-selective. Whether there is an evolutionarily response in a population depends upon the amount of genetic variation in traits associated in particular with allocations to growth and reproduction. Genetic change in such traits is obscured and confused by the purely phenotypic changes in acquisition/allocation schedules resulting from fishery-induced changes in density. Such linked reaction norms of allocation in iteroparous species may be described in terms of allometry. Individual variation in allocation reaction norm in the North Sea sole (Solea soled) was reported by de Veen (1976). In responding to changes in food availability the faster-growing females displayed a steeper reaction-norm allometry between ovarian and somatic weight. Reaction norms to temperature also show allometric differences. Relative gonad size in smaller northern anchovies (Engraulis mordax) is more sensitive to temperature, even to the point of negative allometry during an El Niño year. We may reasonably summarize such variation in a bipartite growth function expressing the relationship of surplus, growth, and reproductive energies as power functions of size (Ware 1980). Both the evolutionary response to selection and its detection depend upon the sources of individual variation in the joint reaction norms of these quantities to changes in food supply, density, and temperature. Only when these joint reaction norms have been identified can their evolutionary change be detected.