ECOPHYSIOLOGY OF MARINE FISH RECRUITMENT - A CONCEPTUAL-FRAMEWORK FOR UNDERSTANDING INTERANNUAL VARIABILITY

Present data and our application of logic do not permit confident rejection of the null hypothesis: Interannual variation in recruitment of marine fishes (typified by certain flatfishes) is independent of ecophysiological factors. Our inability to reject this hypothesis reflects not its likely validity but rather a lack of conceptual structure and appropriate data for realistic evaluation of alternative hypotheses. Therefore, in this paper, we set aside as presently intractable the problem of understanding in any generalizable way the specific effects of environment on interannual variation in marine fish recruitment. Instead, we return to a conceptual scheme first proposed almost 50 years ago by F.E.J. Fry for considering effects of environmental factors on the physiology of fishes. We first extend this scheme to population-level responses, including recruitment, and then even further, to community/ecosystem-level responses. Fry supposed that all of environment can be resolved into five classes of physiological effects-controlling (which set the pace of metabolism), limiting (which constrain maximum metabolism), lethal (which completely interdict metabolism), masking (which increase obligatory metabolic work), and directive (which release and unload metabolism by guiding enviroregulatory responses). We suggest that corresponding effects can be recognized at the levels both of population and community/ecosystem. The key analogy is that environment operates on individuals through metabolism, on populations through recruitment, and on communities/ecosystems through abiotic and biotic diversification. In the context of marine-fish populations, we propose that scope for population increase is the difference between maximum and maintenance recruitment to the spawning stock. Maintenance recruitment is the product of critical spawner density and spawner mortality rate; this product varies with environment as the resultant of controlling effects on the metabolism of individuals, and is increased by loading due to masking factors-e.g, predation-that increase one or both multiplicands. Maximum recruitment is limited by deficiencies of resources, primarily food, but also, potentially, by low spawner density. Population-level lethal factors cause extinction, by reducing population scope to sub-zero values for a time exceeding the generation interval. Directive factors distribute the population in space and time, influencing not only habitat use and zoogeographic range, but also providing context for genetic adaptation and speciation. Exploration of this conceptual scheme from the perspective of flatfish life-history strategies and population dynamics, leads to several testable ecophysiological hypotheses about recruitment.