ECOPHYSIOLOGY OF MARINE FISH RECRUITMENT - A CONCEPTUAL-FRAMEWORK FORUNDERSTANDING INTERANNUAL VARIABILITY

Present data and our application of logic do not permit confident reje ction of the null hypothesis: Interannual variation in recruitment of marine fishes (typified by certain flatfishes) is independent of ecoph ysiological 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. T herefore, in this paper, we set aside as presently intractable the pro blem of understanding in any generalizable way the specific effects of environment on interannual variation in marine fish recruitment. Inst ead, we return to a conceptual scheme first proposed almost 50 years a go by F.E.J. Fry for considering effects of environmental factors on t he physiology of fishes. We first extend this scheme to population-lev el responses, including recruitment, and then even further, to communi ty/ecosystem-level responses. Fry supposed that all of environment can be resolved into five classes of physiological effects-controlling (w hich set the pace of metabolism), limiting (which constrain maximum me tabolism), lethal (which completely interdict metabolism), masking (wh ich increase obligatory metabolic work), and directive (which release and unload metabolism by guiding enviroregulatory responses). We sugge st that corresponding effects can be recognized at the levels both of population and community/ecosystem. The key analogy is that environmen t operates on individuals through metabolism, on populations through r ecruitment, and on communities/ecosystems through abiotic and biotic d iversification. In the context of marine-fish populations, we propose that scope for population increase is the difference between maximum a nd maintenance recruitment to the spawning stock. Maintenance recruitm ent is the product of critical spawner density and spawner mortality r ate; this product varies with environment as the resultant of controll ing effects on the metabolism of individuals, and is increased by load ing due to masking factors-e.g, predation-that increase one or both mu ltiplicands. Maximum recruitment is limited by deficiencies of resourc es, primarily food, but also, potentially, by low spawner density. Pop ulation-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, influe ncing not only habitat use and zoogeographic range, but also providing context for genetic adaptation and speciation. Exploration of this co nceptual scheme from the perspective of flatfish life-history strategi es and population dynamics, leads to several testable ecophysiological hypotheses about recruitment.