CONDITION, BUOYANCY AND THE DISTRIBUTION OF LARVAL FISH - IMPLICATIONS FOR VERTICAL MIGRATION AND RETENTION

A Lagrangian time-stepping model driven by water density, daytime larv al feeding and swimming, and by condition-related larval buoyancy was used to track the vertical position and condition of individual larval cod (Gadus morhua L.) in a stratified water column. The model results can explain the variety of frequencies, phases and amplitudes of vert ical migration (including inverse vertical migrations and increased di spersion at night) observed in field studies. Vertical distributions a nd conditions of post-yolk-sac larvae, derived from the model during d ay and night, are also consistent with comparable field observations. When vertical shear is introduced into the model, a simple localized l arval retention mechanism, directly related to feeding, condition and buoyancy, is revealed. The model results also demonstrate increased sh ear dispersion (dilution) of poor-condition larvae relative to good-co ndition larvae, and may be used to explain the relative paucity of obs ervations of dying or dead larvae in the field. Virtually all of the m odel results are directly testable in the field and/or laboratory, and we show how the findings may be directly applicable to larvae possess ing functional swim bladders and perhaps to freshwater and marine inve rtebrate zooplankton.