Condition, buoyancy and the distribution of larval fish: implications for vertical migration and retention

Abstract

A Lagrangian time-stepping model driven by water density, daytime larval 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 vertical migration (including inverse vertical migrations and increased dispersion at night) observed in field studies. Vertical distributions and conditions of post-yolk-sac larvae, derived from the model during day and night, are also consistent with comparable field observations. When vertical shear is introduced into the model, a simple localized larval retention mechanism, directly related to feeding, condition and buoyancy, is revealed. The model results also demonstrate increased shear dispersion (dilution) of poor-condition larvae relative to good-condition larvae, and may be used to explain the relative paucity of observations of dying or dead larvae in the field, Virtually all of the model results are directly testable in the field and/or laboratory, and we show how the findings may be directly applicable to larvae possessing functional swim bladders and perhaps to freshwater and marine invertebrate zooplankton.