Computer Simulation of Larval Transport Through Tidal Channels: Role of Vertical Migration

A two-dimensional computer model is used to investigate the long-term net advective transport of larvae in tidal channels. Simulations are made over l-year time periods for Adderley Channel in the Exuma Cays of the Bahama Islands, and Aransas Pass on the Texas Gulf coast. Tidal conditions at these two locations are dominated by semi-diurnal and diurnal constituents, respectively. Larval transport is simulated by the product of the predicted tidal current and the larval concentration in each of five layers. The vertical distribution of larvae is a function of vertical migration, which acts to concentrate larvae in top and bottom layers, and turbulent diffusion, which tends to produce vertically uniform concentrations. The effect of turbulent mixing is inversely related to the vertical swimming ability of the larvae and reduces substantially the net transport of slower swimmers. Both light and tidal phase cued migrations are considered. Diel migration results in a distinct annual cycle in the transport of larvae, as diurnal tidal constituents cycle in and out of phase with light and dark conditions. However, little long-term net transport is indicated. Migration according to tidal phase produces a negligible annual cycle, but a substantial quasi-steady net transport of larvae is indicated for both study sites. In other simulations, non-tidal currents are introduced to represent freshwater outflow and low-frequency estuarine-shelf exchanges, and unspecified retention mechanisms are incorporated into the model. For these study sites, both non-tidal transport and larval retention dominate the effect of vertical migration.