Biophysical Mechanisms of Larval Fish Ingress into Chesapeake Bay

Selective tidal stream transport is hypothesized as a dominant mechanism by which larvae of marine animals move through estuarine openings. For larvae moving from the shelf to estuarine habitats, selective tidal stream transport proposes that larvae are higher in the water column during flood tide and lower in the water column during ebb tide. Although a number of studies conclude that selective tidal stream transport is the mechanism responsible for larval ingress, few studies consider alternative mechanisms or consider passive explanations for tidal patterns in larval distributions. We examined the biophysical mechanisms responsible for larval ingress into Chesapeake Bay using an Eulerian approach. We made flux calculations for 3 species and partitioned flux estimates among 3 different ingress mechanisms (wind forcing, residual bottom inflow and tidal). For the Atlantic croaker Micropogonias undulatus (Sciaenidae), all 3 mechanisms of ingress contributed to the net up-estuary flux of larvae, but tidal mechanisms become more important for larger sizes. Net up-estuary flux of the Atlantic menhaden Brevoortia tyrannus (Clupeidae) was dominated by residual bottom inflow and wind forcing. Ingress of the summer flounder Paralichthys dentatus (Paralichthyidae) was dominated by tidal mechanisms, and the importance of tides increased with developmental stage. We found little evidence for the hypothesis that tidal patterns in larval distributions resulted from passive processes (water mass-specific distributions, buoyancy, vertical mixing), thereby supporting the hypothesis that tidal patterns resulted from active behaviors. However, our estimates of vertical mixing were not direct and additional work is needed to examine the role of vertical mixing in influencing vertical distributions in areas with strong tides. We conclude that a combination of wind forcing, residual bottom inflow, and selective tidal stream transport are responsible for the ingress of larval fishes into Chesapeake Bay, and that the relative importance of the 3 mechanisms differs among species and changes with larval development.