Catalogus

A combination of larval behavior and physical factors influence the spatial patterns of settlement of marine organisms. Of particular importance to the settlement process is the blend of passive transport and active responses to water flow near the settlement habitat. Field experiments with the naked goby Gobiosoma bosc, a benthic oyster reef fish, indicated that larvae aggregate in low-flow areas on the downcurrent sides of rocks, and shift position with changing flow directions. Larger aggregations of larvae were found in downcurrent positions at rocks that created larger low-flow zones, and during parts of the tidal cycle with higher ambient flow velocities. Settlement occurred in a highly aggregated pattern that reflected larval distributions. Most settlement measured in a field experiment was adjacent to downcurrent sides of rocks rather than in other positions near rocks, or away from structures that would decrease downcurrent flow velocities. These results suggest that the active response of fish larvae to either direct or indirect effects of flow on reefs may be important to fine-scale spatial patterns of settlement. Because zooplankton densities downcurrent of rocks were similar to, or lower than, densities upcurrent and lateral to rocks, spatial distributions of prey are unlikely to explain larval distributions. Instead, active preference for low-flow areas may enable fish larvae to maintain their position on oyster reefs, the preferred settlement habitat.