LARVAL DISTRIBUTIONS AND THE SPATIAL PATTERNS OF SETTLEMENT OF AN OYSTER REEF FISH - RESPONSES TO FLOW AND STRUCTURE

A combination of larval behavior and physical factors influence the sp atial patterns of settlement of marine organisms. Of particular import ance to the settlement process is the blend of passive transport and a ctive responses to water flow near the settlement habitat. Field exper iments with the naked goby Gobiosoma bose, 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. Larg er aggregations of larvae were found in downcurrent positions at rocks that created larger low-flow zones, and during parts of the tidal cyc le with higher ambient flow velocities. Settlement occurred in a highl y aggregated pattern that reflected larval distributions, Most settlem ent measured in a field experiment was adjacent to downcurrent sides o f rocks rather than in other positions near rocks, or away from struct ures that would decrease downcurrent now velocities. These results sug gest that the active response of fish larvae to either direct or indir ect effects of now on reefs may be important to fine-scale spatial pat terns of settlement. Because zooplankton densities downcurrent of rock s were similar to, or lower than, densities upcurrent and lateral to r ocks, spatial distributions of prey are unlikely to explain larval dis tributions. Instead, active preference for low-flow areas may enable f ish larvae to maintain their position on oyster reefs, the preferred s ettlement habitat.