Observed and simulated swimming trajectories of late-stage coral reef fish larvae off the Florida Keys

The supply of coral reef fish larvae from the open ocean to nearshore reefs is vital for the persistence of local fish populations. Larvae that are competent to settle are often fast swimmers, and their transport to suitable settlement habitat may depend on swimming behavior as well as currents. Our goal was to measure the effects of swimming behavior during the final period of larval transport. We observed late-stage bicolor damselfish Stegastes partitus larvae 1 km offshore of French Reef, Key Largo, Florida, USA, and recorded their in situ swimming trajectories. While all larvae swam remarkably straight, their swimming directions were distributed randomly. Larvae swam at speeds of 2 to 32 cm s–1 and transport due to swimming was of similar magnitude as transport due to cross-shore current, but much less than transport due to alongshore current. We used a simulation model to generate swimming trajectories of much longer duration than would be feasible to directly observe. This enabled us to better quantify the contribution of swimming behavior to larval transport. The observed and simulated swimming trajectories indicate that horizontal swimming by larvae with or without an external reference frame is important at spatial scales of several kilometers.