Piscivore assemblages and predation pressure affect relative safety of some back-reef habitats for juvenile fish in a Caribbean bay

摘要

An important process thought to drive habitat selection during (post-)settlement of coral reef fish is predation. It is assumed that in back-reef habitats such as seagrass beds and mangroves predation is lower than on coral reefs. However, recent studies have suggested that significant piscivore assemblages are present in back-reef habitats. The assumption of reduced predation pressure in back-reef habitats can therefore be debated. We compared piscivore assemblages along the coast of a Caribbean island using underwater visual census surveys in a spatial gradient of coral reefs, seagrass beds and mangroves. We also performed predation experiments in combination with video observations in these habitats using tethered recently settled Haemulon flavolineatum. High piscivore densities and reduced survival showed the presence of significant piscivore assemblages in both reef and back-reef habitats. Both reef-associated piscivores and piscivores that use back-reef habitats as a juvenile habitat contributed to the piscivore assemblage in the investigated gradient. Habitat type had a significant effect on the composition of the piscivore assemblage, density, and prey survival. Piscivore density was highest on the reef, mangroves and notches, all of which are habitats with a high structural complexity. On the contrary, seagrass beds showed lowest piscivore density. Tethering experiments showed lowest survival on the reef but also reduced survival on back-reef habitats located close to the coral reef. Seagrass beds and mangroves located farther away from the reef into the bay showed highest survival. The present study shows that the concept of reduced predation in back-reef habitats used by juvenile fish does not apply to all habitats, since these habitats can harbour significant piscivore assemblages. However, depending on the spatial setting in the seascape (distance to the reef and presence of other habitat types), some Caribbean seagrass beds show low densities of piscivores and increased survival when compared to other habitats.