How important are mangroves and seagrass beds for coral-reef fish?: The nursery hypothesis tested on an island scale

There has been much controversy over the degree to which mangroves and seagrass beds function as nursery habitats for the juveniles of fish species that live on coral reefs as adults. In previous studies we have shown that the juveniles of at least 17 Caribbean reef-fish species are highly associated with bays containing mangroves and seagrass beds as nurseries, and that juveniles of these species are absent in bays lacking such habitats. In this study we therefore hypothesised that on islands lacking these bay nursery habitats, adults of these fish species will be absent or show low densities on the coral reef. Densities of the 17 species were compared between the reefs of Caribbean islands with and without mangroves and seagrass beds. On reefs of islands lacking these habitats, complete absence or low densities were observed for 11 of the 17 species, several of which are of commercial importance to fisheries. This finding suggests a Very important nursery function of such habitats and implies that the densities of several fish species on coral reefs are a function of the presence of nearby bays containing mangroves and seagrass beds as nurseries. The results indicate that degradation or loss of these habitats could have significant impacts on reef-fish stocks in the Caribbean.