The selective utilisation of Rhizophora mangle habitat by juvenile reef fish

Recognised as 'Essential Fish Habitat', red mangrove (Rhizophora mangle) has received much attention in recent years to assess its nursery function. Research suggests that mangrove habitat contributes significantly to coral reef fish populations. Not all mangrove is utilised to its full capacity by juvenile fish, and it may be that, in terms of fisheries sustainability and enhancement, the use of management resources for mangrove habitat protection could be more judiciously targeted on the basis of preferred habitat use. In order to establish the factors influencing fish distribution in Rhizophora mangle, fish populations were studied in a Bahamian tidal lagoon. Biotopic, hydrophysical, structural and biological influences were investigated in natural red mangrove habitat, and by experimental manipulation of artificial mangrove units (AMUs). Reef fish were distributed unevenly over various spatial scales, indicating preferences in some species for particular lagoon biotopes, and various physical and biological characteristics inmangrove prop root habitat. With the exception of tidal flow, other physical attributes such as salinity, pH, dissolved oxygen and water depth had limited influence on fish distribution. In natural mangrove habitat the degree of shade appeared to be the most important factor in attracting fish to mangrove habitat followed by habitat complexity and epiphytic algae biomass. Variations in preference for these attributes between species and size classes suggested that no single attribute was exclusively responsible for a particular locational preference. Experimental manipulations of AMUs indicated a preference for greater shade and root density up to a certain level after which no preference was shown. Preferences varied between species and size classes and also with increased vulnerability to predation. Red mangrove trees perform a number of important functions on tropical and sub-tropical coastlines. They stabilise and protect: the coastline against erosion. supply nutrients to other coastal habitats, and provide habitat for many avian and terrestrial species. Prioritising management in mangrove habitat must be done with care, taking into account all of the habitat's functions.