Spatio-temporal variability in abundance, size and microhabitat associations of early juvenile Nassau grouper Epinephelus striatus in an off-reef nursery system

Nassau grouper Epinephelus striatus is a reef fish that settles in off-reef nursery habitats before emigrating to coral reefs. Because little is known of its habitat use and population dynamics before moving onto reefs, we co nducted monthly visual censuses from May to December (3 to 10 mo post-settl ement) to quantify spatio-temporal variability in density, microhabitat use , and size-structure in a cohort of early juvenile Nassau grouper residing in an off-reef nursery system in the central Bahamas. Juvenile Nassau group er density decreased significantly from August to September, but this decre ase was small compared to a decrease of over 90 % during the first 3 to 4 m o post-settlement, a pattern consistent with Type III survivorship. Through out the study, Nassau grouper had a disproportionately high association wit h the macroalgae Laurencia sp., while other microhabitats (e.g. seagrass, c orals) were usually used in proportion to their availability. Percent cover of Laurencia sp. significantly influenced spatial variability in fish dens ity during May and June (4 to 5 mo post-settlement), but not later in the y ear. Spatial differences in juvenile Nassau grouper density later in the ye ar were correlated with spatial variability in the size distribution of fis h. Sites harboring disproportionately high numbers of large juveniles also had greater juvenile densities. Thus, microhabitat characteristics were det ermined to influence the spatial distribution of Nassau grouper up to 5 mo post-settlement, but the population appears to be influenced by size-depend ent processes (e.g. predation or ontogenetic habitat shifts) after that tim e. Microhabitat characteristics, as well as spatially variable growth rates or size-dependent mortality may cause nursery areas to vary in their contr ibution to the local population at subsequent life history stages.