MEPS 217:145-156 (2001)  -  doi:10.3354/meps217145

ABSTRACT: 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 conducted monthly visual censuses from May to December (3 to 10 mo post-settlement) 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 grouper density decreased significantly from August to September, but this decrease was small compared to a decrease of over 90% during the first 3 to 4 mo post-settlement, a pattern consistent with Type III survivorship. Throughout the study, Nassau grouper had a disproportionately high association with the macroalgae Laurencia sp., while other microhabitats (e.g. seagrass, corals) were usually used in proportion to their availability. Percent cover of Laurencia sp. significantly influenced spatial variability in fish density during May and June (4 to 5 mo post-settlement), but not later in the year. Spatial differences in juvenile Nassau grouper density later in the year were correlated with spatial variability in the size distribution of fish. Sites harboring disproportionately high numbers of large juveniles also had greater juvenile densities. Thus, microhabitat characteristics were determined to influence the spatial distribution of Nassau grouper up to 5 mo post-settlement, but the population appears to be influenced by size-dependent processes (e.g. predation or ontogenetic habitat shifts) after that time. Microhabitat characteristics, as well as spatially variable growth rates or size-dependent mortality may cause nursery areas to vary in their contribution to the local population at subsequent life history stages.

KEY WORDS: Nassau grouper · Epinephelus striatus · Population dynamics · Microhabitat