Ecological processes underlying ontogenetic habitat shifts in a coral reeffish

Distribution of mobile animals may reflect decisions on how to balance conf licting demands associated with foraging and avoiding predators. A simple o ptimality model predicts that mobile animals should respond to changes in m ortality risk (mu) and growth rate (g) by shifting habitats in a way that m aximizes net benefits. In this study, field caging and tethering experiment s quantified habitat-specific growth rates and mortality risk, respectively , for three different sizes of a coral reef fish, Nassau grouper (Epinephel us striatus), during its juvenile tenure in off-reef nursery habitats. Thes e sizes bracketed the size at which this species undergoes an ontogenetic h abitat shift from the interstices of macroalgal clumps ("algal habitat") to areas outside, or adjacent to, macroalgae and other physically complex mic rohabitats ("postalgal habitats"). Experimental results were used in a cost -benefit analysis to test the following alternative (but not mutually exclu sive) a hypotheses: (1) juvenile grouper shift habitats in a way that maxim izes growth rates (g); (2) juveniles shift habitats in a way that minimizes mortality (predation) risk (mu); and (3) if trade-offs exist between maxim izing growth rate and minimizing mortality risk, juveniles shift habitats i n a way that minimizes the ratio of mortality risk to growth rate (mu/g). Results suggested that small fish face a trade-off between living in the re latively safe algal habitat and achieving high growth rates in postalgal ha bitats. The value of mu/g was significantly lower in the algal than postalg al habitats for small fish, which typically reside in the algal habitat, an d significantly lower in postalgal habitats for medium and large fish, whic h typically reside in postalgal habitats. Thus, habitat use by juvenile Nas sau grouper was consistent with the "minimize mu/g hypothesis." These resul ts highlight how behavioral responses to ecological processes, such as chan ging predation risk with body size, determine distribution patterns of mobi le animals.