ECOLOGICAL PROCESSES UNDERLYING ONTOGENETIC HABITAT SHIFTS IN A CORAL REEF FISH

Abstract

Distribution of mobile animals may reflect decisions on how to balance conflicting demands associated with foraging and avoiding predators. A simple optimality model predicts that mobile animals should respond to changes in mortality risk (μ) and growth rate (g) by shifting habitats in a way that maximizes net benefits. In this study, field caging and tethering experiments quantified habitat-specific growth rates and mortality risk, respectively, for three different sizes of a coral reef fish, Nassau grouper (Epinephelus striatus), during its juvenile tenure in off-reef nursery habitats. These sizes bracketed the size at which this species undergoes an ontogenetic habitat shift from the interstices of macroalgal clumps (“algal habitat”) to areas outside, or adjacent to, macroalgae and other physically complex microhabitats (“postalgal habitats”). Experimental results were used in a cost–benefit analysis to test the following alternative (but not mutually exclusive) hypotheses: (1) juvenile grouper shift habitats in a way that maximizes growth rates (g); (2) juveniles shift habitats in a way that minimizes mortality (predation) risk (μ); and (3) if trade-offs exist between maximizing growth rate and minimizing mortality risk, juveniles shift habitats in a way that minimizes the ratio of mortality risk to growth rate (μ/g). Results suggested that small fish face a trade-off between living in the relatively safe algal habitat and achieving high growth rates in postalgal habitats. The value of μ/g was significantly lower in the algal than postalgal habitats for small fish, which typically reside in the algal habitat, and significantly lower in postalgal habitats for medium and large fish, which typically reside in postalgal habitats. Thus, habitat use by juvenile Nassau grouper was consistent with the “minimize μ/g hypothesis.” These results highlight how behavioral responses to ecological processes, such as changing predation risk with body size, determine distribution patterns of mobile animals.