Settlement and beyond: population regulation and community structure of reef fishes

Summary

This chapter examines processes acting at and following settlement from the pelagic phase to reefs. Settlement sets the starting conditions which post-settlement events modify. Recent studies have revealed quite precise habitat selection at settlement, facilitated in some cases by olfactory cues deriving from adults. Many species settle directly to reefs, but some undergo juvenile development in adjacent, nursery habitats. Seagrass beds and mangroves have been widely claimed to be important nurseries for many reef species. However, very few studies have quantified the proportion of reef populations to have passed through such nursery habitats. Available data suggest that the importance of such species is greater in the Caribbean than in the Indo-Pacific. Many species found in these habitats as juveniles undergo daily foraging migrations from the reef to the same habitats in later life; fishery species are important among them. Decline of nursery and foraging habitats close to reefs has been suggested as one cause of recent declines of fisheries in the Caribbean.

A series of models have been proposed to account for the structure of fish communities on reefs, invoking varying degrees of control by post-settlement processes from none to great. Initial models focused on the role of competition for resources. Later models sought to account for small-scale unpredictability in assemblage composition and population size, invoking stochastic or limited settlement from the plankton. None of these models is sufficient alone. Predation following settlement has been shown to be a particularly potent force in structuring assemblages. Competition has been demonstrated to limit population sizes of a number of species, including fishery organisms. Present views converge more on multi-factorial explanations of population regulation based on a keener appreciation of the scale-dependence of processes. Mortality rates and patterns vary among species and within and among reef habitats. Many patterns of habitat use, including those of nursery habitats, can be explained in part as adaptations to reducing predation risk. A broad and robust generalization is that structurally complex habitats support abundant and diverse faunas resulting from protection from predation and greater availability of resources. Reef degradation reduces reef structural complexity and so has serious consequences for fishery productivity. Future research will benefit fisheries by focusing more effort on the effects of reef degradation and on processes that have so far been neglected, such as the importance of post-settlement movement.