Understanding patterns of habitat use in reef fish: implications of ontogenetic shifts in habitat-use for population demography

Abstract

Quantitative surveys of habitat associations of New Zealand's common triplefin fish, Forsterygion lapillum, show that a small-scale ontogenetic shift in habitat use occurs shortly after settlement. Availability of habitat over-used by settler aged F. lapillum (Cystophora) explains ~55% of the variation in settler abundance, while juvenile and adult abundance patterns are not well explained by their respective over-used habitat (encrusting coralline algae). Predation is identified as a possible mechanism underlying the habitat use of settlers. Methodologically similar surveys of a tropical species of labrid (Thalassoma amblycephalum) and a large body of published literature suggest that ontogenetic shifts in habitat use may be taxonomically and geographically widespread. Young T. amblycephalum disproportionately associate with small patch reefs comprised of living coral. In contrast, adults of this species show no clear patterns of association with measured habitat features within the lagoon, and are found higher in the water column. Findings suggest that juvenile T. amblycephalum may seek refuge from the high energy environments occupied by stronger swimming adults, in specific benthic microhabitats. The mechanisms underlying ontogenetic shifts in habitat use and the ecological consequences of these shifts are not well understood. I chose New Zealand's common triplefin (Forsterygion lapillum) as a model species to examine the underlying mechanisms and consequences of age-specific shifts in habitat use. Behavioural "choice" experiments, suggest that newly settled F. lapillum exhibit a preference for the habitat which they over-use. Results of a field experiment that manipulated Cystophora cover at two sites showed that Cystophora abundance can positively affect settler densities, and may limit F. lapillum recruitment if in low abundance. Taken together, these results suggest that: 1) ontogenetic shifts in habitat use may be widespread among fish taxa within both temperate and tropical reef systems; 2) patterns of habitat use may be a result of behavioural preferences; and 3) habitats which are preferred by early stages have the potential to cause population bottlenecks.