Mechanisms of density- and number-dependent population regulation of a coral-reef fish

Abstract

Density-dependent post-settlement losses are a common feature of many reef fish populations, and resent observations suggest that losses may also scale with population size independent of density (i.e. ‘number-dependence’). Despite the potential importance of these two forms of compensatory loss, there have been relatively few attempts to determine the mechanisms underlying these demographic patterns. A factorial experiment was conducted to test whether density- and/or number-dependent losses observed for newly settled six bar wrasse (Thalassoma hardwicke) are attributable to predation or another factor (e.g. migration). Losses of recently settled fish from reefs within predator exclosures were ~26% over a 7-day period and apparently independent of density and number of residents. Losses from reefs that were accessible to predators averaged ~62% over 7 days, and were dependent upon both the density and number of resident fish. Behavioural observations indicate the frequency of agonistic interactions between resident six-bar wrasse scales with the number of fish independent of density. Overall, results attribute both density-and number-dependent losses to predation, and suggest that number-dependent behavioural interactions (perhaps important for the social control of sex change) rather than proximate resource limitation may underlie observed number-dependent mortality.