Mortality is associated with social rank in the clown anemonefish (Amphiprion percula)

Elucidating the causes of post-recruitment mortality is a vital step toward understanding the population dynamics of coral reef fishes. Predation is often considered to be the primary proximate cause of mortality. It has, however, proven difficult to discern the relative contributions of predation and other processes, such as competition for food, shelter, or mates, to patterns of mortality. To determine which other processes might be important drivers of mortality patterns, factors related to mortality in the clown anemonefish Amphiprion percula (Lacepede, 1802) were examined. Patterns of mortality will not be driven by predation in A. percula, because these fish are well protected from predators by their close association with sea anemones. Mortality rates were based on the disappearance of known individuals from a population of 201, in 57 groups, during a 1-year field study (in 1997), in Madang Lagoon, Papua New Guinea. Mortality rate of A. percula was low (14% per annum) compared to other coral reef fish, probably due to the protection from predators afforded by the anemone. Six factors (reef, depth, anemone diameter, number of individuals, density, and standard length) showed no association with the probability of mortality (P>0.05). Rank was the only factor associated with the probability of mortality (P<0.03); low-rank individuals (ranks 4-6) suffered a higher mortality rate than high-rank individuals (ranks 1-3) (P<0.01). The most likely explanation for this pattern was that competition for rank, amongst individuals within an anemone, resulted in some individuals evicting their subordinates. Individuals probably competed for rank because it conferred access to reproduction, and not because it conferred access to food or shelter. Such competition for reproduction will be intense whenever some individuals obtain a greater share of reproduction than others do, and it may be an important process influencing the dynamics of coral reef fish populations.