Predator Responses, Prey Refuges, and Density-dependent Mortality of a Marine Fish

Detection of density dependence in animal populations is a primary goal of population ecology, and the processes causing density dependence play a major role in population regulation. Predation can strongly regulate populations by populational and behavioral responses of predators to their prey. Here I evaluate the existence and strength of density-dependent mortality in local populations of a reef fish, the kelp perch (Brachyistius frenatus), caused by its predator, the kelp bass (Paralabrax clathratus). Specifically, I examine both the functional response and a potential aggregative response by kelp bass as mechanisms underlying predator-induced density-dependent mortality. I calculated the per capita mortality of kelp perch as a result of the functional response of its predator by subjecting a range of densities of kelp perch to a low, constant number of kelp bass and different amounts of giant kelp (Macrocystis pyrifera) as habitat structure and a potential prey refuge. The potential for an aggregative response by kelp bass was determined by exposing densities of kelp perch to natural levels of kelp bass in the field. Per capita mortality of kelp perch in the laboratory was inversely density dependent to density independent with increasing habitat structure. By contrast, per capita mortality in the field was strongly density dependent, with evidence for an aggregative response by kelp bass. Furthermore, emigration and other nonpredatory losses of kelp perch from field plots were negligible, indicating that the product of the functional and aggregative responses by kelp bass induced density-dependent mortality in kelp perch. My results indicated that increasing habitat structural complexity at low densities of kelp perch, coupled with a strong aggregative response by kelp bass, was responsible for the observed pattern of density-dependent mortality. In addition, the distribution and relative abundances of kelp perch and kelp bass at larger spatial scales (entire reefs) were consistent with predation as an important process structuring local populations of kelp perch. The contrasting patterns of short-term per capita mortality found here underscore the need to evaluate both the functional and aggregative responses of predators to their prey. Ultimately, the pattern of mortality expressed will be determined by the relative strengths of these behavioral responses and the degree to which habitat structure provides an effective prey refuge.