Avian predation mediates size-specific survival in a Neotropical annual fish: a field experiment

Abstract

Predation and population density have fundamental size- and sex-specific effects on individual survival and demographic parameters. Given the overlap and interactions between different age cohorts in natural populations, separating the factors related to differential survival and growth based on longitudinal field-collected data is problematic. Using a Neotropical annual fish (Austrolebias minuano) with a single age cohort per generation, we used replicated field enclosures to experimentally test the roles of avian predation and fish population density on survival and growth over adult lifespan. We found that mortality risk was higher in larger males and smaller females when predation was experimentally excluded. Exposure to avian predation eliminated this sex-specific effect of body size on survival. No overall sex difference in survival was found in the experiment, despite a female-biased sex ratio in natural populations. Individually based growth rates were highest in enclosures at low population density with no predation risk. Overall, we demonstrate that annual fish suffer high sex-dependent size-specific mortality that is more strongly related to predation than to density-dependent processes. This has important implications for our understanding of the evolution of senescence and other life history traits in annual fishes.