Effects of Habitat and Body Size on Mortality Rates of Poecilia Latipinna

We examined the effect of body size and habitat on mortality rates of juvenile and adult sailfin mollies (Poecilia latipinna) maintained in field cages that excluded predators. Mortality rates and the extent of size dependence varied among years and among seasons within years for juvenile and adult fish. When mortality rates were size dependent, large individuals had lower mortality rates than small ones. Differences between habitats in mortality rates were present in some experiments but not others. When there were habitat effects under winter conditions, both juveniles and adults had higher survival rates in a saltwater habitat than in a freshwater one. Some adults survived through the winter in all of our experiments. Adult mortality during the growing season exhibited a similar pattern of habitat effects although at lower mortality rates. Juvenile survival to sexual maturity in the growing season showed no consistent effects of habitat. Fish born during the autumn had a much lower probability of survival of sexual maturity than those born in spring or summer, and there was a threshold time beyond which newborn fish were virtually assured of perishing over the winter, presumably due to the inability to achieve a minimum body size capable of withstanding winter conditions. These experiments illustrate that the net direction of selection on molly body size in any generation depends on events that are stochastic on a yearly scale, such as the climatic severity of the winter and the intensity of predation pressure. Our results illustrate the complex interplay of different sources of selective mortality that operate in natural populations, and highlight the importance of ecological investigators of the different agents of mortality for a predictive understanding of the operation of natural selection.