A test of the variable nature of density-dependent mortality in fish populations

Veracity of past analyses on the impact of impingement and entrainment of fish in power-plant cooling systems was questioned. Density-dependent dynamics were hypothesized as variable, not constant, making previous analyses based on constant density-dependent or density-independent mortality in error. This hypothesis was tested with a simulation model based on complex biological and behavioral mechanisms for individual fish, thereby forgoing the need for assuming the kind of mortality exhibited by a fish community prior to the analysis. Yellow perch and walleye in Oneida Lake, New York, were focal points of this test, and data taken in 1972 from Oneida Lake were used to validate the model. Model experiments consisted of varying turbidity, water temperature, impingement and entrainment, and abundance of young perch and walleye. Under most conditions in the experiments, individual growth was unchanged, and predators maintained a high frequency of full stomachs. Exceptions were that water temperature influenced growth and that adult walleyes went hungry when fry of both species were limnetic. Mortality of young perch proved depensatory and that of young walleye density-independent. Mortality of walleye fry was inversely influenced by abundance of young perch. High levels of turbidity greatly enhanced fry survival for both species. Impingement and entrainment reduced survival of young perch and young walleye by 43 and 30 percent, respectively. Expected losses due to density-independent impingement and entrainment were 3 and 2 percent. Depletion of young perch occurred because of their depensatory mortality; depletion of young walleye occurred because their survival was related to abundance of young perch. High abundance of young perch and high turbidity dropped mortality to density~independent levels of about 3 percent. These mitigating effects of turbidity and high abundance showed that mortality could shift from density-dependent to density-independent dynamics. The relationship between abundance of young perch and mortality of young walleye meant that no analysis based on a single species could properly assess impacts of impingement and entrainment in Oneida Lake. The conclusion was that density-dependent and density-independent mortality are variable and are influenced by certain conditions: 1) compensatory mortality is based on density-dependent growth, 2) density-~independent and depensatory mortality are based on density-independent growth, 3) density-independent mortality occurs when predation removes insignificant numbers of prey, and 4) depensatory mortality occurs when predators remove large numbers of prey.