Effect of power plant entrainment on the population dynamics of the bay anchovy ( Anchoa mitchilli)

A model of competing sources of mortality is presented for a bay anchovy ichthyoplankton population subject to entrainment losses. Central to the calculation of entrainment impact through modeling is the generation of evolving spatial-temporal distributions for the various life stages from the egg distribution. The model starts with egg distributions over time and space; these eggs develop through time and are transported passively by estuarine hydrodynamics. Superimposed on these processes are adjustments for active movement, development into the next age category, mortality due to natural causes, and entrainment mortality at the location of the power plant. The fitting process is iterative and uses variations in the spatial/temporal magnitudes of natural mortality within each stage and in the active movement rates to obtain the best fit with observed distributions. In the best fit of the model-generated distributions with observed data, fractional entrainment loss of 76% is estimated for ichthyoplankton that would have reached the juvenile stage without plant-related mortality. Sensitivity analyses that gauge a reasonable range for this estimate from uncertainties in the distribution data, in larval avoidance, and in cooling system mortality bracket the probable loss at 24–76%. However, results from most of the sensitivity runs indicate that more than 50% of the anchovies are lost by the beginning of the juvenile stage.