Assessing coastwide effects of power plant entrainment and impingement on fish populations: Atlantic Menhaden example

A method was developed for assessing coastwide effects of power plant impingement and entrainment on managed fish stocks. The method imbeds an assessment of anthropogenic effects that occur during the first year of life of fish (the period when most entrainment and impingement occur) into a standard age-structured stock assessment that addresses age-l and older fish. The method thereby provides a straightforward means for comparing the effects of entrainment and impingement mortality to other forms of anthropogenic mortality affecting coastwide fish stocks. The method uses standard equations from fishery science that represent the relationships among independent competing forces of mortality, stock abundance, and landings. Power plant mortality is treated like fishing mortality, and power plant losses are treated like fishery landings. The total age-0 natural mortality rate is allocated to the individual age-0 life stages based on a power function relating daily natural mortality rates to age-specific dry weights of fish. An illustrative example of the use of the method is presented for the Atlantic coast stock of Atlantic menhaden Brevoortia tyrannus, which was chosen because it is a coastwide stock, has a coastwide fishery, and is described by high-quality fisheries-dependent data. However, because available entrainment and impingement data were not adequate to support defensible coastwide estimates of the annual numbers killed by entrainment and impingement, actual estimates of the effects of entrainment and impingement on the coastwide Atlantic menhaden stock could not be computed. The method could be used to address the effects of any form of anthropogenic mortality affecting age-0 fish, including loss of habitat, effects of toxic substances, fishing mortality, and fishery bycatch, provided that valid coastwide estimates of the annual numbers of fish killed by the source of mortality are available.