Larval entrainment in cooling water intakes: spatially explicit models reveal effects on benthic metapopulations and shortcomings of traditional assessments

Spatial management of marine populations typically requires consideration of larval dispersal pathways. When a power plant with a cooling water intake system (CWIS) is present, managers must also account for reductions in larval supply to natural habitats due to larval entrainment in the CWIS. To evaluate the consequences of CWIS entrainment for benthic populations, we coupled a transport model for an idealized coastline to a spatially explicit metapopulation model. CWIS entrainment reduced the probability of dispersal to and from sites near the CWIS. However, the reductions in larval supply due to entrainment generally produced only minor, localized effects on adult population density because of postsettlement density-dependent mortality. Only when population densities were already reduced by other forms of adult or larval mortality did entrainment threaten population persistence. Our simulations suggest that subpopulations several kilometres upstream of CWIS make the greatest contribution to metapopulation persistence by countering the effects of CWIS entrainment, and these locations should be the focus of conservation efforts to enhance larval sources. Finally, we show that traditional statistics used to estimate the demographic effects of CWIS entrainment are generally inaccurate and unreliable because they ignore nonlinearities in population dynamics.