Use of population fitness to evaluate the nursery function of juvenile habitats

Abstract

Juveniles of many fish and invertebrate species are able to select among a diverse port folio of nursery habitat alternatives. Environmental heterogeneity among these habitats generates variation in the vital rates of young individuals that may influence overall population dynamics. Therefore, understanding how these habitat options affect population fitness is crucial for identifying habitats that widen bottlenecks in early life histories and promote population persistence. We used cohort analyses and demographic models to explore the population-level consequences of habitat selection by juvenile California halibut Paralichthys californicus in southern California, focusing on population growth rate (γ) as a measure of fitness. Although alternative juvenile habitats (exposed coast and coastal embayments) could contribute an approximately equal number of recruits to the adult stock, positive overall population growth (γ > 1) depended critically on the subpopulations of juveniles that utilized coastal embayments (bays, lagoons, and estuaries). Conversely, the juvenile subpopulation along the exposed coast contributed negatively to overall population growth (γ < 1) in 3 of the 4 years we conducted this study, due to elevated local mortality in that habitat. Life table response experiments confirmed that juvenile growth and survivorship were responsible for differences in A, and that nursery habitat choice could be a key contributor toward overall population fitness. Considering nurseries in a demographic source-sink context could aid conservation efforts by allowing identification or prioritization of the juvenile habitats most critical for population persistence. © Inter-Research 2009.