Individual‐Based Modeling of Delta Smelt Population Dynamics in the Upper San Francisco Estuary III. Effects of Entrainment Mortality and Changes in Prey

Abstract

AbstractWe used an individual‐based model, developed previously for the endangered, endemic Delta Smelt Hypomesus transpacificus, to investigate two factors widely believed to affect its abundance in the San Francisco Estuary: entrainment in large water diversion facilities and declines and species shifts in their zooplankton prey. Previous analyses suggested that these factors had substantial effects on the Delta Smelt population, although evidence is accumulating that other factors, such as contaminants and predation, are also having effects. Simulations were performed for 1995–2005 with either entrainment mortality set to zero or zooplankton biomasses replaced with values sampled from pre‐decline years. The detailed individual‐based and spatial model output was summarized as the annual finite population growth rate (λ). Eliminating entrainment mortality increased the geometric mean λ by 39% through increased survival of larvae and adults. Substituting historical food for present‐day food resulted in variable annual λ values with a geometric mean that was 41% greater than the baseline value (14–81% across 10 alternative food scenarios). Historical food caused higher juvenile consumption and growth rates, leading to larger recruits, earlier maturity, and higher individual fecundity. These results were robust to four sets of simulations using alternative formulations for density dependence, mortality, maturity, and larval growth.