Long‐Term Shifts in the Lateral Distribution of Age‐0 Striped Bass in the San Francisco Estuary

Abstract

Abstract Like several other fishes in the pelagic community of the upper San Francisco Estuary, age‐0 striped bass Morone saxatilis have shown a major decline based on a midwater trawl sampling program that has been conducted for over 40 years. We hypothesized that the apparent decline in age‐0 striped bass might be partially attributable to a behavioral shift away from the channels sampled by the trawls. We found no evidence of an upstream–downstream shift in age‐0 distribution. Instead, age‐0 striped bass distribution remains closely associated with the low‐salinity zone of the estuary. However, the survey data suggest a substantial long‐term distribution shift away from channels and toward shoal areas. The hypothesis that young striped bass are undersampled by midwater trawls is supported by modeling of demographic patterns, which showed that the decline in numbers of age‐0 fish was not consistent with increasing trends in age‐1 fish. We hypothesize that reduced food availability in pelagic habitat is a major cause of apparent behavioral shifts by age‐0 striped bass and some native fishes. Nonetheless, the magnitude of the shift toward shoal habitat does not appear to fully account for the extreme decline in age‐0 striped bass abundance.