Spawning Pacific lamprey in the Napa River. Photo: Joe Bauer
Spawning and rearing habitat for important forage-fish species in San Francisco Bay apparently shifts geographically by many miles depending on how much freshwater is flowing into the Estuary.
In a recent study
, a team of scientists found that in a dry year, Pacific herring and longfin smelt larvae occurred farther up the Estuary than in a wet year, when spawning and recruitment was pushed seaward. The results suggest that the fish have broader geographic ranges than previously believed, a finding that could inform efforts to manage and protect their habitat. Biologists have long assumed that longfin smelt, a protected species in steep decline for decades, spawn strictly in upper reaches of the Estuary. But by towing a weighted hoop net behind a research vessel during the dry year of 2016 and again in the following year of heavy rainfall, the authors of the new study, published in October’s San Francisco Estuary and Watershed Science, put this assumption to rest. In the wet study year, they captured many larval smelt in the shallows of San Francisco Bay. The small size of the young fish indicated that spawning could not have taken place far away. The scientists saw similar patterns among larvae of Pacific herring. The fish spend most of their lives in the open ocean but spawn in coastal estuaries, and the team’s net surveys produced evidence that in dry years “spawning may be occurring further landward than currently monitored by Pacific herring spawning surveys,” the authors write. “Consideration of a wider spawning area could help improve spawning biomass estimates and improve estimates of year-class strength.”