Utilization of Multiple Tracers to Reassess the Life-History, Movements, and Connectivity of Threatened Osmerid Smelts in San Francisco Bay

Osmerids are temperate forage fish that were historically a dominant and characteristic feature of San Francisco Bay and its associated wetlands and estuaries. Over the last 40 years, the San Francisco populations of Longfin Smelt (Spirinchus thaleichthys) and Delta Smelt (Hypomesus transpacificus) have collapsed to just 1% of their historic abundance, resulting in state and federal protections and targeted conservation efforts. Despite many decades of intensive monitoring throughout the region, multiple conflicting descriptions of the life histories, spawning habitats, and movement patterns of these species have persisted, each with important implications for water policy decisions that impact California’s multi-billion-dollar agriculture business and 29 million southern residents.

We have utilized geochemical analyses of otoliths (including 87Sr/86Sr, δ18O, and trace element fingerprinting) and population genetic tools to reconstruct movement patterns, life-history strategies, and assess connectivity among population segments of these imperiled native fishes. Results of otolith geochemical analyses have identified previously undescribed and unique life history patterns for both species that challenge existing life-cycle models and resultant conservation policies. Furthermore, otolith elemental fingerprints have been used to identify distinct natal rearing zones for each species and genetic studies of Longfin Smelt indicate unidirectional gene flow along the California coast. Results indicate broad diversity in individual life history strategies that likely facilitates population resilience within a highly dynamic environment. These results may change our fundamental understanding of the population dynamics of these species and our approaches to managing and protecting their populations.