Classification of Larval and Adult Delta Smelt to Nursery Areas by Use of Trace Elemental Fingerprinting

Different environmental conditions among habitats may generate unique elemental patterns within fish otoliths that can be used to trace the life history as well as the potential natal origin of migratory species. We investigated the use of trace elements in otoliths as natural tags for determining the natal origins of larval and adult delta smelt Hypomesus transpacificus within a single estuary. Larval fish were collected at potential natal sites within the San Francisco Bay Estuary—the North, Central, South, and West Delta areas, Suisun Bay, and Napa River—during May–June 1999; adults were collected in November 1999 throughout Suisun Bay. Using laser ablation inductively coupled plasma mass spectrometry, we assayed trace elements from core (natal) regions of the otolith (Sr:Ca, Mg:Ca, and Ba:Ca ratios). Linear discriminant function analysis (LDFA) was 90.9–100% successful at classifying larval fish to their natal habitats (Napa River, Sacramento River, and Delta). Adults of unknown natal origin were assigned to their natal regions using the larval fingerprints from LDFA and a maximum likelihood mixed‐stock approach. For the 1999 year‐class, we determined that a majority of the population originated from the Delta (77–79%) and a small but significant proportion of the population originated from the Napa River (16–18%) and Suisun Bay (4–8%). These data highlight the value of trace elements as natural tags for determining the natal origins of young fish and the relative contribution of different habitats to the adult population within a single estuary.