Wakasagi in the San Francisco Bay Delta Watershed: Comparative Trends in Distribution and Life-History Traits with Native Delta Smelt

Abstract

Intentional introductions of non-native fishes can severely affect native communities. Wakasagi (Hypomesus nipponensis, referred to as Japanese Pond Smelt) are native to Japan and were once separated from their non-native congener the endangered Delta Smelt (Hypomesus transpacificus) of the San Francisco Estuary (hereon “estuary”) of California. Wakasagi were introduced into California reservoirs in the 20th century as forage fish. Wakasagi have since expanded their distribution downstream to the estuary, but less is known about Wakasagi’s current distribution status, biology in the estuary, and negative influences on Delta Smelt. In this study, we took a comparative approach by synthesizing long-term field monitoring surveys, modeling environmental associations, and quantifying phenology, growth, and diets of Wakasagi and Delta Smelt to describe abundance and range, trends of co-occurrence, and shared ecological roles between smelt species. We found Wakasagi in greatest abundance in the upper watershed below source reservoirs, and in the northern regions of the estuary with the most co-occurrence with Delta Smelt; however, their range extends to western regions of the estuary, and we found evidence of an established population that annually spawns and rears in the estuary. We found these smelt species have similar ecological roles demonstrated by overlaps in habitat use (e.g., an association with higher turbidities and higher outflow), phenology, growth, and diets. Despite similarities, earlier hatching and rearing of Wakasagi during cooler months and reduced growth during warmer drought years suggest this species is unlike typical non-natives (e.g., Centrarchids), and they exhibit a similar sensitivity to environmental variability as Delta Smelt. This sensitivity may be why Wakasagi abundance remains relatively low in the estuary.

Proofing Field and Laboratory Species Identification Procedures Developed for the Non-Native Osmerid Species Wakasagi (Hypomesus nipponensis) Using SHERLOCK-Based Genetic Verification

“…Wakasagi were introduced into reservoirs in northern California in 1959 as forage for trout (Wales 1962;Dill and Cordone 1997) but have since moved downstream into the estuary where they are caught in monitoring surveys with Delta Smelt (Aasen et al 1998;Davis et al 2022). To the untrained eye, Delta Smelt and Wakasagi can look nearly identical (Moyle 1995).…”

“…Even to the trained eye, distinguishing between Delta Smelt, Wakasagi, and potential hybrids can be challenging (Benjamin et al 2018). Disentangling population trends between Delta Smelt and Wakasagi has been particularly important in recent years as the Delta Smelt population has continued to decline, as indicated by record-low catch densities in long-term monitoring surveys, while Wakasagi presence in the estuary has persisted or possibly increased (Davis et al 2022).…”