San Francisco Estuary and Watershed Science

Intentional introductions of non-native fishes can have severe consequences on 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 (CA). Wakasagi were introduced into CA 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 and 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.