| Summary: | <p>The introduced yellowfin goby (YFG) is now common throughout the tidal portion of the Sacramento-San Joaquin River system. We investigated its habitat use, size and diet in the Mokelumne River, an eastern tributary to the system. Catch per unit effort (CPUE) by boat electrofishing was significantly higher in the fall than all other seasons sampled over four years. Habitat type, turbidity and dissolved oxygen were not significantly related to CPUE between years and seasons. Temperatures were negatively related to CPUE in fall sampling only. Delta outflow and CPUE showed a significant negative relationship. Fish captured ranged from 27 mm SL (standard length) (33 mm TL (total length)) to 155 mm SL (196 mm TL) (mean 113 mm SL (138 mm TL)), with largest fish captured during the fall. Seasonal and annual analysis showed a diet composed of a variety of macroinvertebrates, including chironomids, gammarids, aquatic isopods and ephemeropterans with no difference in composition among seasons or years. The YFG’s generalist diet gives it an advantage as a successful invader, but salinity requirements appear to limit its expansion potential.</p>
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