Seasonal and Size-Specific Occupancy of Striped Bass in the Stanislaus River, California.

Striped Bass (Morone saxatilis) monitoring and research in California's Central Valley primarily occurs in the Sacramento River basin and the Sacramento-San Joaquin Delta. A relatively under-studied contingent of the Striped Bass population is present in the San Joaquin River and its tributaries. One such tributary, the Stanislaus River, is an important source of natural production of native anadromous fishes, including fall-run Chinook Salmon (Oncorhynchus tshawytscha), Steelhead (O. mykiss), and Pacific Lamprey (Entosphenus tridentatus). Because Striped Bass are a non-native piscivore, characterizing when and where their distribution overlaps with native fishes is a first step to assessing the potential for negative inter-specific interactions. We compiled incidental observations of Striped Bass made during long-term (1996 to 2021) salmonid monitoring programs performed at different times of the year, and found that Striped Bass were present at monitoring locations throughout most of the year, although not detected in every year. We also used data on Striped Bass captured during boat electrofishing surveys conducted during the 2019, 2020, and 2021 juvenile salmonid emigration season (February to June), for a more detailed evaluation of Striped Bass occupancy within the lower 65 kilometers of the Stanislaus River. Median fork length (FL) of Striped Bass captured from February to April was significantly larger than those captured in May and June (p < 0.001). Using dynamic occupancy models, we showed that Striped Bass larger than 300 mm FL were present, and their occupancy increases earlier in the year than individuals under 300 mm FL. In all 3 years, occupancy estimates were greater than 80% of the lower Stanislaus River by May. Our results suggest a high degree of spatial and temporal overlap with native migratory fishes, which may have important implications for understanding and managing how predation affects juvenile salmon and other native species. [ABSTRACT FROM AUTHOR]