CVPIA Predation Contact Point Study - 2021: Impacts of Water Diversions in the Sacramento – San Joaquin Delta

The Central Valley Project Improvement Act (CVPIA) has led to the implementation of a Decision Support Model (DSM) to assist in the prioritization of CVPIA restoration actions. The fall-run Chinook salmon (Oncorhynchus tshawytscha) DSM depends on a coarse-resolution salmon life-cycle model to predict the population benefits of different restoration actions and scenarios. One critical element of the life-cycle model is how to incorporate predation mortality during the juvenile rearing and outmigration portion of the salmon life-cycle in the Sacramento-San Joaquin Delta (the Delta). Of particular importance to potential restoration activities, is the predation mortality that occurs in proximity to, and as a result of contact points between predator and prey fishes. Water diversions that support agricultural and municipal use result in fish mortality through entrainment and impingement. Additionally, this infrastructure may attract both predators and prey fishes, thereby increasing predation rates and prey mortality near these anthropogenic contact points.
Throughout the spring of 2021, we used ARIS (adaptive resolution imaging sonar; Sound Metrics) sonars to compare piscivore abundance at 30 small water diversions in the north Delta to adjacent shorelines. We used predation event recorders (PERs) to assess the predation risk of juvenile salmonids with linear distance (m) from diversions and other predation drivers in the north Delta. Finally, we used a boat electrofishing survey to determine the piscivore community structure and compare spatial trends in black bass (Micropterus spp.) CPUE and relative abundance throughout these waterways.
Piscivore abundance was greater near small water diversions than at adjacent shorelines and the predation risk of juvenile salmonids increased with proximity to diversions. Additionally, predation risk increased with increasing piscivore abundance and decreasing water depth. The north Delta predator community was dominated by black basses (Micropterus spp.), which likely drove the negative relationship of predation risk with water depth, given habitat requirements of these species. Furthermore, increased smallmouth (Micropterus dolomieu) and Spotted Bass (Micropterus punctulatus) prevalence in our study sites compared to past Delta predation studies, may have diminished the commonly seen temperature effects on predation, given metabolic requirements of these species.