Effects of predator and flow manipulation on Chinook salmon survival in an imperiled estuary

Previous studies indicate that both river flows and predation by non-native predatory fishes, such as largemouth bass, striped bass, and sunfish, can influence survival of out-migrating juvenile salmon in the Delta. This study was the first of its kind in the Delta to manipulate experimentally both predators and flows to assess responses in juvenile salmon survival and migration rate.  Chinook salmon with acoustic tags were released in study reaches before and after predator removal using electrofishing. Salmon survival improved significantly after the first predator removal, but survival rates returned to pre-removal levels one week later. Opening of the Delta Cross Channel (DCC) gates caused hydrodynamic conditions in the study area to change dramatically from tidal, bi-directional flow, to riverine, unidirectional flow. Riverine conditions associated with DCC opening significantly increased salmon migration rate and improved survival for 3 out of 4 salmon release groups. The predator removal experiment demonstrates that reducing the abundance of predators can improve survival, but that the effect attenuates rapidly as new predatory fishes move into the removal area. Management actions that produce a sustained change in predator abundance or species composition appear to be required. That riverine conditions can improve juvenile salmon survival relative to tidal conditions is consistent with previous studies. While the study result is informative, flow changes downstream of the DCC gates are not representative of flows that can be achieved by managed releases of stored waters from reservoirs. For example, to produce a hydrodynamic change in the adjacent Sacramento River equivalent to our DCC experiment (that is, a change from tidal to riverine), Sacramento River flows would have to be increased by 15 times the baseline. That magnitude of flow change occurs naturally during major precipitation events but seems to be impractical and prohibitively costly to provide flow releases of that magnitude from managed reservoir releases.

Cavallo B, Merz J, Setka J. 2012. Effects of predator and flow manipulation on Chinook Salmon (Oncorhynchus tshawytscha) survival in an imperiled estuary. Environmental Biology of Fish DOI 10.1007/s10641-012-9993-5

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