Department of Water Resources Research Project 3

Release Site Predation Study

The California Department of Water Resources (DWR) with technical support from the U.S. Bureau of Reclamation (USBR) and California Department of Fish and Game (DFG), completed a research study to investigate the survival of salvaged fish during the release phase of the Collection, Handling, Transport, and Release (CHTR) process.  Anecdotal observations by recreational anglers have indicated that predatory fish are concentrated near the points of release of fish salvaged at the state and federal export facilities and transported and released into the central Delta.  Field observations have also documented the attraction of predatory birds to the areas when releases of salvaged fish are being made. Preliminary observations using the DIDSON camera system at the State Water Project release sites provide further evidence of the occurrence of predatory fish at the release sites.  The experimental design, methods, and approach for evaluating predator abundance and behavior within the receiving waters at the existing release sites included five different, but interrelated, study methods: predator sampling (electroshocking and avian predation observations), mark-recapture (acoustic & Floy tagging), DIDSON acoustic camera observations, hydroacoustics, and a hypothetical predation risk analysis using bioenergetics.

The primary objective of the Release Site Predation Study was to develop quantitative and qualitative information for use in assessing the potential magnitude of predation mortality within the near-field receiving waters at the release sites.  Field studies focused primarily on the SWP Horseshoe Bend release site.  The Release Site Predation Study was intended to provide additional information on the geographic distribution and behavioral patterns of predatory fish at release sites and to provide the necessary scientific and technical information for assessing predation as a factor affecting survival of salvaged fish.  In the event that predation mortality was identified as a significant factor, the results would provide a foundation of information useful in identifying and evaluating potential alternative technologies designed to reduce or avoid predation mortality of released fish.

We used several different technologies including DIDSON sonar, hydroacoustics, acoustic telemetry of predators, and traditional sampling to evaluate the abundance, composition, behavior, and site fidelity of predators at the salvaged fish release sites

DIDSON and Hydroacoustic surveys were conducted every other month from August 2007 through April 2008 to determine predator abundance and behavior.  Intense surveys using fixed mounted equipment were conducted at the SWP release site at Horseshoe Bend on the Sacramento river, while mobile monitoring using boat mounted equipment was conducted at the 2 other release sites and at 2 control sites.  The results of these surveys will be used to develop a hypothetical predation risk model to estimate the number of salvaged fish potentially lost to predation and to identify potential changes to salvage operations to deter predators.
At the conclusion of each DIDSON/Hydroacoustic survey, predatory fish (striped bass and Sacramento Pikeminnow) were collected (electrofished) and acoustically tagged with Vemco tags by the Department of Fish and Game (under contract with DWR) in the vicinity of the release sites.  These fish were subsequently tracked using an array of VR2 receivers deployed at all the release sites and within the vicinity of Sherman Island.  The results of these tracking efforts will be used to determine whether or not predators exhibited site fidelity or attraction to the release sites.

Data collection for this study was completed in April 2008.  A final DWR technical report will be available in late 2009.  VR2 receivers will be maintained at each of the salvaged fish release sites for the benefit of other consortium researchers.