Department of Water Resources Research Project 2

Steelhead Pre-Screen Loss in Clifton Court Forebay

Clifton Court Forebay is operated as a regulating reservoir within the Sacramento-San Joaquin Delta to improve operations of the State Water Project’s Banks Pumping Plant at the head of the California Aqueduct.  Clifton Court Forebay is tidally filled with water that is diverted from Old River through the operation of five radial gates.  When the radial gates are opened, fish are entrained in Clifton Court Forebay and must make a minimum 2.1 mile crossing of the Forebay before reaching the John E. Skinner Fish Protection Facility.  Losses of fish during movement from the radial gates to the fish protection facility, termed pre-screen loss, include predation by fish and birds.  The NOAA Fisheries OCAP biological opinion (2004) requires investigations to (1) quantify predation losses on juvenile steelhead within Clifton Court Forebay, and (2) identify potential management actions to reduce predation mortality of juvenile steelhead.  The steelhead pre-screen loss investigation is a pre-condition to increasing State Water Project export rates to 8,500 cfs.

The steelhead pre-screen loss investigation is a collaborative effort with participation from the DWR Fishery Improvements Section, DFG Fish Facilities Unit, the USBR Fisheries and Wildlife Resources Group, and Hanson Environmental, Inc.  Multiple tagging and monitoring technologies are being used to address the various research objectives.

A three year experimental field investigation was designed and initiated to:

  1. Evaluate predation losses of juvenile steelhead during passage through Clifton Court Forebay
  2. Evaluate behavior and movement patterns of adult striped bass which have been identified as the primary predatory fish species that would potentially prey on juvenile steelhead within Clifton Court Forebay
  3. Identify physical locations and environmental factors that contribute to increased vulnerability of juvenile steelhead to predation within the Clifton Court Forebay
  4. Develop quantitative estimates of pre-screen losses of juvenile steelhead within the Clifton Court Forebay 

Based on the findings of a 2005 and 2006 pilot study conducted by DWR, DFG, and Hanson Environmental, an experimental design and protocol for conducting the steelhead pre-screen loss study in 2007 was developed and employed .  A stratified mark-recapture experimental design was developed utilizing two complementary tagging technologies, acoustic tags and Passive Integrated Transponder (PIT) tags.  Juvenile steelhead were obtained from the Mokelumne River Fish Hatchery and a small portion of these steelhead were surgically implanted with Vemco V9 acoustic tags prior to release.  The majority of the steelhead were implanted with PIT tags prior to release.  In order to simulate the exposure to the high water velocities and turbulence experienced by wild fish entrained into the Forebay, tagged steelhead were released immediately upstream of the radial gates in groups of 10 or 20, using a specially constructed live-car.  Tagged steelhead were also released directly downstream of the trashracks utilizing a bucket technique.  Releases at both the radial gates and trash racks occurred during the day and night based on routine radial gate and routine pumping operations.  Adult striped bass were identified as the primary predator species responsible for the pre-screen loss of steelhead.  To investigate predator-prey interactions within the Forebay adult striped bass were collected in the Forebay using hook and line and gill nets. The striped bass were then externally tagged using Vemco V13 acoustic tags, and subsequently released back into the Forebay.  Movement of the acoustic tagged juvenile steelhead and adult striped bass was monitored continuously using 31 fixed-position VR2 acoustic receivers deployed adjacent to the radial gates, within the Forebay, within the John E. Skinner Fish Protection Facility holding tanks, and within Old River.  Mobile monitoring, using two VR100 receivers, was also conducted to track the movements of the tagged fish throughout the above locations.  PIT tagged steelhead were detected post salvage at the State Water Project (SWP) release sites near Horseshoe Bend and Curtis Landing via an automated PIT receiver detection system.

The abundance and foraging activity of avian species was also monitored to determine the prevalence of avian predation on fish entrained into the Forebay.  Point count surveys were conducted in Clifton Court Forebay to identify the avian species that occur and forage within the Forebay.

Operational and environmental parameters were monitored during the study and included radial gate operations, SWP export rates, water temperature and turbidity within the Forebay.

The field work for the 2007 full-scale study was completed on June 20, 2007.  A final report was released and is available at   DWR continues to maintain VR2 receivers near the Clifton Court Forebay radial gates in support of the California Fish Tracking Consortium.