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2008 Tracy Research Study Plan

Evaluation of the Abundance of Large Striped Bass Predators in the Primary Channel Between the Trashrack and Louvers at the TFCF

Principle Investigators

Brent Bridges, Fishery Biologist, USBR- Tracy Office BBridges@mp.usbr.gov

Study Summary

Fish entrained into the Tracy Fish Collection Facility (TFCF) must pass by numerous predators at various locations in the system before they reach the holding tank and are released back to the wild. In each of these areas, where predators are known to occur, the predator load must be estimated to determine the impact they are having on the salvage efficiency of the system. Currently, we have no estimate of the number of predators residing between the trashrack and the primary louvers as this area can not be dewatered. The number of predators in this area has the potential to change daily because the louvers are lifted for cleaning, allowing predators in the Delta Mendota Canal (DMC) to travel to this location. Thirty striped bass will be tagged and released into the primary channel at five locations. The Peterson mark-recapture technique will be used to estimate predator abundance in the primary channel. Gill nets will be used for three hours to collect wild and tagged fish in order to determine their relative proportion and derive an estimate of the number of predators present.

Problem Statement

California exports approximately 6.0 million acre ft. of water annually from the Sacramento/San Joaquin River Delta (Delta) to Central and Southern California for agriculture, municipal, and industrial needs. The water is diverted and transported by two primary pumping facilities: Jones Pumping Plant (JPP) and Harvey O. Banks Delta Pumping Plant. Both sites are equipped with fish salvaging facilities upstream of the pumping plants to reduce the number of fish entrained at the pumps. Both the federal Tracy Fish Collection Facility (TFCF) and state Skinner Delta Fish Protective Facility use a behavioral louver-bypass system to guide fish out of the canal and into collection tanks, where they are held until they are transported back to the Delta. Efficiency of the behavioral louvering systems to guide fish through each facility is potentially influenced by large predators residing near the louvers by chasing the prey items through the louvers. In addition, predators can directly influence salvage rates by eating the prey.

The potential negative influence of large striped bass (>45 cm FL) residing in the primary channel (between trashrack and louvers) on fish entering the TFCF is recognized as a problem, yet there is currently no estimate of their abundance. Abundance data would be useful in determining how large an impact the predators are having on the salvage efficiency of the facility. Population estimates should be conducted throughout the year to see how their influence may change with time. Unfortunately, for most of the year the water velocity in the primary channel is too fast to sample effectively. There are times when the water velocity is slowed down for maintenance or research, and during these times sampling could be completed.

The method selected for estimating the number of large striped bass in the primary channel is the Peterson mark-recapture method used for a closed population of fish. The primary channel is nearly a closed system for large striped bass > 18 in FL (45 cm) when the louvers in the primary channel are not being cleaned. This is because fish of this size can not swim through either the 1in (2.5 cm) diameter louvers or 2 1/8 in (5.3 cm) trashrack. The only location the predators can travel is into the primary bypasses and these can be closed for short durations while the study is being completed.

Pumping at the JPP should be limited to no more than three pumps for this study in order to facilitate the release and re-capture of fish and ensure that the net does not become tangled on facility equipment. Success of the study would also depend on our ability to contain the predators in the primary channel. This would require that the tagged fish be graded to ensure that they cannot fit through the louver arrays (1 in spacing) and through the trash-rack (2 1/8 in (5.3 cm) spacing). In addition, the four bypass tubes will also have to be closed for the duration of the experiment (4 hours; 1 hour acclimation/mixing, 3 hour fish with gillnets).

Guiding Question/Hypothesis

How fast can the water flow in the primary channel be and still enable sampling for the mark and re-capture experiment?
How many striped bass predators take up residence at the TFCF between the trashrack and primary louvers?
What are the species sizes and sex of predators?
Where are the predators residing? (i.e., behind the trashrack, along the northern wall, or at the end of the channel between bypasses 3 and 4).

Assumptions and Study Limitations

The primary assumption of this study is that the tagged predators, once released into the channel, will be caught at the same frequency as the wild fish.
The wild and domestic predators share the same distribution in the primary channel.
Predators in the primary channel consist primarily of larger individuals, in order to be capable of maintaining their position in the channel.
The primary canal with the bypasses closed represents a closed system with no escape routes.

Methods/Approach

Thirty striped bass will be caught by fishing within the primary channel and held in the Tracy Aquaculture Facility (TAF) in a large 1,500-g tank. Measurements (fork length, [mm], width [mm], and wet-weight [Kg]) will be recorded and all fish will be assigned a unique tag for their release location. Only fish sufficiently large to be retained by the trash rack (> 2.5”, 6.3 cm width) will be selected for the study. This will ensure that the predators released remain within the confines of the primary channel.

The predator release will take place immediately after the morning primary louver cleaning (0900 hr) if there are three or fewer pumps on at the Jones Pumping Plant. Once cleaning is complete the four bypass tubes will be closed to make the primary channel a closed system. The louver cleaning cycle is critical as the predators can escape from the primary channel into the DMC when the louvers are raised for cleaning. The predators will be released in the primary channel behind the trashrack and will be evenly distributed among five locations:

North side of trashrack
South side of trashrack
Bypass number four
North wall 1/3 distance from trashrack
North wall 2/3 distance from trashrack

One hour after releasing the predators, three 20 Lb monofilament, gill nets (6” stretch mesh) will be used to fish in the primary channel. We will collect as many tagged and wild predators as possible for three hours. All fish caught will be measured, sexed, and the tags recorded; these will likely die as a result of the gill net.

Coordination

This study proposal will be given to Tracy Technical Advisory Team (TTAT) to review before any data is collected. All necessary permits will be obtained to do this work including ESA Sect 10 permits. This work will be coordinated with other Tracy researchers and operational staff at the TFCF. Work will be performed on site during VAMP (April 15, 2008) and other time intervals when the JPP has reduced flow (e.g. maintenance). Data collection will continue for up to three years and a draft report for peer review will be finished in January 2011. The draft report for TTAT will be completed in March 2011.

Endangered Species Take Issues

This study will not involve handling or capturing endangered or threatened species; however, salmonid “take,” as defined by the National Marine Fisheries Service, will occur. Therefore, a Section 10 permit has been submitted for this work. No permits are being applied for delta smelt take as it is covered under our biological opinion.

Data Analysis/Interpretation

Predators are able to hold up at numerous positions within the facility. The data collected are only estimating the number of predators in the primary channel and not in the entire facility. The gill nets are not 100% effective in capturing fish and this is why a mark-recapture technique is necessary to quantify the total number of predators. This project is a starting point to understanding the predator impact at this location in the facility.

The Lincoln/Peterson Mark Recapture Method will be used to estimate the number of large predators residing in the primary canal. The equation compares the ratio of total number of marked individuals divided by our population estimate to the number of tagged fish caught in the gill net divided by the total number of fish caught in the gill net. Note our tagged fish are included in the population estimate and will have to be corrected for.
formula
The equation above provides an estimate of the population size. A 95% confidence interval (95% CI) will be determined to learn how confident we are in the estimate. The equation below will be used to calculate the standard error of our population estimate (N) and then a 95% CI will be estimated:

formula

N = population estimate
C = Total caught in the gill net
M2 = Number of marked fish caught

Dissemination of Results

A Tracy Series Report volume will be prepared and published upon completion of the study. Progress updates and presentations will be provided internally upon request by TTAT and other interagency technical forums. Data collection will continue for up to three years and a draft report for peer review will be finished in January 2011. The draft report for TTAT will be completed in March 2011.

Updated December 17, 2007