Behavior and Modeling of Predator-Prey Interactions in The Dalles Dam Tailrace

The Problem

Recent juvenile salmonid survival studies conducted at The Dalles Dam (Columbia River) suggest that survival at this dam may be reduced at higher levels of spill. Conditions in the tailrace are relatively unique compared to other projects in the basin. The river environment is relatively shallow, with bedrock islands and a slough-like habitat on the south side of the river. At higher levels of spill, juvenile salmonids may be redirected into island habitats where their vulnerability to predators such as northern pikeminnow (Ptychocheilus oregonensis) and smallmouth bass (Micropterus dolomieu) may be higher. We hypothesize that predation may be an important component of the lower relative survival observed at higher levels of spill. Predation by resident fish is known to be a substantial cause of juvenile salmonid mortality, especially in dam tailraces. Behavior and habitat use by northern pikeminnow, the primary predator on juvenile salmonids, may be

Northern pikeminnow and juvenile salmon removed from gut.

influenced by changes in facility operations; less is known, however, about smallmouth bass behavior in near-dam areas. Predators are also opportunistic and respond to changes in the availability and abundance of juvenile salmonids as prey items. Thus, if dam operations concentrate or direct juvenile salmonids into eddies, low velocity areas, or near structures occupied by predators, risk of mortality may be elevated beyond that incurred through other dam passage routes.

Objectives

The goal of this subtask is to provide fishery managers and decision makers with detailed information on the ecology of predatory fishes (northern pikeminnow and smallmouth bass) and juvenile salmonids in The Dalles Dam tailrace. Such information will help identify potential predation risks to juvenile salmonids in relation to project operations, spill, and total discharge, and thereby assist the region in developing operational procedures and guidelines. Our specific objectives are 1) use radio telemetry to determine the distribution and movement of northern pikeminnow and smallmouth bass in The Dalles Dam tailrace and 2) use radio telemetry to determine the distribution and movement of juvenile salmon in The Dalles Dam tailrace, concurrent with monitoring of predator movements.

Methodology

Radio-tagged smallmouth bass.

This study uses radio telemetry to monitor the movements of juvenile salmon and predators. Predators are collected within the study area by electroshocking and angling. Radio transmitters are surgically implanted into the body cavity. Fish are released after they recover from anesthesia, but data collection does not begin for 7 d, to allow time for the fish to recover.

Radio-tagging a smolt.

Juvenile salmon are collected at the Smolt Monitoring Facility (SMF) at John Day Dam and transported to The Dalles Dam. Small (< 2 g) radio transmitters are implanted into the fish by inserting them into the stomach. The transmitter has a 30 cm trailing antenna, which is bent near the mouth of the fish so that it trails behind the fish as it swims. Radio-tagged juveniles are held in river water for 24 h following tag implantation, and are then released into the sluiceway.

Radio-tagged fish (both juvenile salmon and predators) are monitored by both boat tracking and by fixed yagi antenna arrays. The fixed arrays line the downriver aspect of the dam to monitor each spillbay and turbine unit. These arrays are linked to a data-logging telemetry receiver. Fixed arrays allow us to constantly monitor the near-field movements of fish. To get specific

Tracking radio-tagged fish in the field.

route of travel information, we monitor the movements of fish through intensive boat tracking. A boat is equipped with a yagi antenna, telemetry receiver and a GPS. The signal from the fish is located, and the boat positions itself directly over the fish's location (as best as possible). The GPS coordinate of this position is recorded. Fish locations are input into a Geographic Information System for display and analyses.

Highlights and Key Findings

This study was designed to build upon predator radio-tracking work that was completed in the early 90s. Previous studies have monitored pikeminnow movements near The Dalles Dam in order to assess the risk of predation to juvenile salmon. The movements of the predators were monitored, and assumptions were made about the potential for predator-prey interactions. Less is known about smallmouth bass behavior in near-dam areas. This study will improve our understanding of bass movements and will show predator and prey movement paths under the same conditions. These data can be used to generate predictive models to improve our understanding of factors that influence juvenile salmon survival.

Currently, the data from the field component of this study have not been analyzed, therefore no key findings are available.

Where Are We Headed In 2003

We completed the field component of this study in September 2002 and we are currently proofing, analyzing and presenting the study findings. An annual report of research will be available in January 2003. A presentation of our research findings was given at the Anadromous Fish Evaluation Program Review (sponsored by the U.S. Army Corps of Engineers) in November 2002. As resources allow, further analyses may be conducted in an effort to model the behavior of predators and prey in this tailrace.

Project Contact

Theresa Liedtke
U.S. Geological Survey
Western Fisheries Research Center
Columbia River Research Laboratory
5501-A Cook-Underwood Rd.
Cook, WA 98605-9717

Email: theresa_liedtke@usgs.gov
Phone: 509-538-2299
Fax: 509-538-2843

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