The Distribution and Flux of Fish in the Forebay of The Dalles Dam in 2003

Abstract

In spring and summer 2003, the Pacific Northwest National Laboratory led a team that conducted mobile and fixed hydroacoustic surveys in the forebay of The Dalles Dam for the U.S. Army Corps of Engineers-Portland District. This research was part of the Corps' Anadromous Fish Evaluation Program. The surveys provided information on the distribution and movement of smolt-sized fish relative to ambient factors such as flow, bathymetry, or diel cycle in the forebay at The Dalles Dam. A proposal for the use of a guidance structure in the forebay at The Dalles Dam, a modified version of a similar structure located at Lower Granite Dam, has recently been suggested by the U.S. Army Corps of Engineers and regional resource managers as a potential approach to improve the passage survival of juvenile salmon at the dam. The structure would be designed to divert fish from the powerhouse to the spillway. This project provided baseline data for the development of a behavioral guidance structure and surface bypass alternatives for juvenile salmon at The Dalles Dam. We sampled the forebay of The Dalles Dam one day and night each week for six weeks in the spring and another six weeks in the summer. Two research vessels were used. Each pushed a raft outfitted with sampling gear consisting of two split-beam transducers, four single-beam transducers, one acoustic Doppler current profiler (ADCP), a pitch-roll-heading indicator, and a differential global positioning system (GPS). The split-beam transducers provided information on the location, size, and movement of fish. The ADCP sampled the flow environment in which each fish was detected. One 12° split-beam transducer was aimed downward. The other split-beam transducers and the four single-beam transducers were all forward-looking 6° beams that were aimed to provide intensive sampling in the upper 9 m of the water column. The rafts were secured 7.5 m forward of the bow of the research vessels (15 m from the vessel's outboard motor) to minimize fish avoidance behavior. Mobile sampling was conducted from a research vessel and raft moving in a zigzag pattern extending from 180 m above the spillway to 1.8 km upstream of the spillway along 26 transects during each sampling period. A second research vessel sampled at 15 fixed-point locations for ten minutes at each point. From the fixed sampling we determined the rate and the direction of fish movement past those points (flux). Using the combined mobile and fixed sampling methods we were able to determine the distribution of smolt-sized fish and their movement patterns in the forebay. Smolt-sized fish were defined as those with a return signal of greater than-56 dB re||1µPa and less than-34 dB for spring fish (90-320 mm) or less than-45 dB for summer fish (90-105 mm). The species of smolt-sized fish that were targeted for springtime samples were juvenile steelhead trout (Oncorhynchus mykiss), juvenile yearling Chinook salmon (O. tshawytscha), juvenile coho salmon (O. kisutch), and juvenile sockeye salmon (O. nerka). Summertime samples were ...

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