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Fine-scale behavioral responses of Pacific salmonid smolts as they encounter divergence and acceleration of flow

Abstract
We assessed fine-scale behavioral responses of the smolts of four Pacific salmonid species to open and constricted channels in a flume. Natural migrants encountered two geometrically similar parallel channels with different hydraulic conditions representing constricted and open treatments. Observation of route selection under alternate discharge scenarios provided evidence of behavioral choice by smolts. As expected, the majority of smolts passed through the open channel in a ratio consistent with flow. After controlling for the influence of flow, both initial channel selection and subsequent channel rejection was higher for the constricted channel; rejection was probably due to fish detecting an area of rapidly accelerating flow. The majority of smolts traveled downstream headfirst and faster than the mean midcolumn water velocity. Those that faced the flow passed at a slower rate and tended to select the open treatment. The few yearling Chinook salmon Oncorhynchus tshawytscha and coho salmon O. kisutch smolts that did not pass through the treatment channels, but held position within the flume, were larger than their conspecifics that passed downstream. Large steelhead (anadromous rainbow trout O. mykiss) smolts and subyearling Chinook salmon were more likely to pass the constricted channel than smaller fish. These results suggest that efforts to effectively guide fish with diversion structures will require understanding how the structures alter the local hydraulic environment and, thus, influence fish behavior
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2005-01-01
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