Factors Affecting the Homing of Fall Chinook Salmon from Columbia River Hatcheries

Abstract
Straying results in an exchange of individuals between wild and hatchery salmon populations and is important because it could affect their genetic differences. Understanding what factors affect straying could help in the development of procedures for controlling the influence of specific stocks on nearby populations. We explore the effects of release date and the transportation of juvenile Chinook salmon Oncorhynchus tshawytscha on straying by adults from two Columbia River hatcheries. Early and late releases from Washougal Hatchery produced much higher straying levels than intermediate release dates, and this pattern was consistent among brood years. Fish reared at Grays River Hatchery and released from Washougal Hatchery strayed significantly more than local (i.e., Washougal) fish of the same brood year and release date. Our results and those from previous studies indicate that the sequence of imprinting events is characterized by the combination of the time at which the fish experience given locations and the physiological state of the fish at that time. Changes in release date and transportation disrupt the sequence and timing of these events, with varying effects on homing. Hatchery rearing and release techniques can sometimes be modified to control straying and minimize the impact of hatchery fish on other hatchery and wild stocks.

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