Abstract
Pacific salmon (Oncorhynchus spp.) are generally anadromous fishes that spawn in fresh water. When young salmon have grown and matured to what is called the smolt stage (ocean ready), they move into the ocean to mature and grow into adulthood. When they have reached sexual maturity, they return to fresh water to spawn and then die. Salmon typically home to their natal streams when returning to fresh water to spawn. Straying, however, is a natural behavior for a small fraction of individuals in a population, and may even have an adaptive advantage under some circumstances. Straying can also occur as a result of various factors including natural habitat disruption, modification of the watershed, or human intervention in salmonid reproduction. In the winter of 2006/2007, tens of thousands of late-fall-run Chinook salmon (Oncorhynchus tshawytscha) reared in the Coleman National Fish Hatchery, a US Fish and Wildlife Service facility on Battle Creek in the upper Sacramento River basin, were released at several downstream locations as part of a Sacramento-San Joaquin River Delta survival study. Two years later, in the winter of 2008/2009, at the end of the annual Department of Fish and Game lower American River escapement survey for fall-run Chinook salmon, a new pulse of fish was found to be spawning in the American River. These fish turned out to be stray late-fall-run Chinook salmon from the Coleman National Fish Hatchery, spawning in the American River where the fall-run Chinook salmon were completing their spawning run. Late-fall-run Chinook salmon have not been known to spawn in the American River and understanding the reason for this unusual behavior was the basis for this project. Currently the only run of Chinook salmon in the lower American River is the fall-run. The Department does not want late-fall-run Chinook salmon to establish themselves in the river because of potential disruption of fall-run Chinook salmon nest success due to an overlap in run timing, potential interbreeding, and limited available spawning habitat. This study was based on the hypothesis that salmon released in close proximity to the mouth of the American River are more likely to stray into the river during their return spawning migration than fish released farther from the river’s mouth. Coded-wire tag inland return data from for the 2006 brood year of late-fall-run Chinook collected from 2007/2008 through 2010/2011 were used for this study. The tags were collected primarily from salmon found during river escapement surveys and those that returned to hatcheries in the Sacramento River watershed. The return data were analyzed using Chi-square statistical analyses to determine if there was a difference in the number of salmon straying into the American River with respect to the distance they were released from the mouth of that river, and a Spearman noncollated rank analysis was used to describe the overall relationship between release distance from the American River and percent straying into the river. Results indicated that straying did increase with proximity of release location to the mouth of the American River and with respect to downstream releases in general. No salmon released in the vicinity of the Coleman National Fish Hatchery were recovered in the lower American River. This study indicates that release location should be carefully evaluated if future downstream releases are conducted by Sacramento River watershed hatcheries.