Graduate Thesis Or Dissertation
 

Downstream Rearing Chinook Salmon (Oncorhynchus tshawytscha) in the Upper Mainstem of the John Day River

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  • Juvenile Chinook Salmon (Oncorhynchus tshawytscha) leave the natal reach at different times and move downstream at different rates which can increase their productivity by allowing them to utilize habitats on different temporal scales and spreading risk of localized stochastic events. This life history diversity can also allow fish to adapt to changing river conditions, thus is an important part of conserving salmon populations. In the spring and summer of 2016 and 2017, I investigated juvenile spring Chinook Salmon in the John Day River, Oregon to identify the abundance, summer distribution, and relative survival of fish following a life history pattern in which they rear in habitats downstream of their natal reach in their first spring (DSR), as opposed to fish that rear in their natal reach and do not move downstream until their second spring (NRR). DSR fish in the John Day River must find cold water refuge to survive lethal water temperatures in the mainstem. Fish were captured in the spring and marked with PIT tags to track their movements and estimate their abundance and survival. In the summer, multiple pass snorkel surveys were conducted in tributaries to investigate summer distribution. NRR fish were also captured and marked in the same time frame to compare the relative survival rates of the two life history types. More than 30,000 and 11,000 sub-yearling fish migrated in 2016 and 2107 respectively, and mainstem water temperature exceeded lethal levels in June of both years. The majority of tributaries within the study area were utilized as thermal refuges, but some of these were dewatered later in the summer. Approximately 13% of tagged fish were detected moving into tributaries in the spring. In most tributaries, summer distribution was limited to the first 1.5 km upstream of the mouth, though in warmer tributaries fish moved upstream farther, with high fish densities detected as far as 43 km from the mainstem John Day River. Averaged across the two years, fish survival to the Columbia River as smolts was 20 times lower for DSR fish marked in the mainstem John Day River than for NRR fish from the same cohort, but for DSR fish that moved into tributaries, survival was only 2.5 times lower than NRR fish, and in 2017 there was no statistical difference between these rates. DSR fish in the John Day River followed an unusual life history variation, and they made longer upstream movements into non-natal tributaries than documented elsewhere. Fish following a DSR life history pattern may have been quite successful in the mainstem John Day River prior to heavy water withdrawals, decreased channel complexity, and altered riparian habitats, but current conditions appear to impair their success compared to NRR fish. The high abundance of DSR fish and the relative performance of those that find thermal refuge in tributaries shows that DSR fish have the potential to make valuable contributions to smolt production despite conditions in the mainstem, but 87% do not appear to find adequate refuge to survive. Increasing the quality and quantity of summer habitat for DSR fish is important to increasing their success and the overall population productivity.
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  • Region 17 Pacific Northwest
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