|Title:||Factors affecting sockeye salmon returns to the Columbia River in 2008|
|Author/Editor:||John G. Williams, Steven G. Smith|
|Institution:||Report of the Northwest Fisheries Science Center, National Marine Fisheries Service|
In 2008, more than 213,000 adult sockeye salmon Oncorhynchus nerka returned to the Columbia River Basin. This is the highest return since 1959. As in the previous 40 years, greater than 99% of these fish were destined for the Upper Columbia River. Nonetheless, the estimated 805 adults passing Lower Granite Dam marked the highest return there since 1968.
...Here we report analyses of each of these life cycle components to investigate their influence on the high observed return. This was to analyze the variation in adult return rates across recent years under contemporary conditions of the mainstem hydropower system. We made no attempt to relate these returns to those from early periods before or during dam construction.
Direct estimates of smolt-to-adult returns (SAR) for the total Columbia River population were possible, but for the Snake River population only an Index SAR could be calculated for juvenile outmigrations from 1998 to 2006. Most migrating Snake River sockeye juveniles were collected at Snake River dams and transported by barge to below Bonneville Dam, while nearly all Upper Columbia River sockeye juveniles migrated through the hydropower system and were not transported.
Snake River sockeye Index SARs were substantially lower than Columbia River sockeye SARs, in part simply because the migration distance incorporated into estimated SARs was longer for Snake River fish. Snake River Index SARs were estimated from arrivals of both juveniles and adults at Lower Granite Dam, while the Columbia River SARs were estimated from arrivals of juveniles at McNary Dam and adults at Bonneville Dam.
A number of additional factors could have influenced the difference in SARs. For example, adults returning to the Snake River were largely of hatchery origin, while those from the Columbia River were mostly wild. Furthermore, these stocks are from different Evolutionarily Significant Units (ESUs). Therefore, we expect them to exhibit inherent differences that may influence stock productivity. These include genetic differences that control growth, size, and migration timing. Finally, the Snake River stock represents fish at the limit of their natural range, with the longest migrations and attaining the highest elevations to reach spawning sites.
There are several lines of evidence suggesting that changes in ocean productivity led to the high adult return observed in 2008. Estimated SARs for Upper Columbia and Snake River sockeye salmon stocks were highly significantly correlated (R2 = 0.87, P < 0.01). In addition, there was no correlation between sockeye salmon SARs and indices of mainstem flow and percentage spill at hydropower projects in the Columbia River from McNary to Bonneville Dams. This suggests that the primary factors influencing the variation in annual adult returns acted downstream from Bonneville Dam and on both stocks in common. There was no evidence that adult escapements in 2008 were influenced by changes in ocean or river harvest.
In summary, the results discussed here provide a consistent pattern to explain the large return of adult sockeye to the Columbia River in 2008. Based on these results, we conclude that the factors responsible for the high return largely acted on fish downstream of Bonneville Dam and during the marine component of their life cycle, and not in the river upstream of Bonneville Dam.
|Theme:||Recovery and rebuilding of marine and coastal species|
Characterize the population biology of species, and develop and improve methods for predicting the status of populations.