Concurrent Sessions C: Ecological Consequences of Partial Passage - Measuring Outside the Box: An Example of Delayed Effects Associated With Fish Passage in Steelhead

Ecologists have long recognized the importance of scale when investigating processes, and selecting an appropriate scale of inquiry is critical for robust environmental monitoring. Evaluation of fish passage structures is often limited to the local scale, focusing on the question did fish pass? However, indirect and delayed effects have the potential to affect migratory fish populations on longer spatial and temporal scales. Juvenile salmonids out-migrating from the Snake River basin encounter eight dams and a large portion of these juveniles are collected at the first three dams and transported downstream on barges. Previous studies have demonstrated that barging affects adult upstream migration behavior and fate. Specifically, adults that were barged as juveniles have higher apparent mortality in the main stem river and stray at higher rates compared to adults that were not barged as juveniles. We developed a simple demographic model to assess the potential effects of elevated straying on donor (straying from) and recipient (receiving strays) populations. We evaluated scenarios with a range of donor and recipient population sizes and straying rates. The model demonstrates that small recipient populations may be composed of a majority of strays from donor populations under realistic parameter values. The model results are also consistent with empirical observations from the Deschutes River, an Oregon tributary to the Columbia River. These results and other recent studies suggest small populations of conservation concern may be negatively affected by straying from donor populations depending on the relative fitness of natives, strays and hybrids in the recipient environment. More broadly, the results demonstrate that actions taken to mitigate the effects of impoundment may manifest only years and hundreds of kilometers away from the impact site