Associations among Fish Length, Dam Passage History, and Survival to Adulthood in Two At‐Risk Species of Pacific Salmon

Abstract

Threatened or endangered salmon and steelhead originating in the Snake River basin must pass through a series of eight major hydroelectric dams during their seaward migration. Understanding the effects of specific dam passage routes on lifetime survival for these stocks is essential for successful management. Juvenile fish may pass these dams via three primary routes: (1) spillways, (2) turbines, or (3) juvenile bypass systems, which divert fish away from turbines and route them downstream. Bypass systems may expose fish to trauma, increased stress, or disease. However, numerous studies have indicated that direct survival through bypass systems is comparable to and often higher than that through spillways. Some researchers have suggested that the route of dam passage affects mortality in the estuary or ocean, but this is complicated by studies finding that fish size affects the route of passage. We tested whether passage through bypass systems was associated with the probability of adult return after accounting for fish length and other covariates for two species of concern. We also investigated the association between fish length and the probability of bypass at dams and how this relationship could lead to spurious conclusions regarding effects of bypass systems on survival if length is ignored. We found that (1) larger fish had lower bypass probabilities at six of seven dams; (2) larger fish had a higher probability of surviving to adulthood; (3) bypass history had little association with adult return after accounting for fish length; and (4) simulations indicated that spurious effects of bypass on survival may arise when no true bypass effect exists, especially in models without length. Our results suggest that after fish leave the hydropower system, bypass passage history has little effect on mortality. Our findings underscore the importance of accounting for fish size in studies of dam passage or survival.

Comment: Associations among Fish Length, Dam Passage History, and Survival to Adulthood in Two At‐Risk Species of Pacific Salmon

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Faulkner et al (2019) presented data and analyses to support a hypothesis that bypass encounters by juvenile Chinook Salmon Oncorhynchus tshawytscha and steelhead O. mykiss at hydropower dams are not harmful (e.g.,

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“…Our concerns with Faulkner et al (2019) are associated with five primary issues: (1) there was evidence of a negative bias in survival associated with the fish tagged at Lower Granite Dam (Snake River) versus those tagged upstream, (2) sample sizes were inadequate to test for bypass effects on survival for fish tagged above Lower Granite Dam, (3) major drivers of bypass probability at dams were ignored, (4) length does not explain differences in survival among disparate population groups in the Columbia River basin, and (5) indices of freshwater outmigration conditions continue to explain patterns of marine survival with or without information on individual length. The first two issues represent components of one of our primary arguments.…”

“…We take issue with their conclusions, finding substantive flaws in the basis for this hypothesis related to both the data and the methods of analysis. Faulkner et al (2019) has been publicized widely throughout the region (e.g., NOAA Fisheries West Coast Region 2019), with reference at times pointing to the article as a matter of fact. However, little attention has been focused on the approach or the data underlying their basic conclusions.…”

“…However, little attention has been focused on the approach or the data underlying their basic conclusions. Given the relevance of this topic to management objectives both regionally and, for that matter, wherever impounded rivers exist, we believe that it is essential to illustrate where the data and methods have led to erroneous conclusions; we are concerned that indiscriminate acceptance of Faulkner et al (2019) may mislead important decisions surrounding management and conservation.…”

Survival of prey growing through gape-limited and apex predators

“…In total, the evidence suggests that significant ocean mortality can occur across a range of ages and sizes. However, comparisons of early life size with adult returns of individually tagged salmon clearly indicate that large smolts have higher adult survival [16,17]. One limitation in resolving the effects of size on fish recruitment has been the lack of a theoretical framework that accounts for the changing spectrum of predators experienced by rapidly growing species like salmon.…”