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.

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Water, sediment, and channel catfish lctalurus punctatus were sampled from seven farms experiencing Proliferative gill disease (PGD) outbreaks in spring 1992 . At each farm, samples were collected from the pond that experienced the outbreak (PGD-positive pond) and from another pond where no PGD was observed (control pond) . Seven species of Actinosporea were detected in the oligochaete populations of the ponds examined . The only actinosporean species significantly (P :< 0 .05) correlated with clinical PGD outbreaks was Aurantiactinomyxon sp . (type 1 ) . Population densities of aquatic oligochaetes in the pond sediments, including Dero digitata, the host of Aurantiactinomyxon sp ., were surveyed during the PGD outbreaks and over the following year. Population densities of all oligochaetes, densities of D . digitata, and proportions of the total oligochaete population represented by D . digitata were all significantly higher (P < 0 .05) in ponds experiencing clinical outbreaks of PGD than in control ponds . After the PGD outbreaks, total oligochaete density returned to levels not significantly different from those of control ponds . During the study, PGD organisms were observed in fish from control ponds, and the prevalence of Aurantiactinomyxon sp . (type 1 ) infection of D . digitata was similar to that in the PGD-positive ponds . However, the control ponds with PGD-positive fish had lower densities of D . digitata . Water analysis showed that PGD-positive ponds had significantly (P < 0 .05) lower turbidity than control ponds . Organic content of soil in positive and control ponds did not significantly (P < 0 .05) differ. photometers and Hach reagents for ions and tur-1 To whom correspondence should be addressed.

Migratory fish populations are impacted worldwide by river impoundments. Efforts to restore populations will benefit from a clear understanding of survival and migration process over a wide-range of river conditions. We developed a model that estimates travel time and survival of migrating juvenile salmonids (Oncorhynchus spp.) through the impounded Snake and Columbia rivers in the northwestern United States. The model allows users to examine the effects of river management scenarios, such as manipulations of river flow and spill, on salmonid survival. It has four major components: dam passage and survival, reservoir survival, fish travel time, and hydrological processes. The probability that fish pass through specific routes at a dam and route-specific survival probabilities were based on hydroacoustic, radio telemetry, PIT tag, and acoustic tag data. We related reservoir mortality rate (per day and per km) to river flow, water temperature, and percentage of fish passing through spillways and then fit the relationships to PIT-tag survival data. We related fish migration rate to water velocity, percentage of fish passing through spillways, and date in the season. We applied the model to two threatened ''Evolutionarily Significant Units'' (as defined under the US Endangered Species Act): Snake River spring/ summer Chinook salmon (O. tshawytscha Walbaum) and Snake River steelhead (O. mykiss Walbaum). A sensitivity analysis demonstrated that for both species survival through the hydropower system was responsive to water temperature, river flow, and spill proportion. The two species, however, exhibited different patterns in their response. Such information is crucial for managers to effectively restore migratory fish populations in regulated rivers.

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