Bonneville Dam Research Articles

Visual and genetic stock identification of a test fishery to forecast Columbia River spring Chinook salmon stocks 2 weeks into the future.

Modern fisheries management strives to balance opposing goals of protection for weak stocks and opportunity for harvesting healthy stocks. Test fisheries can aid management of anadromous fishes if they can forecast the strength and timing of an annual run with adequate time to allow fisheries planning. Integration of genetic stock identification (GSI) can further maximize utility of test fisheries by resolving run forecasts into weak- and healthy-stock subcomponents. Using 5 years (2017-2022) of test fishery data, our study evaluated accuracy, resolution, and lead time of predictions for stock-specific run timing and abundance of Columbia River spring Chinook salmon (Oncorhynchus tshawytscha). We determined if this test fishery (1) could use visual stock identification (VSI) to forecast at the coarse stock resolution (i.e., classification of "lower" vs. "upriver" stocks) upon which current management is based and (2) could be enhanced with GSI to forecast at higher stock resolution. VSI accurately identified coarse stocks (83.3% GSI concordance), and estimated a proxy for abundance (catch per unit effort, CPUE) of the upriver stock in the test fishery that was correlated (R 2 = 0.90) with spring Chinook salmon abundance at Bonneville dam (Rkm 235). Salmon travel rates (~8.6 Rkm/day) provided predictions with 2-week lead time prior to dam passage. Importantly, GSI resolved this predictive ability as finely as the hatchery broodstock level. Lower river stock CPUE in the test fishery was correlated with abundance at Willamette Falls (Rkm 196, R 2 = 0.62), but could not be as finely resolved as achieved for upriver stocks. We described steps to combine VSI and GSI to provide timely in-season information and with prediction accuracy of ~12.4 mean absolute percentage error and high stock resolution to help plan Columbia River mainstem fisheries.

Abundance Trends for Adult Pacific Lamprey in Western Oregon (USA): Historic Declines, Recent Increases, and Relative Contributions from Coastal Rivers

AbstractThe anadromous Pacific Lamprey Entosphenus tridentatus is native to the northern Pacific Ocean and its drainages, returning each spring to spawn in streams along the West Coast of North America. We analyzed abundance trends of adult Pacific Lamprey from dam counts and redd surveys in western Oregon, USA. We compared a series of generalized additive models (GAMs) to describe lamprey abundance trends. Lamprey counts varied considerably among dams but showed steep declines over 70+ years (1949–2019) at Bonneville Dam on the Columbia River, over 50+ years (1965–2019) at Winchester Dam on the North Umpqua River, and over 17 years (1993–2009) at Gold Ray Dam on the Rogue River. By contrast, redd surveys displayed synchronous abundance trends of different magnitudes among geographic management areas on the Oregon coast, with an overall increasing abundance over 13 years (2007–2019). This corroborates pre‐existing notions that Pacific Lamprey spawning in Oregon coast rivers are from a single mixed population. The most supported GAM included a fixed effect for location and allowed abundance to vary over years as a function of location. These results suggest that abundance trends among dam counts are more variable than coastal redd surveys and that dam counts and redd surveys across this large region are not useful proxies for each other. Furthermore, counts across dams may be more variable because they record prespawning lamprey and thus may be recording a mixture of cohorts. Redd surveys, by contrast, provide a metric of a single cohort of spawning lamprey.

Rapid Migration and High Survival of Adult Pacific Lampreys in Reservoirs

AbstractDams and reservoirs present diverse hazards for riverine fishes, including slowed migration, reduced or blocked access to essential habitats, and direct and delayed mortality risks. Many of these hazards are evident for Pacific Lamprey Entosphenus tridentatus in the heavily dammed Columbia River basin, where the species’ distribution and abundance has sharply declined. In a 3‐year study, we used acoustic telemetry to investigate behavior and apparent survival of 784 adult Pacific Lampreys in Columbia River reservoirs and turbulent dam tailraces, two environments where previous monitoring efforts have been somewhat ineffectual. We released Pacific Lampreys upstream and downstream from Bonneville Dam to test whether Pacific Lamprey passage at one dam negatively affected upstream dam and reservoir passage metrics (i.e., delayed effects on migration rate or reach passage success) and whether upstream‐released groups migrated further than their downstream‐released counterparts. Overall, results showed that Pacific Lampreys migrated very rapidly (median rates = 50–62 km/d) in reservoir reaches and slowly (1–3 km/d) in reaches with tailraces or fishways. Pacific Lampreys also had much higher passage success in reservoir reaches (reach × year estimates = 0.93–0.99) than in reaches with tailraces or fishways (most estimates = 0.58–0.82). We found little statistical evidence for negative delayed effects of dam passage. Pacific Lampreys released in the Bonneville forebay were far more likely to pass upstream dams (~26–57%) than were those released in the Bonneville tailrace (~20–36%). We conclude that tailraces and fishways constrained the upstream distribution of adult Pacific Lampreys, that reservoirs were not associated with high mortality during active upstream migration, and that translocation past individual dams can increase the upstream migration distance of individuals and population distribution. Areas of continuing uncertainty include behavior and survival during overwintering (i.e., prespawn holding) and the potential delayed effects of tailrace and fishway passage on Pacific Lamprey fitness.

Epidemiological models to control the spread of information in marine mammals.

Socially transmitted wildlife behaviours that create human-wildlife conflict are an emerging problem for conservation efforts, but also provide a unique opportunity to apply principles of infectious disease control to wildlife management. As an example, California sea lions (Zalophus californianus) have learned to exploit concentrations of migratory adult salmonids below the fish ladders at Bonneville Dam, impeding endangered salmonid recovery. Proliferation of this foraging behaviour in the sea lion population has resulted in a controversial culling programme of individual sea lions at the dam, but the impact of such culling remains unclear. To evaluate the effectiveness of current and alternative culling strategies, we used network-based diffusion analysis on a long-term dataset to demonstrate that social transmission is implicated in the increase in dam-foraging behaviour and then studied different culling strategies within an epidemiological model of the behavioural transmission data. We show that current levels of lethal control have substantially reduced the rate of social transmission, but failed to effectively reduce overall sea lion recruitment. Earlier implementation of culling could have substantially reduced the extent of behavioural transmission and, ultimately, resulted in fewer animals being culled. Epidemiological analyses offer a promising tool to understand and control socially transmissible behaviours.

Network analyses reveal intra‐ and interspecific differences in behaviour when passing a complex migration obstacle

Summary Network theory offers new perspective on movement data by evaluating the relationships between animal movements (links) and detection locations (nodes) in spatially complex systems, including human‐altered landscapes. We applied network analyses to intra‐ and interspecific movement patterns in the migration behaviour and dam passage success of two anadromous fish species, Pacific lamprey Entosphenus tridentatus Gairdner and Chinook salmon Oncorhynchus tshawytscha Walbaum, when moving through a large multifishway hydroelectric project (Bonneville Dam, USA). Network analyses revealed greater variation in movement for Pacific lamprey compared with Chinook salmon. Salmon that passed the dam had networks consisting of more direct passage routes with fewer overall movements compared with lamprey that passed the dam. Lamprey that did not pass the dam exhibited a wide range of behaviours, from approaching only one fishway site to testing all possible passage routes. Accounting for the time spent in the network improved the ability to detect biological differences in network structure for lamprey that did and did not pass the dam. The movement patterns likely resulted from different behavioural responses to complex environmental and internal factors affecting a philopatric species (Chinook salmon) vs. a non‐philopatric species (Pacific lamprey) when moving through an engineered environment designed primarily for salmon. Synthesis and applications. Our case study highlights the potential for network analyses to link questions of basic movement ecology with monitoring of movement and behaviour in human‐altered landscapes. Network analyses can thus serve as a valuable tool for describing movement and behaviour in the face of environmental change and assessing the effectiveness of mitigation efforts at spatially complex obstacles to animal movement.

Fishway Bottleneck Relief Models: a Case Study using Radio‐Tagged Pacific Lampreys

AbstractStructural or operational changes to fishways can improve fish passage efficiency, but there is often uncertainty regarding which improvements will be most effective. We developed a “bottleneck relief” model using Kaplan–Meier methods to help managers assess where remediation efforts are likely to provide the largest increases in fishway passage. The simulation model uses a matrix of observed efficiency estimates from fishway subsections and incorporates multiple passage attempts by individual tagged fish. In a case study application to test the model, we used radiotelemetry data from 2,170 adult Pacific Lampreys Entosphenus tridentatus at Bonneville Dam (Columbia River, Washington–Oregon), which features multiple fishways and low lamprey passage success (mean fishway passage efficiency ∼0.50; n = 10 years). The model was run iteratively to test potential dam passage benefits from improving efficiency in 20 individual fishway segments. The highest benefits were for improvements at top‐of‐fishway segments and at sites where passage routes converged. Benefits were lower for segments used by fewer fish, segments located downstream from serious bottlenecks, and segments with a limited scope for improvement (i.e., efficiency was already high). The model provides a flexible and objective method for assessing complex fish passage problems and informing remediation decision making.Received December 19, 2013; accepted March 11, 2014

Fishway passage bottleneck identification and prioritization: a case study of Pacific lamprey at Bonneville Dam

Fishways designed for salmonids often restrict passage by non-salmonids, and effective tools are needed both to identify passage problems for nontarget species and to inform remediation planning. In this case study, we used migration histories from 2170 radio-tagged adult Pacific lamprey (Entosphenus tridentatus) to identify locations of poor passage (bottlenecks) at a large, multifishway dam. Over 10 years, 49% of tagged lamprey that entered fishways failed to pass the dam. Models accounting for repeated attempts by individual lamprey indicated successful passage strongly depended on attempted passage route. Success also varied with time of fishway entry, water temperature, and lamprey body size. Most failed passage attempts terminated in lower fishway segments, but extensive seasonal shifts in bottleneck locations were detected. Ranking metrics helped prioritize bottlenecks and identified sites where structural or operational modifications should improve lamprey passage. Our integration of spatially intensive monitoring with novel analytical techniques was critical to understanding the complex relationships among fishway features, environmental variation, and lamprey behavior. The prioritization framework can be applied to a wide range of fish passage assessments.

Movement of Radio‐Tagged Adult Pacific Lampreys during a Large‐Scale Fishway Velocity Experiment

AbstractOptimization of fishways to pass multiple species is challenging because life history, swimming ability, and behavior often differ among species. For example, high fishway water velocities designed to attract adult Pacific salmon Oncorhynchus spp. at Columbia River dams inhibit fishway entrance and passage success of adult Pacific lampreys Lampetra tridentata, a species of conservation concern. We tested whether reduced water velocities (∼1.2 m/s, 0.15 m of head) at Bonneville Dam fishway openings improved entrance efficiency and other passage metrics for radio‐tagged Pacific lampreys compared with control velocities (>1.98 m/s, 0.46 m of head) and near‐zero (“standby”) velocities. Lamprey entrance efficiencies were significantly higher in the reduced‐velocity treatment (26–29%) than in the control (13–20%) or standby (5–9%) treatment. In some years, significantly more Pacific lampreys passed through fishway collection channels and transition pools and reached the fish ladder during reduced‐velocity treatment conditions, indicating that benefits extended beyond fishway entrances. However, overall passage efficiency at the dam was relatively unchanged, suggesting that additional passage bottlenecks for Pacific lampreys exist upstream from fishway entrances. The experiment demonstrated how operational changes can improve passage performance and how exploiting behavioral differences among species can improve multispecies management.

One Sea Lion's Worth - Evaluating the Role of Values in Section 120

In 1994, the Marine Mammal Protection Act (MMPA) was amended to include a provision that would allow the lethal take of pinnipeds that were causing 'significant negative impact' to salmonid species listed by the Endangered Species Act. Implicit in Section 120 and its placement in the MMPA implementation are competing values in the desire to protect entire salmonid populations at the cost of individual pinnipeds. This article uses the most recent application of Section 120 at Bonneville Dam on the Columbia River to explore the role of interest group values on the interpretation of Section 120, which in turn affected the application of the statute. By analyzing the underlying values of interest groups, this article explores how values affect the various proposed 'solutions' to the Bonneville Dam situation, and suggests reform that will limit the value judgments of the Section 120 process. I. INTRODUCTION II. A SEA LION NAMED HERSCHEL--THE ORIGIN OF SECTION 120 A. Setting the Stage--The Battle at the Ballard Locks B. The Passage of Section 120 C. Section 120's First Application--The Ballard Locks Revisited III. HISTORY REPEATS--THE BONNEVILLE DAM CASE STUDY A. The States' Application for Lethal Removal B. Competing Definitions--The Pinniped-Fishery Interaction Task Force vs. the Marine Mammal Commission 1. The Pinniped-Fishery Interaction Task Force's Findings and Recommendations a. Determining Significant Negative Impact--Applying Section 120's Four Requirements to Bonneville Dam b. The Task Force's Lethal Take Recommendations 2. The Marine Mammal Commission's Comments on the Task Force's Report C. Synthesizing the Competing Definitions 1. NMFS's Findings and Permit Terms 2. The Courts Weigh in--Humane Society v. Gutierrez IV. ANALYSIS: SECTION 120'S PLACE IN THE MARINE MAMMAL PROTECTION ACT A. Statutory Ambiguity and the Competing Values of the MMPA B. The Effect of Different Goals on Statutory Interpretation C. The Role of Interest Groups in the Application of Section 120 V. CONCLUSION I. INTRODUCTION When it comes to the preservation of America's wildlife, there may be no greater protections than those afforded by the Endangered Species Act of 1973 (ESA) and the Marine Mammal Protection Act of 1972 (MMPA). Both acts seek to protect biodiversity by reducing lethal take, habitat destruction, and other direct human impacts on wildlife populations. But humans also have indirect impacts on wildlife, including the creation of peculiar situations where one protected species threatens the survival of another protected species. A prominent illustration of such a situation occurred in the 1980s, when MMPA-protected California sea lions began to prey on the small steelhead run at Seattle's Ballard Locks. Unable to deter the sea lions from decimating the fish population, wildlife managers simultaneously found their hands tied by the no-take provisions of the MMPA. (1) Faced with the startling prospect of predation by a protected species ... threatening the existence of another species, (2) Congress added Section 120 to the MMPA in order to allow lethal removal of individually identifiable pinnipeds (3) which are having a significant negative impact on the decline or recovery of salmonid fishery stocks. (4) The amendment was not without its critics; environmental groups such as the Humane Society of the United States viewed the amendment as the first step towards a government-authorized cull of marine mammals, (5) an ironic addition to a statute whose goal was to protect the nation's marine mammal populations. Despite these concerns, Section 120's authorization of lethal take of pinnipeds to protect at-risk salmonid populations has not resulted in a 'cull' of marine mammals. …

Development of Pacific lamprey fishways at a hydropower dam

Abstract Traditional fishways do not accommodate the passage needs of all migrating species. In the north-western United States, structures designed to aid adult Pacific lamprey, Lampetra tridentata (Gairdner), passage are critically needed. The structures described here were fabricated in modular units and installed at Bonneville Dam on the Columbia River (235 km). They featured a series of aluminium ramps interspersed with rest boxes that prevented lamprey from moving back downstream. The effects of various design changes and structure operation (water volume delivered) were assessed using lamprey counts and passive integrated transponder detections. Up to 40% of the lamprey entered the structures and 90–100% of these passed through to the exit. Lowering water volume pumped to the structures had little effect on lamprey performance, but passage improved when a 3.8-m-long, steep (40°) ramp was replaced with two, 1.4-m-long, 45° ramps. Pacific lamprey ascended the 8- to 9-m-high structures and entered the dam forebay in less than 1 h. The success of these prototypes was attributed to site selection and attention to lamprey-specific performance.

Hydraulic Alterations Resulting from Hydropower Development in the Bonneville Reach of the Columbia River

We used a two-dimensional (2D) hydrodynamic model to simulate and compare the hydraulic characteristics in a 74-km reach of the Columbia River (the Bonneville Reach) before and after construction of Bonneville Dam. For hydrodynamic modeling, we created a bathymetric layer of the Bonneville Reach from single-beam and multi-beam echo-sounder surveys, digital elevation models, and navigation surveys. We calibrated the hydrodynamic model at 100 and 300 kcfs with a user-defined roughness layer, a variable-sized mesh, and a U.S. Army Corps of Engineers backwater curve. We verified the 2D model with acoustic Doppler current profiler (ADCP) data at 14 transects and three flows. The 2D model was 88% accurate for water depths, and 77% accurate for velocities. We verified a pre-dam 2D model run at 126 kcfs using pre-dam aerial photos from September 1935. Hydraulic simulations indicated that mean water depths in the Bonneville Reach increased by 34% following dam construction, while mean velocities decreased by 58%. There are numerous activities that would benefit from data output from the 2D model, including biological sampling, bioenergetics, and spatially explicit habitat modeling.

Barging Effects on Sensory Systems of Chinook Salmon Smolts

AbstractTo avoid mortality caused by passage through dam turbines and spillways, juvenile Chinook salmon Oncorhynchus tshawytscha are annually transported downstream by barge through the federal hydropower system on the Snake and Columbia rivers. Survival of transported fish is higher than that of in‐river migrants; however, transported fish experience higher rates of postrelease mortality. Increased mortality could result from a decrease in the ability to detect or avoid predators due to stressors associated with the barge environment. This study examined the effects of barging on juvenile Chinook salmon olfaction and auditory function, two sensory systems involved in predator detection. We focused on dissolved metals known to be toxic to the salmon olfactory system and on the level of noise from the barge, which could impair the auditory system. Experimental groups included animals collected (1) before barge loading (control group), (2) at the Bonneville Dam bypass system (migrant fish), (3) immediately after barge transport, and (4) within 7 d postbarging and at or after 7 d postbarging. Measured concentrations of dissolved metals from the water within the barge were below established water quality criteria for the protection of aquatic life. Moreover, ultrastructural examination of the olfactory epithelium surface showed no evidence of injury to olfactory sensory neurons. Noise in the barge holding tanks had levels up to 136 dB referenced to 1 μPa (root mean square) with primary energy below 400 Hz. Auditory sensitivity was measured using the auditory‐evoked potentials (AEP) technique. We found a small but statistically significant threshold shift for fish collected within 7 d postbarging, while in the 7‐d‐and‐later postbarging group the AEP thresholds were similar to the control. Our findings indicate that the olfactory systems of transported Chinook salmon are intact and probably functional, while the auditory sensitivities are compromised with probable recovery.

Fallback by Adult Sockeye Salmon at Columbia River Dams

AbstractWe implanted radio transmitters into sockeye salmon Oncorhynchus nerka in 1997 to determine the (1) fallback percentage and rate at eight Columbia River dams, (2) effect of fallback on adult counts at each dam, (3) relations between spillway discharge and fallback, (4) relations between injuries and fallback, and (5) relations of fallback and survival to spawning tributaries. The rate of fallback, that is, the total number of fallback events at a dam divided by the number of fish known to have passed the dam, ranged from 1.9% to 13.7% at the eight dams. The rate of fallback was highest at Bonneville Dam, the dam with the most complex fishway. Fallback produced overcounts of 2% to 7% at most dams. Fallback was weakly related to spill volume at Bonneville Dam. Significantly more sockeye salmon with head injuries fell back than fish without head injuries. About 40% of the sockeye salmon had injuries from marine mammals, but these injuries were not associated with the rate of fallback. The rate of survival was similar between fish that fell back (68.0%) and fish that did not fall back (67.5%). We suggest that fisheries managers adjust counts for fallback but note that these relationships were obtained under high‐discharge conditions. We conclude that fallback biases dam counts and that the relationship between spawning success and fallback should be an area of future research.

Use of Electromyogram Telemetry to Assess Swimming Activity of Adult Spring Chinook Salmon Migrating Past a Columbia River Dam

AbstractElectromyogram (EMG) radiotelemetry was used to estimate the swim speeds of spring Chinook salmon Oncorhynchus tshawytscha migrating upstream past a Columbia River dam. Electrodes from EMG transmitters were surgically implanted in the red muscle of fish captured at Bonneville Dam, and output from the tags was calibrated to defined swim speeds for each fish in a tunnel respirometer. The fish were then released below Bonneville Dam and radio‐tracked as they migrated through the tailraces, fishways, and forebays of the dam. On average, swim speed was significantly higher when tagged salmon were moving through tailraces than when they were moving through other parts of the dam. Specifically, swim speeds for fish in tailraces (106.4 cm/s) were 23% higher than those of fish in fishways (84.9 cm/s) and 32% higher than those of fish in forebays (80.2 cm/s). Swim speeds were higher in fishways during the day than during the night, but there were no diel differences in swim speeds in tailraces and forebays. During dam passage, Chinook salmon spent the most time in tailraces, followed by fishways and forebays.

Migration Depths of Adult Spring and Summer Chinook Salmon in the Lower Columbia and Snake Rivers in Relation to Dissolved Gas Supersaturation

AbstractHigh spill volume at dams can create supersaturated dissolved gas conditions that may have negative effects on fish. Water spilling over Columbia and Snake River dams during the spring and summer creates plumes with high dissolved gas that extend downstream of dam spillways and throughout reservoirs and creates gas‐supersaturated conditions throughout the water column. During the spring and summer of 2000, 228 adult Chinook salmon Oncorhynchus tshawytscha were tagged at Bonneville Dam with archival radio data storage transmitters (RDSTs) that recorded depth and water temperature as the fish migrated through dams and reservoirs of the lower Columbia and Snake rivers. Swimming depths from 131 of the 228 adult spring and summer Chinook salmon tagged with RDSTs were used to estimate the potential for gas bubble formation given in‐river dissolved gas concentrations and hydrostatic compensation. We found that adult spring and summer Chinook salmon spent a majority of the time at depths that would have provided adequate hydrostatic compensation for in‐river dissolved gas conditions during this study, which were at or slightly below long‐term averages. Adult spring and summer Chinook salmon spent a majority of their time at depths deeper than 2 m, interspersed with periods lasting minutes at depths shallower than 2 m. Statistical associations were weak between the percent and duration of time fish occupied depths near the surface and dissolved gas concentrations, suggesting a lack of behavioral avoidance. Collectively, these data suggest little potential for negative effects of gas supersaturation on adult spring and summer Chinook salmon under average river conditions, despite the fact that fish tissues were probably supersaturated with dissolved gases. However, additional research over a broader range of dissolved gas conditions is needed to confirm that short, but frequent, exposure to conditions conducive to gas bubble formation does not affect survival and reproductive potential.

Empty Nets: Indians, Dams, and the Columbia River

Empty Nets is a disturbing history of broken promises and justice delayed. It chronicles a native people's fight to maintain their livelihood and culture in the face of an indifferent federal bureaucracy and hostile state governments. In 1939, the U.S. Government promised to provide Columbia River Indians with replacements for traditional fishing sites flooded in the backwater of the Bonneville Dam. Roberta Ulrich recounts the Indians sixtyyear struggle, in the courts and on the river, to persuade the government to keep its promise. From the beginning, the battle was intertwined with the tribes larger effort to assert treatyguaranteed fishing rights. Ulrich deftly examines a host of other issues?including declining salmon runs, industrial development, tribal selfgovernment, and recreation that became enmeshed in the tribes? pursuit of justice. Her broad and incisive account ranges from descriptions of the dam's disastrous effec ts on a salmondependent culture to portraits of the plights of individual Indian families. Descendants of those to whom the promise was made and ac tivists who have s pent their lives working to acquire the sites reveal the remarkable patience and resilience of the Columbia River Indians. In a new epilogue, Ulrich updates the story of the treaty fishing sites? now all nearly completed, nd describes political and cultural developments since 1999, including a major new component: the planned reconstruc tion of the Celilo Indian Village. And yet des pite the everchanging circumstances surrounding the treaty sites, the tribes? objec tive remains the same. In the words of Donald Sampson, former executive direc tor of the Columbia River InterTribal Fish Commission, ?Our people's desire is simple to preserve the fish, to preserve our way of life, now and for future generations.?

Index of Predation on Juvenile Salmonids by Northern Pikeminnow in the Lower Columbia River Basin, 1994–1996

We estimated relative abundance of northern pikeminnow Ptychocheilus oregonensis and relative consumption of juvenile salmonids Oncorhynchus spp. by northern pikeminnow at standardized sites in the lower Columbia and lower Snake rivers from 1994 to 1996. Indexes of abundance and consumption were compared with indexes measured from 1990 to 1993 to evaluate changes in predation concurrent with a predator control program in the lower Columbia basin. Reductions in indexes of northern pikeminnow abundance, consumption, or both resulted in mean 1994–1996 predation index values that were 44–91% lower than mean 1990–1993 values throughout the lower Columbia basin. Consumption of juvenile salmonids by surviving northern pikeminnow has not increased in response to predator control efforts. Spatial patterns were consistent among years, being greatest downstream from Bonneville Dam, intermediate in lower Columbia River reservoirs, and lowest at Snake River sites. Reductions in relative predation were consistent with changes in northern pikeminnow population structure associated with harvest, although annual variation in river flow, dam spill, and juvenile salmonid passage may have magnified reductions in predation.

Environmental Changes Affecting the Migratory Timing of American Shad and Sockeye Salmon

The rate of evolution and the scope of phenotypic plasticity can be assessed by studying the adaptation of an introduced population to a new habitat and the response of established populations to progressive environmental change. Adult American shad (Alosa sapidissima), and introduced species, and sockeye salmon (Oncorhynchus nerka), a native species, migrate up the Columbia River (northwestern United States) in late spring and early summer to spawn. Based on records from Bonneville Dam, the river's spring warming has occurred progressively earlier since ≈ 1950, coinciding with a reduction in spring discharge. The date when 50% of the shad migrated past the dam is correlated with this shift inthermal and flow regimes; they now ascend the river ≈38 d earlier than they did in 1938. However, the mean temperature that they experience has actually decreased by 1.8$deg;C in 45 yr, indicating that the change in their migratory timing has outstripped the rate of environmental change. The upriver migration of sockeye salmon is also earlier than in past years, but their change in time (≈6 d since 1949) lags behind the rate of environmental change, and they are now experiencing ≈2.5°C warmer temperatures than in past years. We hypothesize that the differences in response to changing environmental conditions between these species arise from differences in their migration patterns and early life histories. Shad spawn soon after they enter the river in its main stem where environmental conditions of the larvae will closely mirror those experienced by upstream—migrating adults. They may therefore have evolved a migratory pattern that allows greater behavioral response to environmental fluctuations than sockeye salmon, which spawn in distant locations many months after their upriver migration. Sockeye salmon migration may be more strongly controlled by innate responses to photoperiod, migrating at the time of year which is best on average because conditions in the lower river will not be indicative of those to be experienced by incubating embryos and juveniles.

White Sturgeon Spawning and Rearing Habitat in the Lower Columbia River

Abstract Estimates of spawning habitat for white sturgeons Acipenser transmontanus in the tailraces of the four dams on the lower 470 km of the Columbia River were obtained by using the Physical Habitat Simulation System of the U.S. Fish and Wildlife Service's Instream Flow Incremental Methodology to identify areas with suitable water depths, water velocities, and substrates. Rearing habitat throughout the lower Columbia River was assessed by using a geographic information system to identify areas with suitable water depths and substrates. The lowering of spring and summer river discharges from hydropower system operation reduces the availability of spawning habitat for white sturgeons. The four dam tailraces in the study area differ in the amount and quality of spawning habitat available at various discharges; the differences are due to channel morphology. The three impoundments and the free-flowing Columbia River downstream from Bonneville Dam provide extensive areas that are physically suitable for rea...

Differential Predation by Northern Squawfish (Ptychocheilus oregonensis) on Live and Dead Juvenile Salmonids in the Bonneville Dam Tailrace (Columbia River)

Juvenile salmonids (Oncorhynchus spp.) that have been killed or injured during dam passage may be highly vulnerable or preferred prey of predators that aggregate below dams. Salmonid loss due to predation will be overestimated using gut content analysis if some prey were dead or moribund when consumed. To examine this issue, field experiments were conducted in the Bonneville Dam tailrace (Columbia River) to compare rates of capture of live and dead juvenile salmonids by northern squawfish (Ptychocheilus oregonensis). Known numbers of coded-wire-tagged live and dead chinook salmon (O. tshawytscha) were released into the tailrace on six nights. Northern squawfish were collected after each release and their gut contents were examined for tags. When 50% of salmon released were dead, northern squawfish consumed 62% dead salmon. When 10% of salmon released were dead, comparable with dam passage mortality, 22% of the tags found in northern squawfish digestive tracts were from dead salmon. These results indicate that predator feeding behavior and prey condition are important considerations when estimating the impact of predation on a prey population.