Columbia River Reservoirs Research Articles

Relating social networks, ecological health, and reservoir basin governance

AbstractThe Columbia River Basin is a complex social‐ecological system, spanning political, legal, socio‐economic, geographic, and biophysical boundaries. Outreach to others in social networks develops fundamental communications needed for sustainable collaborations in adaptive management. However, operationalizing and comparing measures of social processes and outcome success in biophysical indicators remains challenging for resource managers. Using survey‐based research, we examined the interactions for water resource governance of five Columbia River reservoir basins in the northwestern US and Canada: Lakes Chelan, Roosevelt, Pend Oreille, Koocanusa, and člq'etkw (Flathead). Respondents included: water resource professionals working for Tribes/First Nations, federal, state, or provincial departments in water quality and/or fisheries, and people who engage in the networks on behalf of area businesses, government offices, public services, non‐profit organizations, and other entities. Perceived social process metrics in these governance networks included the levels of collaboration and inclusiveness, common goals, common strategies, identifying issues, implementing action, and the adequacy of available scientific data. We evaluated social measures relative to participant‐reported changes in physical lake health indicators. Qualitative data‐enhanced understanding of basin‐specific differences. Correlations of social by ecological measures varied widely between basins. Even moderate to strong functionality parameters did not scale well from individual to cross‐basin levels, as many correlations vanished with data aggregated. However, data analysis at the basin scale revealed important variability across the region in scope and governance functionality. Process indicators such as identifying issues and implementing action yielded stronger relationships for 10‐year horizons than for 2 years, reflecting the lag‐time in resource action.

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.

Linking functional response and bioenergetics to estimate juvenile salmon growth in a reservoir food web.

Juvenile salmon (Oncorhynchus spp.) use of reservoir food webs is understudied. We examined the feeding behavior of subyearling Chinook salmon (O. tshawytscha) and its relation to growth by estimating the functional response of juvenile salmon to changes in the density of Daphnia, an important component of reservoir food webs. We then estimated salmon growth across a broad range of water temperatures and daily rations of two primary prey, Daphnia and juvenile American shad (Alosa sapidissima) using a bioenergetics model. Laboratory feeding experiments yielded a Type-II functional response curve: C = 29.858 P *(4.271 + P)-1 indicating that salmon consumption (C) of Daphnia was not affected until Daphnia densities (P) were < 30 · L-1. Past field studies documented Daphnia densities in lower Columbia River reservoirs of < 3 · L-1 in July but as high as 40 · L-1 in August. Bioenergetics modeling indicated that subyearlings could not achieve positive growth above 22°C regardless of prey type or consumption rate. When feeding on Daphnia, subyearlings could not achieve positive growth above 20°C (water temperatures they commonly encounter in the lower Columbia River during summer). At 16–18°C, subyearlings had to consume about 27,000 Daphnia · day-1 to achieve positive growth. However, when feeding on juvenile American shad, subyearlings had to consume 20 shad · day-1 at 16–18°C, or at least 25 shad · day-1 at 20°C to achieve positive growth. Using empirical consumption rates and water temperatures from summer 2013, subyearlings exhibited negative growth during July (-0.23 to -0.29 g · d-1) and August (-0.05 to -0.07 g · d-1). By switching prey from Daphnia to juvenile shad which have a higher energy density, subyearlings can partially compensate for the effects of higher water temperatures they experience in the lower Columbia River during summer. However, achieving positive growth as piscivores requires subyearlings to feed at higher consumption rates than they exhibited empirically. While our results indicate compromised growth in reservoir habitats, the long-term repercussions to salmon populations in the Columbia River Basin are unknown.

From salmon to shad: Shifting sources of marine‐derived nutrients in the Columbia River Basin

AbstractLike Pacific salmon (Oncorhynchus spp.), nonnative American shad (Alosa sapidissima) have the potential to convey large quantities of nutrients between the Pacific Ocean and freshwater spawning areas in the Columbia River Basin (CRB). American shad are now the most numerous anadromous fish in the CRB, yet the magnitude of the resulting nutrient flux owing to the shift from salmon to shad is unknown. Nutrient flux models revealed that American shad conveyed over 15,000 kg of nitrogen (N) and 3,000 kg of phosphorus (P) annually to John Day Reservoir, the largest mainstem reservoir in the lower Columbia River. Shad were net importers of N, with juveniles and postspawners exporting just 31% of the N imported by adults. Shad were usually net importers of P, with juveniles and postspawners exporting 46% of the P imported by adults on average. American shad contributed &lt;0.2% of the total annual P load into John Day Reservoir, but during June when most adult shad are migrating into John Day Reservoir, they contributed as much as 2.0% of the P load. Nutrient inputs by American shad were similar to current but far less than historical inputs of Pacific salmon owing to their smaller size. Given the relatively high background P levels and low retention times in lower Columbia River reservoirs, it is unlikely that shad marine‐derived nutrients affect nutrient balances or food web productivity through autotrophic pathways. However, a better understanding of shad spawning aggregations in the CRB is needed.

Early detection monitoring for larval dreissenid mussels: how much plankton sampling is enough?

The development of quagga and zebra mussel (dreissenids) monitoring programs in the Pacific Northwest provides a unique opportunity to evaluate a regional invasive species detection effort early in its development. Recent studies suggest that the ecological and economic costs of a dreissenid infestation in the Pacific Northwest of the USA would be significant. Consequently, efforts are underway to monitor for the presence of dreissenids. However, assessments of whether these efforts provide for early detection are lacking. We use information collected from 2012 to 2014 to characterize the development of larval dreissenid monitoring programs in the states of Idaho, Montana, Oregon, and Washington in the context of introduction and establishment risk. We also estimate the effort needed for high-probability detection of rare planktonic taxa in four Columbia and Snake River reservoirs and assess whether the current level of effort provides for early detection. We found that the effort expended to monitor for dreissenid mussels increased substantially from 2012 to 2014, that efforts were distributed across risk categories ranging from high to very low, and that substantial gaps in our knowledge of both introduction and establishment risk exist. The estimated volume of filtered water required to fully census planktonic taxa or to provide high-probability detection of rare taxa was high for the four reservoirs examined. We conclude that the current level of effort expended does not provide for high-probability detection of larval dreissenids or other planktonic taxa when they are rare in these reservoirs. We discuss options to improve early detection capabilities.

Trophic Interactions and Consumption Rates of Subyearling Chinook Salmon and Nonnative Juvenile American Shad in Columbia River Reservoirs

AbstractWe used a large lampara seine coupled with nonlethal gastric lavage to examine the diets and estimate consumption rates of subyearling Chinook Salmon Oncorhynchus tshawytscha during July and August 2013. During August we also examined the diet and consumption rates of juvenile American Shad Alosa sapidissima, a potential competitor of subyearling Chinook Salmon. Subyearling Chinook Salmon consumed Daphnia in July but switched to feeding on smaller juvenile American Shad in August. We captured no juvenile American Shad in July, but in August juvenile American Shad consumed cyclopoid and calanoid copepods. Stomach evacuation rates for subyearling Chinook Salmon were high during both sample periods (0.58 h−1 in July, 0.51 h−1 in August), and daily ration estimates were slightly higher than values reported in the literature for other subyearlings. By switching from planktivory to piscivory, subyearling Chinook Salmon gained greater growth opportunity. While past studies have shown that juvenile American Shad reduce zooplankton availability for Chinook Salmon subyearlings, our work indicates that they also become important prey after Daphnia abundance declines. The diet and consumption data here can be used in future bioenergetics modeling to estimate the growth of subyearling Chinook Salmon in lower Columbia River reservoirs.Received July 27, 2016; accepted November 22, 2016 Published online February 1, 2017

Migration Depths of Juvenile Chinook Salmon and Steelhead Relative to Total Dissolved Gas Supersaturation in a Columbia River Reservoir

AbstractThe in situ depths of juvenile salmonids Oncorhynchus spp. were studied to determine whether hydrostatic compensation was sufficient to protect them from gas bubble disease (GBD) during exposure to total dissolved gas (TDG) supersaturation from a regional program of spill at dams meant to improve salmonid passage survival. Yearling Chinook salmon O. tshawytscha and juvenile steelhead O. mykiss implanted with pressure‐sensing radio transmitters were monitored from boats while they were migrating between the tailrace of Ice Harbor Dam on the Snake River and the forebay of McNary Dam on the Columbia River during 1997–1999. The TDG generally decreased with distance from the tailrace of the dam and was within levels known to cause GBD signs and mortality in laboratory bioassays. Results of repeated‐measures analysis of variance indicated that the mean depths of juvenile steelhead were similar throughout the study area, ranging from 2.0 m in the Snake River to 2.3 m near the McNary Dam forebay. The mean depths of yearling Chinook salmon generally increased with distance from Ice Harbor Dam, ranging from 1.5 m in the Snake River to 3.2 m near the forebay. Juvenile steelhead were deeper at night than during the day, and yearling Chinook salmon were deeper during the day than at night. The TDG level was a significant covariate in models of the migration depth and rates of each species, but no effect of fish size was detected. Hydrostatic compensation, along with short exposure times in the area of greatest TDG, reduced the effects of TDG exposure below those generally shown to elicit GBD signs or mortality. Based on these factors, our results indicate that the TDG limits of the regional spill program were safe for these juvenile salmonids.

Community context and the influence of non-indigenous species on juvenile salmon survival in a Columbia River reservoir

Non-indigenous species (NIS) have been called biological pollutants, which implies that reducing their numbers should reduce negative impacts. To test this hypothesis, we used food web models, parameterized with data from field studies, to ask how reducing the number of NIS co-occurring with endangered salmon would affect salmon mortality. Our analyses indicate that predation on Upper Columbia River spring chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss juveniles was affected very little by NIS reduction. The effects of removing NIS were partly or totally offset by indirect food web interactions, and were subtle compared to effects of native predator management. We predict that the most effective way of reducing predation on salmon smolts will involve managing native predators and targeted removals of specific NIS. Minimizing impact of established NIS thus entails not only reducing NIS prevalence, but also considering background management practices and community context.

EFFECTS OF CLIMATE CHANGE ON HYDROLOGY AND WATER RESOURCES IN THE COLUMBIA RIVER BASIN1

ABSTRACT: As part of the National Assessment of Climate Change, the implications of future climate predictions derived from four global climate models (GCMs) were used to evaluate possible future changes to Pacific Northwest climate, the surface water response of the Columbia River basin, and the ability of the Columbia River reservoir system to meet regional water resources objectives. Two representative GCM simulations from the Hadley Centre (HC) and Max Planck Institute (MPI) were selected from a group of GCM simulations made available via the National Assessment for climate change. From these simulations, quasi‐stationary, decadal mean temperature and precipitation changes were used to perturb historical records of precipitation and temperature data to create inferred conditions for 2025, 2045, and 2095. These perturbed records, which represent future climate in the experiments, were used to drive a macro‐scale hydrology model of the Columbia River at 1/8 degree resolution. The altered streamflows simulated for each scenario were, in turn, used to drive a reservoir model, from which the ability of the system to meet water resources objectives was determined relative to a simulated hydrologic base case (current climate). Although the two GCM simulations showed somewhat different seasonal patterns for temperature change, in general the simulations show reasonably consistent basin average increases in temperature of about 1.8–2.1°C for 2025, and about 2.3–2.9°C for 2045. The HC simulations predict an annual average temperature increase of about 4.5°C for 2095. Changes in basin averaged winter precipitation range from ‐1 percent to + 20 percent for the HC and MPI scenarios, and summer precipitation is also variously affected. These changes in climate result in significant increases in winter runoff volumes due to increased winter precipitation and warmer winter temperatures, with resulting reductions in snowpack. Average March 1 basin average snow water equivalents are 75 to 85 percent of the base case for 2025, and 55 to 65 percent of the base case by 2045. By 2045 the reduced snowpack and earlier snow melt, coupled with higher evapotranspiration in early summer, would lead to earlier spring peak flows and reduced runoff volumes from April‐September ranging from about 75 percent to 90 percent of the base case. Annual runoff volumes range from 85 percent to 110 percent of the base case in the simulations for 2045. These changes in streamflow create increased competition for water during the spring, summer, and early fall between non‐firm energy production, irrigation, instream flow, and recreation. Flood control effectiveness is moderately reduced for most of the scenarios examined, and desirable navigation conditions on the Snake are generally enhanced or unchanged. Current levels of winter‐dominated firm energy production are only significantly impacted for the MPI 2045 simulations.

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.

Growth and mortality of age-0 northern squawfish, Ptychocheilus oregonensis, rearing in shoreline habitats of a Columbia River reservoir

We investigated growth and mortality of age-0 northern squawfish during early rearing in shallow shoreline habitats. Larvae and juveniles (n=22914) were collected by weekly seining at three sample sites in the upper John Day Reservoir, Columbia River, during June through early September 1994–1996. Using a length-based ageing method, it was estimated that the exponential growth rate (G) for a common growth stanza (10–28 mm standard length SL) was significantly higher in 1994 (G=0.047) than in 1996 (G=0.037). Growth rate in 1995 could not be estimated, but was probably intermediate between 1994 and 1996 based on mean standard lengths of fish collected at the end of each sampling season (46.3, 40.0, and 32.0 mm SL in 1994, 1995, and 1996, respectively). For many fish species, variations in early growth can influence survival through size-selective mortality processes. Consistent with this possibility, our estimates of instantaneous mortality rates (Z) demonstrated that larvae and juveniles had significantly higher mortality in 1996 than in 1994 (Z=0.103 in 1994, versus Z=0.138 in 1996). Enhanced growth and lower mortality in 1994 were associated with a number of interrelated environmental conditions – comparatively low flows and turbidities, abundant instream vegetative cover, and high near-shore water temperatures.

Evaluation of a New Miniature Pressure-Sensitive Radio Transmitter

Abstract A miniature pressure-sensitive radio transmitter (tag) was evaluated and field tested as a tool for determining the depths of juvenile salmonids. The tag had an effective radiated power of −19.7 decibels (1 mW reference), dimensions of 23 mm × 7 mm, and a weight of 2.2 g in air. The pulse rate of the tag increased with pressure, resulting in an expected tag life of approximately 11 d at the water surface and 7.5 d at 10.5 m. The tags were accurate to within 16 mm with 95% of observations within ±0.32 m of the true depth. The resolution of the tags was 0.2 m. Errors in indicated depth resulting from differences between the calibration and operating temperatures were minimized by means of a correction factor. Tags surgically implanted in juvenile steelhead Oncorhynchus mykiss indicated a depth 0.2 m less than the same tags in water. This difference was not affected by pressure or temperature and was rectified by adjusting data from tags in fish. A test tag in a Columbia River reservoir was detected...

Seasonal distribution, movements and habitat associations of northern squawfish in two lower Columbia River reservoirs

We tracked 335 northern squawfish implanted with radio transmitters in the Columbia River in May–December 1993 and May–September 1994. Most fish were released near The Dalles and John Day dams, with the remaining fish released into reservoir areas away from dams. We used boats with mounted Yagi antennas, fixed site receiver stations near the dams and aerial surveys to track movements of tagged northern squawfish. Northern squawfish were commonly associated with water <5 m deep, water velocities <1 m/s and were <45 m from shore. The average movement from the release site was 19.5 km; fish released near dams remained closer to their release sites than fish released into reservoirs. Short-term movements (successive observations within 24 h) comprised 75% of all detected movements, represented fish moving short distances (mean=0.96 km) between dams and down-river areas and corresponded to increased passage of juvenile salmonids. Long-term movements (successive observations exceeding 24 h) were mainly attributed to fish released into the reservoirs moving up-river to a dam, and fish moving between the two dam tailrace areas. A strong up-river movement trend terminating at John Day Dam tailrace in June suggested that spawning occurred nearby. Reduced numbers of observations of fish in the autumn suggested that northern squawfish moved away from dam areas and into deeper water for the winter. © 1997 John Wiley & Sons, Ltd.

Seasonal distribution, movements and habitat associations of northern squawfish in two lower Columbia River reservoirs

We tracked 335 northern squawfish implanted with radio transmitters in the Columbia River in May–December 1993 and May–September 1994. Most fish were released near The Dalles and John Day dams, with the remaining fish released into reservoir areas away from dams. We used boats with mounted Yagi antennas, fixed site receiver stations near the dams and aerial surveys to track movements of tagged northern squawfish. Northern squawfish were commonly associated with water <5 m deep, water velocities <1 m/s and were <45 m from shore. The average movement from the release site was 19.5 km; fish released near dams remained closer to their release sites than fish released into reservoirs. Short-term movements (successive observations within 24 h) comprised 75% of all detected movements, represented fish moving short distances (mean=0.96 km) between dams and down-river areas and corresponded to increased passage of juvenile salmonids. Long-term movements (successive observations exceeding 24 h) were mainly attributed to fish released into the reservoirs moving up-river to a dam, and fish moving between the two dam tailrace areas. A strong up-river movement trend terminating at John Day Dam tailrace in June suggested that spawning occurred nearby. Reduced numbers of observations of fish in the autumn suggested that northern squawfish moved away from dam areas and into deeper water for the winter. © 1997 John Wiley & Sons, Ltd.

Differences in the Dynamics and Potential Production of Impounded and Unimpounded White Sturgeon Populations in the Lower Columbia River

White sturgeons Acipenser transmontanus were sampled in three lower Columbia River reservoirs from 1987 to 1991 to describe population dynamics, the ability of these stocks to sustain harvest, and differences among reservoir and unimpounded populations. Significant differences were observed among reservoirs in white sturgeon abundance, biomass, size composition, sex ratio, size of females at maturity, growth rate, condition factor, and rate of exploitation. No differences among reservoirs were detected in fecundity, natural mortality rate, or longevity, in part because of sampling difficulties. Recruitment rates and densities in reservoirs were inversely correlated with growth rate, condition factor, and size of females at maturity. Differences in population dynamics resulted in substantial differences in sustainable yields. Maximum yields per recruit were predicted at annual exploitation rates between 5 and 15%. Most characteristics of reservoir populations were less than or equal to optima reported for the unimpounded lower river; as a result, yield per recruit, reproductive potential per recruit, and the number of recruits were less in reservoirs than in the unimpounded river. Comparisons with pristine standing stocks suggest that the unimpounded river may approximate preimpoundment conditions for white sturgeon. We conclude that potential yield from impounded populations has been reduced by dam construction, which restricts populations to river segments that may not include conditions optimal for all life stages. Alternatives for enhancement of reservoir populations might include improved passage at dams, increased spring flow to improve spawning success, transplants from productive populations, hatchery supplementation, and more intensive harvest management.

Variability in Biological Characteristics of Northern Squawfish in the Lower Columbia and Snake Rivers

Abstract We described and compared population structure, growth, mortality, and reproduction of northern Squawfish Ptychocheilus oregonensis among four reservoirs of the lower Columbia River and among four reservoirs of the lower Snake River. We also pooled data for comparisons among three larger areas: Columbia River reservoirs, Snake River reservoirs, and the unimpounded Columbia River downstream from Bonneville Dam. Females made up 76–100% of northern squawfish 380 mm fork length or greater in individual reservoirs, and 87–99% of fish 380 mm or greater in the three combined areas. Females lived longer and grew faster than males in all reservoirs and areas and had lower annual mortality rates in all but two reservoirs. Proportional stock density was highest and annual mortality rates were generally lowest in Columbia River reservoirs. Northern Squawfish downstream from Bonneville Dam had a lower mean relative weight and higher relative fecundity (eggs per unit fish weight) than populations in impoundmen...

Changes in Catch Rate and Diet of Northern Squawfish Associated with the Release of Hatchery-Reared Juvenile Salmonids in a Columbia River Reservoir

Abstract We investigated the distribution and predation activities of northern Squawfish Ptychocheilus oregonensis at three locations where hatchery-reared juvenile salmonids Oncorhynchus spp. were released in Bonneville Pool, a Columbia River reservoir. Significant increases in northern Squawfish catch rate occurred at all three locations after releases. The timing and duration of elevated catch rates appear to be closely related to the release and subsequent residence time of the hatchery-released fish in the sampling area. Northern Squawfish caught after salmonid releases had a significantly higher frequency of occurrence and mean number of juvenile salmonids in their diet compared with fish caught before releases. Consumption indices, relative measures of consumption rates, were also higher at each location after release, Our results suggest that northern Squawfish aggregate to feed on hatchery-released juvenile salmonids in the spring. Management activities to reduce predation on juvenile salmonids s...

Importance of Spatial Pattern in Estimating Predation on Juvenile Salmonids in the Columbia River

The impact of piscivores in aquatic systems is often estimated by assuming that predation rate and predator density can be characterized as means throughout large, homogeneous areas. Predation losses of juvenile Pacific salmonids Oncorhynchus spp. migrating through Columbia River reservoirs were previously estimated with the assumption that each reservoir consisted of one or two homogeneous areas. Data from the John Day Reservoir and throughout the river system showed that predation rate and predator density vary greatly between near-dam and midreservoir areas, suggesting that reservoirs in the Columbia River should be divided into at least three or four areas for estimating salmonid losses. For example, the estimated number of salmonids annually eaten by northern squawfish Ptychocheilus oregonensis in John Day Reservoir decreased from 2.9 million when all samples in the reservoir were pooled into one area, to 1.4 million when samples were partitioned among four areas. Variance about the estimates also decreased steadily with finer partitioning. Mortality of juvenile salmon from predation was substantial with any type of partitioning; however, spatial variation in predation rates and other density-dependent processes may be especially important in river models of migrating juvenile salmon that repeatedly apply predation rates in a series of reservoirs or river reaches.

A stochastic, compartmental model of the migration of juvenile anadromous salmonids in the Columbia River Basin

A probabilistic model is developed which describes the juvenile migration of Pacific salmon and steelhead trout ( Oncorhynchus sp.) in the Columbia River Basin in western North America. The downstream passage of smolts through a series of reservoirs or river reaches is modeled as an irreversible particle diffusion process through a series of compartments. The probability of live passage from one compartment to the next t time units after having entered the compartment can be viewed as the product of the probability of having survived to time t, the probability of transiting the compartment in t time units, and the probability of a successful transition between compartments at time t. From this basic premise, a general passage model is developed. Focusing on survival through Columbia and Snake River reservoirs (as opposed to dam-related mortality), a specific model which uses a Poisson death process and a gamma distribution to describe transmit time is presente. Analytical solutions to the single-compartment system are derived with extensions to the multi-compartment system. Empirical data from the Columbia River is used to demonstrate the utility of the model as well as to highlight problems that exist in fitting the model using existing data. Graphical techniques are presented for using the model to assess the effectiveness of management actions that are designed to improve the reservoir survival of smolts.

Management Implications of a Model of Predation by a Resident Fish on Juvenile Salmonids Migrating through a Columbia River Reservoir

Abstract We constructed a model ofpredation by northern squawfish Ptychocheilus oregonensis on juvenile salmonids migrating through John Day Reservoir. The model predicts salmonid survival as a function of number and distribution of northern squawfish, number and timing of juvenile salmonids entering the reservoir, salmonid residence time, water temperature, and flow. The model predicted survival similar to independent estimates for 1983–1986 and also approximated differences among areas and months. Uncertainty analyses showed that the number of salmonids surviving predation may vary ±5% with normal annual variation in predator number, temperature, and flow. Survival in 1983–1986 was near the average predicted from 30 years of historic environmental data. Sensitivity analyses implied that the best avenues ofreducing predation are to reduce the number of northern squawfish, pass salmonids earlier in the year, and maintain sizes of runs of juvenile salmonids at or above present levels. Survival of salmonids...