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COMPARATIVE SURVIVAL STUDY (CSS) of PIT-Tagged Spring/Summer Chinook and Steelhead In the Columbia River Basin: Ten-year Retrospective Summary Report
Abstract
The Comparative Survival Study (CSS) Oversight Committee prepared this report to
address the recommendation provided by the Independent Scientific Advisory Board (ISAB) to
prepare a retrospective synthesis of the methods and results to date on spring/summer Chinook
and steelhead in the Columbia Basin. This ten-year summary report describes study methods,
results and conclusions based on ten years of monitoring efforts. The Passive Integrated
Transponder (PIT) data used in the CSS are analyzed retrospectively, incorporating all juvenile
and adult recovery data available for the period 1996 through 2006.
The Ten-Year Retrospective Summary Report analyzes the available PIT-tag data withinand
across-years, assessing the effects of migration routes, environmental conditions and
migration timing on juvenile reach survival rates and Smolt-to-Adult Return rates (SAR). These
analyses provide for improved understanding of survival rates and the effects of various
environmental conditions and management actions on those rates.
Synopsis of Key Findings
• Juvenile travel times, instantaneous mortality rates and survival rates through the
hydrosystem are strongly influenced by managed river conditions including flow, water
travel time and spill levels.
• Statistical relationships were developed that can be used to predict the effects of
environmental factors and management strategies on migration and survival rates of
juvenile yearling Chinook and steelhead.
• The CSS results indicate that the SAR of transported fish relative to in-river migrants
(TIR) varied across species and between wild and hatchery origins. Wild spring/summer
Chinook on average showed no benefit from transportation, except in the severe drought
year (2001). Hatchery spring/summer Chinook responded to transportation with higher
TIR averages across hatcheries than wild Chinook. Wild and hatchery steelhead
responded to transportation with the highest TIR. Substantial differential delayed
transport mortality (D < 1.0) was evident for both species and across wild and hatchery
groups for each species.
• Overall SARs for wild spring/summer Chinook and wild steelhead fell short of the
Northwest Power and Conservation Council (NPCC) SAR objectives (2% minimum, 4%
average for recovery).
• SAR values for these Snake River Basin groups were only one quarter those of similar
downriver populations that migrated through a shorter segment of the Federal Columbia
River Power System (FCRPS).
The above lines of evidence for Snake River reach survivals, SARs by passage route,
overall SARs, and downriver SARs relative to the NPCC objectives, indicate that
collecting and transporting juvenile spring/summer Chinook and steelhead at Snake River
Dams did not compensate for the effects of the FCRPS.
• The overall SARs are also insufficient to meet broad sense recovery goals that include
providing harvestable surplus for wild Snake River Basin spring/summer Chinook and
steelhead.
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• Adult upstream migration survival is affected by the juvenile migration experience.
Adults that were transported from Lower Granite Dam as smolts exhibited a 10% lower
adult upstream survival rate than either in-river migrants or those transported from Little
Goose or Lower Monumental Dams.
• Simulations results indicate that Cormack-Jolly-Seber parameter estimates are robust in
the presence of temporal changes in survival or detection probabilities.
Given the different responses of wild Chinook and wild steelhead to transportation, it
would seem that maximization of survival of both species cannot be accomplished by
transportation as currently implemented.
• Our analyses on in-river survival rates indicate that improvements in in-river survival can
be achieved through management actions that reduce the water travel time or increase the
average percent spilled for Snake River yearling Chinook and steelhead in the Lower
Granite to McNary reach. The effectiveness of these actions varies over the migration
season.
• Higher SARs of Snake River wild yearling Chinook were associated with faster water
travel times during juvenile migration through the FCRPS, cool broad-scale ocean
conditions, and near-shore downwelling during the fall of the first year of ocean
residence.
... In-river migrants from the Snake River inevitably pass eight dams on their way to the Pacific Ocean, whereas fish from the John Day River encounter only three. Schaller et al. (2007) found that when out-migration timing of smolts originating from the Snake and John Day rivers overlapped at Bonneville Dam, rates of adult return were consistently higher for John Day River fish despite no clear evidence of a systematic size difference between the two groups. More recently, Schaller et al. (2014) again compared the performance of Snake and John Day River populations of Chinook Salmon. ...
... More recently, Schaller et al. (2014) again compared the performance of Snake and John Day River populations of Chinook Salmon. They considered two approaches: the first, updating Schaller et al. (2007), assessed differences in life cycle survival among fish that encountered three dams (John Day River fish) versus those that encountered eight dams (Snake River fish); and the second examined the differential response of John Day and Snake River populations to completion of the Federal Columbia River Power System (FCRPS). Echoing the findings of Schaller et al. (2007), John Day River fish performed dramatically better in FIGURE 3. Model-predicted bypass probability at Little Goose Dam (Snake River) for wild yearling Chinook Salmon tagged above Lower Granite Dam as a function of spill proportion. ...
... They considered two approaches: the first, updating Schaller et al. (2007), assessed differences in life cycle survival among fish that encountered three dams (John Day River fish) versus those that encountered eight dams (Snake River fish); and the second examined the differential response of John Day and Snake River populations to completion of the Federal Columbia River Power System (FCRPS). Echoing the findings of Schaller et al. (2007), John Day River fish performed dramatically better in FIGURE 3. Model-predicted bypass probability at Little Goose Dam (Snake River) for wild yearling Chinook Salmon tagged above Lower Granite Dam as a function of spill proportion. Bypass probabilities are plotted at three different length categories (10th, 50th, and 90th percentiles). ...
... In addition to being affected by ocean conditions, mortality of salmon can also occur as a result of downstream and upstream passage through freshwater systems (Schaller et al. 2007(Schaller et al. , 2014Petrosky and Schaller 2010;Haeseker et al. 2012). Discharge-dependent costs of locomotion and temperature-mediated costs of metabolism are thought to be highly influential on in-river mortality (e.g., Smith et al. 2003;Rand et al. 2006;Crossin et al. 2008;Naik and Jay 2011;Martins et al. 2012). ...
... In some systems, such as the Columbia River basin, juvenile salmon also must transit through an extensive hydropower system during their out-migration ( Figure 1). Juvenile fish are either spilled over the dams or enter the powerhouse, at which point they either go through turbines or go into collection systems from which they are routed back to the river or transported (Schaller et al. 2007(Schaller et al. , 2014Petrosky and Schaller 2010). The route of passage through the Federal Columbia River Power System (hereafter, hydropower system) has been found to impact the survival of Columbia River juvenile Chinook Salmon O. tshawytscha and steelhead O. mykiss (e.g., Schaller et al. 2007Schaller et al. , 2014Petrosky and Schaller 2010;Haeseker et al. 2012). ...
... Juvenile fish are either spilled over the dams or enter the powerhouse, at which point they either go through turbines or go into collection systems from which they are routed back to the river or transported (Schaller et al. 2007(Schaller et al. , 2014Petrosky and Schaller 2010). The route of passage through the Federal Columbia River Power System (hereafter, hydropower system) has been found to impact the survival of Columbia River juvenile Chinook Salmon O. tshawytscha and steelhead O. mykiss (e.g., Schaller et al. 2007Schaller et al. , 2014Petrosky and Schaller 2010;Haeseker et al. 2012). ...
Snake River Sockeye Salmon Oncorhynchus nerka were declared endangered in 1991 after several years of
decreasing abundance. Several factors, including poor marine survival, likely contributed to the decline of Snake River Sockeye Salmon. Little is known about their migration and ocean distribution and the factors influencing their production. We sampled (1) coastal waters from southern British Columbia (BC) to southeast Alaska during June–July, October–November, and February–March 1998–2011; and (2) Oregon and Washington coastal waters during May–June and September 2007–2010. In total, 8,227 juvenile Sockeye Salmon were captured. Despite their extremely low abundance relative to other stocks, 15 coded-wire-tagged juveniles from Redfish Lake were recovered since 2007, primarily in spring and summer surveys off the BC coast. Genetic analyses revealed that an additional eight Redfish Lake juveniles were also present in this area during summer. Snake River smolts undertook a rapid northward migration that brought them well beyond the Columbia River estuary and plume, exposing them to ocean conditions prevailing off BC. Through a multimodel inference approach, we characterized associations between the number of returning adults and a suite of ocean and river variables. Seven ocean variables and five river variables were chosen for the model selection analysis (e.g., copepod biomass anomalies, coastal upwelling indices, date of the spring transition, river discharge, river temperature, and the proportion of smolts transported through the hydropower system). Although adult returns were highly correlated with smolt abundance, our analyses suggest that ocean conditions encountered during the first growing season (as indexed by copepod anomalies) contribute to the variability in total adult returns. There was also evidence for a negative effect of transporting smolts through the hydropower system, with the caveat that we used transportation data for steelhead O. mykiss as a proxy.
... Transported smolts avoid most of the direct mortality of in-river migrants but experience the injuries and stresses of collection systems at the transport dam, crowding and exposure to pathogens in holding raceways and barges, and altered estuary arrival timing (Budy et al. 2002;Van Gaest et al. 2011). There are numerous lines of evidence that barged fish may incur additional mortality after release as a result of these stresses of collection and transport and the altered estuary arrival timing (Budy et al. 2002;Schaller et al. 2007;Tuomikoski et al. 2011). ...
... Spatial evaluations have contrasted survival rate patterns of Snake River populations with those of reference populations that pass fewer FCRPS dams. These evaluations have generally found that the survival rate of the Snake River population group was a small fraction of that for the reference group (Peters and Marmorek 2001;Schaller and Petrosky 2007;Hinrichsen and Fisher 2009). However, the relevance of upriver and downriver spatial population comparisons that infer common climatic influences to estimate FCRPS impacts was questioned by Zabel and Williams (2000), Levin and Tolimieri (2001), and Williams et al. (2005). ...
... The addition of 11 populations to the seven index populations greatly increases the geographic scope to include multiple populations within each of the four MPGs of the Snake River ESU upstream of Lower Granite Dam. For populations downstream from the Snake River, we updated the SR data for three John Day River populations from the mid-Columbia ESU with unpublished information provided by Oregon Department of Fish and Wildlife, following the convention employed in Schaller and Petrosky (2007). ...
Evidence suggests Snake River stream-type Chinook salmon (Oncorhynchus tshawytscha) experience substantial delayed mortality in the marine environment as a result of their outmigration experience through the Federal Columbia River Power System (FCRPS). We analyzed mortality patterns using methods that incorporated downriver reference populations passing fewer dams, and temporal approaches that were independent of reference populations. Our results from the alternative spatial and temporal methods consistently corroborated with spawner–recruit residuals and smolt-to-adult survival rate data sets, indicating that Snake River salmon survived about one quarter as well as the reference populations. Temporal analysis indicated that a high percentage (76%) of Snake River juvenile salmon that survived the FCRPS subsequently died in the marine environment as a result of their outmigration experience. Through this and previous studies, it is evident that delayed hydrosystem mortality increases with the number of powerhouse passages and decreases with the speed of outmigration. Therefore, a promising conservation approach would be to explore management experiments that evaluate these relationships by increasing managed spill levels at the dams during the spring migration period.
Abundances of important and imperiled fishes of the Snake River Basin continue to decline. We assessed the rationale for breaching the four lower Snake River Basin dams to prevent complete loss of these fishes, and to maximize their likelihood of recovery. We summarize the science surrounding Sockeye Salmon (Oncorhynchus nerka), Chinook Salmon (O. tshawytscha), steelhead (O. mykiss), Bull Trout (Salvelinus confluentus), White Sturgeon (Acipenser transmontanus), and Pacific Lamprey (Entosphenus tridentatus). From this, we drew ten conclusions: (1) development of the Columbia River System (including the Snake River Basin) has converted mainstem rivers into reservoirs, altering fish behavior and survival; (2) most populations currently record their lowest abundance; (3) the Columbia River System dams reduce productivity of diadromous fishes in the highest-quality spawning grounds that could buffer against future climate dynamics; (4) past actions have done little to reduce impacts or precipitate recovery; (5) the Columbia River System constrains survival and productivity of salmon, steelhead and Bull Trout; (6) Snake River Basin salmon and steelhead remain at high extinction risk; (7) eliminating migration impediments and improving mainstem habitats are essential for maintaining genetic diversity and improving Bull Trout persistence; (8) the lower Snake River Basin dams preclude passage of adult White Sturgeon, constraining gene flow and recruitment; (9) the lower Snake River Basin dams impede dramatically passage of adult and juvenile Pacific Lamprey, and (10) Snake River Basin Pacific Lamprey is at high risk of extirpation. Breaching the four lower Snake River Basin dams is an action likely to prevent extirpation and extinction of these fishes. Lessons from the Columbia River System can inform conservation in other impounded rivers.
Abundances of important and imperiled fishes of the Snake River Basin continue to decline. We assessed the rationale for breaching the four lower Snake River Basin dams to prevent complete loss of these fishes, and to maximize their likelihood of recovery. We summarize the science surrounding Sockeye Salmon (Oncorhynchus nerka), Chinook Salmon (O. tshawytscha), steelhead (O. mykiss), Bull Trout (Salvelinus confluentus), White Sturgeon (Acipenser transmontanus), and Pacific Lamprey (Entosphenus tridentatus). From this, we drew ten conclusions: (1) development of the Columbia River System (including the Snake River Basin) has converted mainstem rivers into reservoirs, altering fish behavior and survival; (2) most populations currently record their lowest abundance; (3) the Columbia River System dams reduce productivity of diadromous fishes in the highest-quality spawning grounds that could buffer against future climate dynamics; (4) past actions have done little to reduce impacts or precipitate recovery; (5) the Columbia River System constrains survival and productivity of salmon, steelhead and Bull Trout; (6) Snake River Basin salmon and steelhead remain at high extinction risk; (7) eliminating migration impediments and improving mainstem habitats are essential for maintaining genetic diversity and improving Bull Trout persistence; (8) the lower Snake River Basin dams preclude passage of adult White Sturgeon, constraining gene flow and recruitment; (9) the lower Snake River Basin dams impede dramatically passage of adult and juvenile Pacific Lamprey, and (10) Snake River Basin Pacific Lamprey is at high risk of extirpation. Breaching the four lower Snake River Basin dams is an action likely to prevent extirpation and extinction of these fishes. Lessons from the Columbia River System can inform conservation in other impounded rivers.
We analyze and compare productivity and survival rates of stream‐type Chinook Salmon Oncorhynchus tschawystcha populations from the upper and middle ranges of their distribution in the Columbia River basin. These two groups of populations undergo vastly different exposures during migration through the Federal Columbia River Power System (FCRPS). Declines of the Snake River populations, listed as Threatened under the U.S. Endangered Species Act (ESA), have been associated with development and operation of the FCRPS. In contrast, John Day River stream‐type Chinook Salmon populations, which were less affected by the FCRPS, have declined to a lesser extent and are not listed. Smolt‐to‐adult survival rates (SARs) accounted for a majority of the variation in life‐cycle survival rates of Snake River Chinook Salmon. Productivity declined to 13% and 44% of historical productivity levels for Snake River and John Day River populations, respectively. A synthesis of previous studies contrasting anthropogenic impacts between the two regions supports the conclusion that FCRPS impacts explain the large difference in population productivity. Our results suggest that SARs of 4% would result in an expected productivity of up to 70% of historical levels (an SAR level consistent with regional restoration objectives). SARs have been shown to be highly influenced by conditions within the FCRPS (e.g., water velocity and passage through dam powerhouses). Marine conditions also influence SARs; however, meaningful management actions are only available to affect conditions within the FCRPS. Given the importance of SARs to overall life‐cycle productivity, recovery and restoration strategies need to prioritize actions that have potential to substantially increase SARs by addressing the significant impacts of mainstem dams. This study highlights the importance of considering river management options in the face of increasingly variable and warming ocean conditions.
Pacific salmon Oncorhynchus spp. from the Snake River basin experience a wide range of environmental conditions during their freshwater, estuarine, and marine residence, which in turn influence their survival rates at each life stage. In addition, researchers have found that juvenile out-migration conditions can influence subsequent survival during estuarine and marine residence, a concept known as the hydrosystem-related, delayed-mortality hypothesis. In this analysis, we calculated seasonal, life-stage-specific survival rate estimates for Snake River spring–summer Chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss and conducted multiple-regression analyses to identify the freshwater and marine environmental factors associated with survival at each life stage. We also conducted correlation analyses to test the hydrosystem-related, delayed-mortality hypothesis. We found that the freshwater variables we examined (the percentage of river flow spilled over out-migration dams and water transit time) were important for characterizing the variation in survival rates not only during freshwater out-migration but also during estuarine and marine residence. Of the marine factors examined, we found that the Pacific Decadal Oscillation index was the most important variable for characterizing the variation in the marine and cumulative smolt-to-adult survival rates of both species. In support of the hydrosystem-related, delayed-mortality hypothesis, we found that freshwater and marine survival rates were correlated, indicating that a portion of the mortality expressed after leaving the hydrosystem is related to processes affected by downstream migration conditions. Our results indicate that improvements in life-stage-specific and smolt-to-adult survival may be achievable across a range of marine conditions through increasing spill percentages and reducing water transit times during juvenile salmon out-migration.Received June 23, 2010; accepted July 1, 2011
Petrosky CE, Schaller HA. Influence of river conditions during seaward migration and ocean conditions on survival rates of Snake River Chinook salmon and steelhead. Ecology of Freshwater Fish 2010: 19: 520–536. © 2010 John Wiley & Sons A/S
Abstract – Improved understanding of the relative influence of ocean and freshwater factors on survival of at-risk anadromous fish populations is critical to success of conservation and recovery efforts. Abundance and smolt to adult survival rates of Snake River Chinook salmon and steelhead decreased dramatically coincident with construction of hydropower dams in the 1970s. However, separating the influence of ocean and freshwater conditions is difficult because of possible confounding factors. We used long time-series of smolt to adult survival rates for Chinook salmon and steelhead to estimate first year ocean survival rates. We constructed multiple regression models that explained the survival rate patterns using environmental indices for ocean conditions and in-river conditions experienced during seaward migration. Survival rates during the smolt to adult and first year ocean life stages for both species were associated with both ocean and river conditions. Best-fit, simplest models indicate that lower survival rates for Chinook salmon are associated with warmer ocean conditions, reduced upwelling in the spring, and with slower river velocity during the smolt migration or multiple passages through powerhouses at dams. Similarly, lower survival rates for steelhead are associated with warmer ocean conditions, reduced upwelling in the spring, and with slower river velocity and warmer river temperatures. Given projections for warming ocean conditions, a precautionary management approach should focus on improving in-river migration conditions by increasing water velocity, relying on increased spill, or other actions that reduce delay of smolts through the river corridor during their seaward migration.
Using yearling chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss tagged with passive integrated transponders (PITs), we estimated passage survival through bypass systems, turbines, and spill bays with and without flow deflectors at Snake River dams relative to survival of fish released into the tailrace below the dam. Actively migrating fish were collected and marked with PIT tags at Snake River dam smolt collection facilities. Groups of tagged fish were then released through hoses into different passage routes; releases were coincident with a tailrace release approximately 1–2 km below the dam. Relative survival was estimated by the use of tag–recapture models for paired releases from detections of individual PIT-tagged fish at juvenile collection or detection facilities at downstream dams. Detection sites included Little Goose, Lower Monumental, McNary, John Day, and Bonneville dams, depending on the release location and year. Standard errors of relative survival probability estimates were generally less than 3.0% through all potential passage routes. The estimated relative survival was highest through spill bays without flow deflectors (98.4–100%), followed by spill bays with flow deflectors (92.7–100%), bypass systems (95.3–99.4%), and turbines (86.5–93.4%). These estimates of relative survival, which include both the direct and indirect effects of passage, are generally higher than past estimates but similar to other recent estimates determined with modern techniques under present dam configurations and operating conditions.
Past research indicates that on an annual basis, smolts of stream-type Chinook salmon Oncorhynchus tshawytscha collected at Snake River dams and transported by barge to below Bonneville Dam have greater post-hydropower system mortality than smolts that migrate in-river. To date, this difference has most commonly been attributed to stress from collection and transportation, leading to decreased disease resistance or predator avoidance ability. Using both hatchery and wild passive integrated transponder (PIT) tagged Chinook salmon, we explored two mechanisms that either separately or jointly contributed to an alternative explanation: Altered timing of ocean entry and lost growth opportunity leading to size-selective predation. Based on weekly estimates of in-river survival and adult return rates of smolts that were transported or that migrated in-river between Lower Granite and Bonneville dams, we found greater post-hydropower system mortality for smolts transported early in the season but greater mortality for in-river migrating smolts later in the season. Migrants took 2–4 weeks to travel between the two dams, while transported fish took less than 2 d. Thus, fish leaving Lower Granite Dam under the two transit modes encountered different conditions downstream from Bonneville Dam. Further, wild and hatchery migrants grew 6–8 and 5–6 mm, respectively, while transported fish had no apparent growth in the less than 2-d barge ride. Using length data and regression equations of size selectivity, we found that transported smolts were more vulnerable to predation by northern pikeminnow Ptychocheilus oregonensis (freshwater) and Pacific hake Merluccius productus (marine) than were migrants; this was particularly true for the smaller wild smolts transported early in the season. We concluded that the most parsimonious explanation for differential post-hydropower system mortality of transported Chinook salmon smolts related not to effects of stress but to differential size and timing of ocean entry.
We compared the release-to-adult returns of coded-wire-tagged groups of fall chinook salmon Oncorhynchus tshawytscha in a 2 × 2 factorial experimental design: subyearlings and yearlings released directly from a hatchery versus those barged below two main-stem hydroelectric dams on the Snake River, Washington. Releases comprised six brood years over a 6-year period. In every release year, chinook salmon released as yearlings performed better than subyearlings; this was true both for returns to the Snake River and for their contribution to Pacific Ocean and lower Columbia River fisheries. We found no significantly consistent differences in rates of return between transported and on-station releases for either age-class. Return rates of both subyearlings and yearlings varied significantly with year of release. Median migration speeds of branded chinook salmon released on-station varied directly with Snake River flows. However, there was no general relation between flow (and attendant spill) in the Snake and lower Columbia rivers and subsequent release-to-adult returns, both for subyearlings and yearlings. Likewise, transportation past two dams did not improve the adult returns of yearlings in low-flow conditions, and it had limited but inconsistent benefits to subyearlings when Snake River flow and spill were low. The advantages of transportation past two main-stem dams were probably offset by ( 1 ) additional handling stress of loading and unloading fish for transportation and (2) the short duration of transport relative to the time required to acclimate to barge hauling. Most notable was the significant variation in annual rates of return: 1,500% for subyearlings and 700% for yearlings. Both age-groups that migrated in 1985 returned at high rates. Transported chinook salmon strayed to freshwater areas outside the Snake River basin at a significantly higher rate (5.9%) than those released on-station (0.3%). Conversely, chinook salmon released on-station strayed to locations upstream of the hatchery in the Snake River basin at a higher rate (7.8%) than chinook salmon that were transported (2.4%). Overall, the stray rate for all treatment groups to locations outside the Snake River basin was 1.8%.
Straying results in an exchange of individuals between wild and hatchery salmon populations and is important because it could affect their genetic differences. Understanding what factors affect straying could help in the development of procedures for controlling the influence of specific stocks on nearby populations. We explore the effects of release date and the transportation of juvenile Chinook salmon Oncorhynchus tshawytscha on straying by adults from two Columbia River hatcheries. Early and late releases from Washougal Hatchery produced much higher straying levels than intermediate release dates, and this pattern was consistent among brood years. Fish reared at Grays River Hatchery and released from Washougal Hatchery strayed significantly more than local (i.e., Washougal) fish of the same brood year and release date. Our results and those from previous studies indicate that the sequence of imprinting events is characterized by the combination of the time at which the fish experience given locations and the physiological state of the fish at that time. Changes in release date and transportation disrupt the sequence and timing of these events, with varying effects on homing. Hatchery rearing and release techniques can sometimes be modified to control straying and minimize the impact of hatchery fish on other hatchery and wild stocks.
Snake River stream-type Chinook salmon Oncorhynchus tshawytscha exhibited substantial delayed mortality despite recent improvements in oceanic and climatic conditions. These salmon declined sharply with the completion of the Columbia River hydrosystem in addition to other anthropogenic impacts and changes in oceanic conditions. Previous analytical approaches have compared management options for halting the population decline. The predicted benefits of these options on salmon recovery hinged on whether the source of the mortality that takes place in the estuary and during early ocean residence is related to earlier hydrosystem experience during downstream migration (i.e., delayed hydrosystem mortality). We analyzed the spatial and temporal patterns of mortality for Chinook salmon populations to determine whether delayed mortality for the Snake River populations decreased during the recent period of favorable oceanic and climatic conditions. We found that Snake River stream-type Chinook salmon populations continued to exhibit survival patterns similar to those of their downriver counterparts but survived only one-fourth to one-third as well. The hypothesis that delayed mortality decreased and became negligible with more favorable oceanic conditions appears inconsistent with the patterns we observed for the common year effect and our estimates of delayed mortality of in-river migrants. A plausible explanation for this persistent pattern of delayed mortality for Snake River populations is that it is related to the construction and operation of the hydrosystem.
We used 11 years of parr-to-smolt survival estimates from 33 Snake River sites to demonstrate that despite a number of confounding factors higher numbers of past habitat remediation or enhancement actions are associated with higher parr-to-smolt survival of endangered wild Snake River spring−summer (stream-type) Chinook salmon Oncorhynchus tshawytscha. Information-theoretic weights were applied to help distinguish between statistical models based on their relative plausibility. In the models with the highest estimated weights, actions taken to improve fish habitat showed a positive association with increased parr-to-smolt survival. However, because the actions were not sited randomly on the landscape, and because the actions may also have influenced other potentially important covariates, it is difficult to separate habitat action effects from effects due to other important factors.
Migration rates of yearling chinook salmon (Oncorhynchus tshawytscha) through free-flowing and impounded stretches of the Snake and Columbia Rivers were compared during periods of low and moderate river discharge. Generally, the rate of migration was directly related to the water flows; it was 21 km/day at the low river discharge (Columbia—4,248 m/sec; Snake—1,416 m/sec), and 37 km/day during moderate river discharge (Columbia—8,495 m/sec; Snake—2,265 m/sec). Migration rates through most free-flowing and impounded stretches of the Snake and Columbia Rivers were similar with the one exception that in McNary Reservoir (Columbia River) fish moved only about one-third as fast as elsewhere.Marked yearling chinook salmon from the Salmon River took 32 days to travel the 669 km to Bonneville Dam during the low river discharge in 1966. New impoundments may more than double the travel time now required during low river flows.
This paper examines average annual survival of juvenile spring–summer chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss during migration through the hydropower system of the Snake and Columbia rivers from 1966 to 1980 and 1993 to 1999. In each year, survival was estimated from observations of marked fish in a portion of the hydropower system corridor. We expanded these estimates to calculate an annual estimate of survival over the entire system (head of uppermost reservoir to tailrace of lowermost dam). Temporal changes in the hydropower system were compared with trends in estimated survival to evaluate the effects of dams on survival of downstream migrants. When only four dams were in place (1966–1967), estimates of survival through the hydropower system were 32–56%. Four additional dams were constructed between 1968 and 1975. Survival estimates during the 1970s typically were 10–30% for spring–summer chinook salmon, but less than 3% in the drought years of 1973 and 1977. From 1993 to 1999, after structural and operational changes in the hydropower system, survival estimates of spring–summer chinook salmon and steelhead ranged from 31% to 59%. Smolt-to-adult return rates of Snake River wild spring–summer chinook salmon from the middle to late 1960s generally exceeded 4% but decreased during the 1970s. Although survival through the hydropower system in the 1990s is substantially greater than that in the 1970s, adult return rates in the 1990s have remained low. Thus, in the 1990s, the cause of the continuing low adult return rates does not seem to be related to direct mortality of downstream migrant fish within the hydropower system.