Geology topics

Ryan G. Tomka

Publications and source records attributed to Ryan G. Tomka.

9 recordsLinked to original sources

USGS2020

A standard operating procedure for the preparation of purposely killed juvenile salmon used to test survival model assumptions

This document describes a standard operating procedure (SOP) for the preparation of purposely killed juvenile salmon, implanted with telemetry transmitters, to be released into rivers, lakes, or streams to test one of the survival model assumptions. Procedures for releases of purposely killed fish (hereinafter dead fish releases) were developed by staff from the U.S. Geological Survey’s Columbia River Research Laboratory, on the basis of laboratory experiments and practical experience with telemetry studies in the Columbia River Basin. Initially, we used extended exposure to high dose anesthetic baths to euthanize fish for dead fish releases. This approach was selected on the basis of euthanization procedures described in the literature for studies that required an effective and rapid procedure, such as stress physiology assessments. Ultimately, this technique was deemed insufficient because detection records suggested that some fish seemed to revive and continue their migration with limited effect. That is, the detection histories of dead fish were very similar to those of live fish. To overcome this challenge, we adapted our procedures to require a combination of euthanization procedures on individual fish to ensure that there was no opportunity for revival. A combination of euthanization procedures for dead fish releases was used in one study in Germany. This SOP has been used by the U.S. Geological Survey to test survival model assumptions in several field studies and has consistently performed well. In addition, limited laboratory tests were completed to ensure that no live juvenile salmon were found in holding tanks for 24 hours following the procedures described in this SOP.

Open-File Report↗

USGS2020

Behavior and movement of adult winter steelhead ( Oncorhynchus mykiss ) in the upper Cowlitz River Basin, Washington, 2017–18

Executive Summary A 2-year radiotelemetry study was completed to monitor the movements of adult winter steelhead ( Oncorhynchus mykiss ) in the upper Cowlitz River Basin. A reintroduction program was established to restore healthy and harvestable populations of steelhead because volitional access to the area was blocked in the 1960s after construction of dams in the lower river. A trap-and-haul program is used to move adult steelhead and salmon ( Oncorhynchus spp.) upstream, around the dams and large reservoirs, and release them in the upper basin to spawn naturally. Fish are released into Lake Scanewa, the uppermost reservoir in the system, and into the Cowlitz and Cispus Rivers. The goal of this study was to describe the behavior, movement, and tributary use of adult steelhead in the upper Cowlitz River Basin to assist in evaluating the trap-and-haul program and reintroduction program. We were specifically interested in learning more about the locations where steelhead spawn. Individual fish were assigned one of four fates, based on their location during the spawning period: Cowlitz River, Cispus River, Lake Scanewa, or fallback below the dam that impounds the reservoir. A total of 215 steelhead were tagged for the 2017–18 study. Of these, 5 fish regurgitated their transmitters before or shortly after release, so 210 tagged fish were used for analyses, including 121 fish (57.6 percent) released into the Cispus River and 89 fish (42.4 percent) released into Lake Scanewa. The Cowlitz River release site was not evaluated. Hatchery-origin (HOR) and natural-origin (NOR) steelhead were included in the study. Within the first 10 days after release, most steelhead had moved at least 2 river kilometers away from their release site. Fish released into Lake Scanewa, however, moved from their release site significantly sooner than fish released into the Cispus River, regardless of origin or sex. Most radio-tagged steelhead (93–100 percent) made no more than one trip between the reservoir and one of the rivers prior to spawning. Steelhead were predominantly assigned fates in the river closest to where they were released, but origin also played a role. Steelhead released into the Cispus River were assigned Cispus River fates more than 80 percent of the time, including both origins and both years. About 13 percent of NOR fish had fates in the Cowlitz River, but the proportion was lower for HOR fish (0–2 percent). Fallback was the least common fate for Cispus-released steelhead (three HOR fish in 2017), followed by the reservoir fate, ranging from 5 to 9 percent in 2017, and zero in 2018. The steelhead released into Lake Scanewa were almost exclusively NOR fish, which had primarily Cowlitz River fates (55–57 percent). We released only six HOR steelhead into Lake Scanewa, and they all had Cowlitz River fates. The reservoir fate was uncommon for both release sites, and it was consistently lower in 2018 compared to 2017. Flow conditions were higher in 2017, which may have affected steelhead movement patterns or timing. The three HOR steelhead released in the Cispus River in 2017 were the only fish that fell back over Cowlitz Falls Dam, which represented 1.4 percent of the total study fish, 5.7 percent of the Cispus-released fish in 2017, and 9.4 percent of the Cispus-released HOR fish in 2017. About 62 percent of the steelhead released in Lake Scanewa spawned in the Cowlitz River, with the remaining 38 percent in the Cispus River. Within the Cowlitz River, close to one-half (about 47 percent) of the fish spawned in the upper reach, and about 16 percent spawned in the lower reach. Steelhead released into Lake Scanewa that spawned in the Cispus River were minimal in reach 5 (4 percent) and distributed in approximately equal proportions in the middle (reach 6; 16 percent) and upper (reach 7; 18 percent) reaches. Steelhead released into the Cispus River spawned almost exclusively in the Cispus River (94 percent). The upper Cispus River reach (reach 7) was used by more fish (72 percent) than the middle Cispus reach (reach 6; 16 percent). Taken together, the upper Cowlitz River reach (reach 4) and upper Cispus River reach (reach 7) accounted for more than 71 percent of the spawning locations for radio-tagged steelhead. When the middle Cispus River reach (reach 6) is included with the upper reaches, they account for 87 percent of our described spawning sites. We described 12 tributaries in the Cowlitz River and 8 tributaries in the Cispus River where steelhead spawned. After spawning, about 53–63 percent of steelhead moved downstream as kelts, either being collected at Cowlitz Falls Dam or being detected downstream from the dam. More fish were collected from both release sites in 2018 compared to 2017. Across release sites and years, more NOR fish and females moved downstream as kelts. This study added to the understanding of the behavior and movement of adult steelhead in the upper Cowlitz River Basin, supporting the findings of previous studies in the basin and describing spawning sites in the system’s two main rivers and their tributaries. Future research efforts in this system may use additional telemetry studies, genetic analyses, and spawning ground surveys to provide further insights into the progress of the reintroduction effort.

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USGS2019

Movements of juvenile Pacific Lamprey (Entosphenus tridentatus) in the Yakima and Columbia Rivers, Washington, 2018—A pilot study using acoustic telemetry

Telemetry has been an invaluable tool to improve our understanding of adult Pacific Lamprey ( Entosphenus tridentatus ) movements and to guide management approaches to protect and restore this species of concern. Juvenile and larval lamprey, however, are much smaller than adults, and have not been monitored with telemetry because available transmitters have traditionally been too large. With funding from the U.S. Department of Energy and the U.S. Army Corps of Engineers, the Pacific Northwest National Laboratory developed a prototype micro-transmitter of appropriate size for use in small fish such as juvenile lampreys and eels. Through a collaborative research approach, we used these prototype transmitters to do a pilot level evaluation of juvenile lamprey (macrophthalmia) movements in the Yakima and Columbia Rivers in 2018. Our project monitored tagged lamprey using acoustic monitoring arrays installed and maintained for juvenile salmon ( Oncorhynchus spp.) migration studies done by our partners. The study was done in the lower Yakima River, Washington, from river mile 111 to the river mouth, and in the Columbia River, from the Yakima River mouth to Camas, Washington, downstream of Bonneville Dam. We released four groups of tagged lamprey from May 9 to 15, 2018. Two groups were released at the upper site (located at the State Route 24 bridge, about 4.5 river miles upstream of Wapato Dam), and two groups were released at the lower site (about 1.7 miles upstream of the Yakima River mouth). We detected 95.6 percent of the tagged lamprey, with more individuals detected in the Columbia River than in the Yakima River. Lamprey arrived at Bonneville Dam in an average of 8.0–9.6 days from the upper site (300 river miles) and in an average of 6.5 days from the lower site (193 river miles). Lamprey moved through the study area at an average rate of 30–35 miles per day and generally remained at each detection site for less than about 20 minutes. Most lamprey (63 percent) arrived at detection sites during periods of darkness, but some travel occurred during daylight and transitional light periods.

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USGS2018

Evaluation of sockeye salmon after passage through an innovative upstream fish-passage system at Cle Elum Dam, Washington, 2017

Executive Summary The Bureau of Reclamation (Reclamation) and the Washington State Department of Ecology (Ecology), working with the Yakima River Basin Water Enhancement Project Workgroup (composed of representatives of the Yakama Nation; Federal, State, county, and city governments; environmental organizations; and irrigation districts), developed the Yakima Basin Integrated Plan (Integrated Plan). The Integrated Plan identifies a comprehensive approach to water resources and ecosystem restoration improvements in the Yakima Basin to be implemented over a 30-year period. The Integrated Plan includes seven elements: Reservoir fish passage, Structural and operational changes to existing facilities, Surface water storage, Groundwater storage, Habitat/watershed protection and enhancement, Enhanced water conservation, and Market reallocation. The first listed element, reservoir fish passage, will be expensive and take many years to accomplish. Reclamation and Ecology decided to look at new and innovative means to provide passage that could help reduce project cost and construction timing while maintaining survival rates of traditional upstream passage facilities. Reclamation contracted with the U.S. Geological Survey to do a study to evaluate the outcome of passage through one innovative fish-passage system at Cle Elum Dam, the first Integrated Plan reservoir fish-passage project being implemented.

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USGS2017

Preliminary evaluation of the behavior and movements of adult spring Chinook salmon in the Chehalis River, southwestern Washington, 2014

Recent interest in flood control and restoration strategies in the Chehalis River Basin has increased the need to understand the current status and ecology of spring Chinook salmon ( Oncorhynchus tshawytscha ). Spring Chinook salmon have the longest exposure of all adult Chinook salmon life histories to the low-flow and high water temperature conditions that typically occur during summer. About 100 adult spring Chinook salmon were found dead in the Chehalis River in July and August 2009. Adult Chinook salmon are known to hold in cool-water refugia during warm summer months, but the extent to which spring Chinook salmon might use thermal refugia in the Chehalis River is unknown. A preliminary evaluation of the movements and temperature exposures of adult spring Chinook salmon following their return to the Chehalis River was conducted using radiotelemetry and transmitters equipped with temperature sensors. A total of 12 spring Chinook salmon were captured, radio-tagged, and released in the main-stem Chehalis River between May and late June 2014. Tagged fish were monitored from freshwater entry through the spawning period using a combination of fixedsite monitoring locations and mobile tracking. Water temperature and flow conditions in the main-stem Chehalis River during 2014 were atypical compared to historical averages. Mean monthly water temperatures between March and August 2014 were higher than any decade since 1960 and mean monthly discharge was 90–206 percent of the discharge in previous years. Overall, 92 percent of the tagged fish were detected, with a mean of 102 d in the detection history of tagged fish. Seven tagged fish (58 percent) moved upstream, either shortly after release (5–8 d, 57 percent), or within about a month (34–35 d, 29 percent). One fish (14 percent) remained near the release location for 98 d before moving upstream. The final fates for the seven fish that moved upstream following release included six fish that were assigned a fate of spawner and one fish with an unknown fate. Tagged fish showed limited movements during the peak water temperatures in July and August, and were not frequently detected at sites where water temperatures exceeded 21 °C . The mouths of the Skookumchuck and Newaukum Rivers were commonly used by tagged fish for extended periods during peak water temperatures and study fish with a fate of spawner were last detected in these tributaries. This pilot study represents a substantial contribution to the understanding of spring Chinook salmon in the Chehalis River Basin, and provides information for the design and execution of future evaluations. The water temperatures and flow conditions during the 2014 study period were not typical of the historical conditions in the basin and the numbers of tagged fish monitored was relatively low, so results should be interpreted with those cautions in mind.

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USGS2016

Behavior and movements of adult spring Chinook salmon ( Oncorhynchus tshawytscha ) in the Chehalis River Basin, southwestern Washington, 2015

Recent interest in flood control and restoration strategies in the Chehalis River Basin has increased the need to understand the current status and ecology of spring Chinook salmon. Based on the extended period between freshwater entry and spawn timing, spring Chinook salmon have the longest exposure of all adult Chinook salmon life histories to the low-flow and high water temperature conditions that typically occur during summer. About 100 adult spring Chinook salmon were found dead in the Chehalis River in July and August 2009. Adult Chinook salmon are known to hold in cool-water refugia during warm summer months, but the extent to which spring Chinook salmon might use thermal refugia in the Chehalis River is unknown. The movements and temperature exposures of adult spring Chinook salmon following their return to the Chehalis River were investigated using radiotelemetry and transmitters equipped with temperature sensors, combined with water temperature monitoring throughout the basin. A total of 23 spring Chinook salmon were radio-tagged between April and early July 2015; 11 were captured and released in the main-stem Chehalis River, and 12 were captured and released in the South Fork Newaukum River. Tagged fish were monitored with a combination of fixed-site monitoring locations and regular mobile tracking, from freshwater entry through the spawning period. Water temperature and flow conditions in the main-stem Chehalis River during 2015 were atypical compared to historical averages. Mean monthly water temperatures between March and July 2015 were higher than any decade since 1960 and mean daily flows were 30–70 percent of the flows in previous years. Overall, 96 percent of the tagged fish were detected, with a mean of 62 d in the detection history of tagged fish. Of the 11 fish released in the main-stem Chehalis River, six fish (55 percent) moved upstream, either shortly after release (2–7 d, 50 percent), or following a short delay (12–18 d, 50 percent). One fish released in the main-stem Chehalis River remained near the release location for 64 d before moving upstream. The final fates for the seven fish that moved upstream in the main-stem Chehalis River included two fish with unknown fates, two fish with a fate of pre-spawn mortality, and three fish that were assigned a fate of spawner. Four (36 percent) of the radio-tagged Chinook salmon released in the main-stem Chehalis River showed limited movement from their release sites, and were assigned fates of unknown (one fish), pre-spawn mortality (one fish), and spit/mortality (2 fish). The 12 spring Chinook salmon released in the South Fork Newaukum River remained in the South Fork Newaukum River throughout the study period. Five (42 percent) of these fish were actively moving through the spawning period and were assigned a fate of spawner. Seven (58 percent) of these fish were detected for a period following release, but their detection histories ended prior to the spawning period. The fates assigned to these seven fish included two fish with spit/mortality fates and five fish with fates of pre-spawn mortality. Tagged fish in both the Chehalis River and the South Fork Newaukum River showed limited movements during the peak water temperatures in July and August, and were not frequently detected at sites where water temperatures were greater than 21 °C. Pre-spawn mortality due to predation or harvest may be an important factor in the Chehalis River Basin as it was the assigned fate for 27 percent of the fish released in the main-stem Chehalis River and 42 percent of the fish released in the South Fork Newaukum River. This study represents a substantial contribution to the understanding of spring Chinook salmon in the Chehalis River Basin. The water temperatures and flow conditions during the 2015 study period were not typical of the historical conditions in the basin and the numbers of tagged fish monitored was relatively low, so results should be interpreted with those cautions in mind.

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USGS2014

Evaluation of juvenile salmonid behavior near a prototype weir box at Cowlitz Falls Dam, Washington, 2013

Collection of juvenile salmonids at Cowlitz Falls Dam is a critical part of the effort to restore salmon in the upper Cowlitz River because the majority of fish that are not collected at the dam pass downstream and enter a large reservoir where they become landlocked and lost to the anadromous fish population. However, the juvenile fish collection system at Cowlitz Falls Dam has failed to achieve annual collection goals since it first began operating in 1996. Since that time, numerous modifications to the fish collection system have been made and several prototype collection structures have been developed and tested, but these efforts have not substantially increased juvenile fish collection. Studies have shown that juvenile steelhead (Oncorhynchus mykiss), coho salmon (Oncorhynchus kisutch), and Chinook salmon (Oncorhynchus tshawytscha) tend to locate the collection entrances effectively, but many of these fish are not collected and eventually pass the dam through turbines or spillways. Tacoma Power developed a prototype weir box in 2009 to increase capture rates of juvenile salmonids at the collection entrances, and this device proved to be successful at retaining those fish that entered the weir. However, because of safety concerns at the dam, the weir box could not be deployed near a spillway gate where the prototype was tested, so the device was altered and re-deployed at a different location, where it was evaluated during 2013. The U.S. Geological Survey conducted an evaluation using radiotelemetry to monitor fish behavior near the weir box and collection flumes. The evaluation was conducted during April–June 2013. Juvenile steelhead and coho salmon (45 per species) were tagged with a radio transmitter and passive integrated transponder (PIT) tag, and released upstream of the dam. All tagged fish moved downstream and entered the forebay of Cowlitz Falls Dam. Median travel times from the release site to the forebay were 0.8 d for steelhead and 1.2 d for coho salmon. Most fish spent several days in the dam forebay; median forebay residence times were 4.4 d for juvenile steelhead and 5.7 d for juvenile coho salmon. A new radio transmitter model was used during the study period. The transmitter had low detection probabilities on underwater antennas located within the collection system, which prevented us from reporting performance metrics (discovery efficiency, entrance efficiency, retention efficiency) that are traditionally used to evaluate fish collection systems. Most tagged steelhead (98 percent) and coho salmon (84 percent) were detected near the weir box or collection flume entrances during the study period; 39 percent of tagged steelhead and 55 percent of tagged coho salmon were detected at both entrances. Sixty-three percent of the tagged steelhead that were detected at both entrances were first detected at the weir box, compared to 52 percent of the coho salmon. Twelve steelhead and 15 coho salmon detected inside the weir box eventually left the device and were collected in collection flumes or passed the dam. Overall, collection rates were relatively high during the study period. Sixty-five percent of the steelhead and 80 percent of the coho salmon were collected during the study, and most of the remaining fish passed the dam and entered the tailrace (24 percent of steelhead; 13 percent of coho salmon). The remaining 11 percent of steelhead and 7 percent of coho salmon did not pass the dam while their transmitters were operating. We were able to confirm collection of tagged fish at the fish facility using three approaches: (1) detection of radio transmitters in study fish; (2) detection of PIT-tags in study fish; (3) observation of study fish by staff at the fish facility. Data from all three methods were used to develop a multistate mark-recapture model that estimated detection probabilities for the various monitoring methods. These estimates then were used to describe the percent of tagged fish that were collected through the weir box and collection flumes. Detection probabilities of PIT-tag antennas in the collection flumes were 0.895 for juvenile steelhead and 0.881 for juvenile coho salmon, although radiotelemetry detection probabilities were 0.654 and 0.646 for the two species, respectively. The multistate model estimates showed that all steelhead and most coho salmon (94.5 percent) that were collected at the dam entered the collection system through the flumes rather than through the weir box. None of the tagged steelhead and only 5.5 percent of the tagged coho salmon were collected through the weir box. These data show that juvenile steelhead and coho salmon collection rates were much higher through the collection flumes than through the weir box. Low detection probabilities of tagged fish in the fish collection system resulted in uncertainty for some aspects of our evaluation. Missing detection records within the collection system for fish that were known to have been collected resulted in four tagged steelhead and seven tagged coho salmon being removed from the dataset, which was used to assess discovery rates of the weir box and collection flumes. However, the multistate model allowed us to provide unbiased estimates of the percentage of tagged fish that were collected through each route, and these data showed that few fish were collected through the weir box. Overall, the fish collection system performed reasonably well in collecting juvenile steelhead and coho salmon during the 2013 collection season. Fish collection efficiency estimates from the Washington Department of Fish and Wildlife showed that steelhead collection efficiency was slightly higher than the 10-year average (46 percent compared to 42 percent), whereas coho salmon collection efficiency was more than twice as high as the 10-year average (63 percent compared to 30 percent). However, the performance of the weir box was poor because most fish were collected through the collection flumes.

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USGS2012

Behavior and passage of juvenile salmonids during the evaluation of a behavioral guidance structure at Cowlitz Falls Dam, Washington, 2011

Executive Summary A radiotelemetry evaluation was conducted during April–October 2011 to describe movement patterns, forebay behavior, and passage of juvenile steelhead, coho salmon, and Chinook salmon at Cowlitz Falls Dam, Washington. The primary focus of the study was to describe fish behavior near a behavioral guidance structure (BGS) and floating surface collector (FSC) deployed upstream of Cowlitz Falls Dam. A secondary focus was to determine the proportion of tagged fish that were detected near spillbays 2 and 3 on the dam, because this location has been proposed for deploying weir boxes as an additional dam-based collection alternative in the future. Juvenile steelhead ( Oncorhynchus mykiss ), coho salmon ( Oncorhynchus kisutch ), and Chinook salmon ( Oncorhynchus tshawytscha ) were collected and tagged at the Cowlitz Falls Fish Collection Facility and transported upstream where they were released into the Cowlitz and Cispus Rivers. We radio-tagged and released 110 juvenile steelhead, 110 juvenile coho salmon, and 110 juvenile Chinook salmon and monitored their movements in and around the BGS/FSC complex, at the dam, and downstream of the dam. We used detection records and a Markov chain model to calculate probabilities of movement between specific areas in the forebay of Cowlitz Falls Dam. These areas are referred to as states and the Markov chain model was used to create a series of tables, called transition matrices, that contained estimated probabilities of movement between states. These probabilities were insightful for understanding how radio-tagged fish moved near the BGS, FSC, and spillbays. Most tagged fish (89–91 percent) moved downstream of release sites (9 or 22 rkm upstream of the dam) and were detected in the dam forebay during the study period. Tagged fish that encountered the BGS on their first approach to the dam were distributed across the forebay, which supports the concept of using a BGS to concentrate fish near a collector entrance in the dam forebay. We found that 14 percent of the steelhead, 18 percent of the coho salmon, and 17 percent of the Chinook salmon encountered the FSC discovery area without BGS guidance on their first trip through the forebay. The BGS guided 36 percent of the steelhead, 22 percent of the coho salmon, and 46 percent of the Chinook salmon to the FSC discovery area when fish first entered the forebay, which resulted in 40–63 percent (by species) of the tagged fish arriving at the FSC discovery area. Movement patterns along the BGS showed that fish were likely to guide along the device, but also demonstrated the tendency of fish to move under the BGS and downstream to Cowlitz Falls Dam. Differential distribution among sucker species within the Williamson River Delta and between the delta and adjacent lakes indicated that shortnose suckers likely benefited more from the restored Williamson River Delta than Lost River or Klamath largescale suckers ( Catostomus snyderi ). Catch rates in shallow-water habitats within the delta were higher for shortnose and Klamath largescale sucker larvae than for larval Lost River suckers in 2008, 2009, and 2010. Shortnose suckers also comprised the greatest portion of age-0 suckers captured in the Williamson River Delta in all 3 years of the study. The relative abundance of age-1 shortnose suckers was high in our catches compared to age-1 Lost River suckers in 2009 and 2010. Tagged fish that arrived at Cowlitz Falls Dam were distributed across the dam face but a high percentage of each species (65 percent of steelhead; 61 percent of coho salmon; 71 percent of Chinook salmon) arrived on the northern side of the dam. Movement probabilities near spillbays 1 and 4 showed a strong preference for tagged fish to move from the outer edges of the dam towards the center of the dam where they were detected at the debris barrier (range of probabilities = 0.690–0.841). We found that 76 percent of the steelhead, 61 percent of the coho salmon, and 92 percent of the Chinook salmon were detected at spillbays 2 or 3 during the study. This behavior supports the strategy of weir box deployments in spillbays 2 and 3 for future dam-based collection options. Tagged fish that arrived at the dam commonly moved upstream and were detected at the BGS or FSC discovery area. This behavior provided a secondary opportunity for fish to encounter the FSC discovery area and we found that in total, 72 percent of the steelhead, 48 percent of the coho salmon, and 92 percent of the Chinook salmon were detected near the FSC while residing in the forebay. Overall, 88 percent of the steelhead, 76 percent of the coho salmon, and 95 percent of the Chinook salmon that entered the forebay were detected near the FSC or in spillbays 2 and 3. Turbine passage was the most common passage route for tagged fish at Cowlitz Falls Dam during 2011. We found that 40 percent of the steelhead, 52 percent of the coho salmon, and 33 percent of the Chinook salmon passed through turbines. An additional 22 percent of the steelhead and 32 percent of the coho salmon passed through turbines or spillways when both passage routes were available. Fish collection numbers were relatively low during 2011 compared to long-term averages. In total, 37 percent of the steelhead, 14 percent of the coho salmon, and 23 percent of the Chinook salmon that entered the forebay were collected, primarily through collection flumes. The FSC collected a single radio-tagged fish (a Chinook salmon) in 2011.

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USGS2011

Evaluation of angler effort and harvest of rainbow trout (Oncorhynchus mykiss), Lake Scanewa, Washington, 2010

A creel evaluation was conducted in Lake Scanewa, a reservoir on the Cowlitz River, to monitor catch rates of rainbow trout (Oncorhynchus mykiss) and determine if the trout fishery was having negative impacts on juvenile anadromous salmon (Oncorhynchus spp.) in the system. The trout fishery, which is supported by releases of 20,000 fish (2 fish per pound) per year from June to August, was developed to mitigate for the construction of the Cowlitz Falls Dam in 1994. The trout fishery has a target catch rate of at least 0.50 fish per hour. Interviews with 1,214 anglers during the creel evaluation found that most anglers targeted rainbow trout (52 percent) or Chinook and coho salmon (48 percent). The interviewed anglers caught a total of 1,866 fish, most of which were rainbow trout (1,213 fish; 78 percent) or coho salmon (311 fish; 20 percent). We estimated that anglers spent 17,365 hours fishing in Lake Scanewa from June to November 2010. Catch rates for boat anglers (1.39 fish per hour) exceeded the 0.50 fish per hour target, whereas catch rates for shore anglers (0.35 fish per hour) fell short of the goal. The combined catch rates for all trout anglers in the reservoir were 0.96 fish per hour. We estimated that anglers harvested 7,584 (95 percent confidence interval = 2,795-12,372 fish) rainbow trout during the study period and boat anglers caught more fish than shore anglers (5,975 and 1,609 fish, respectively). This estimate suggests that more than 12,000 of the 20,000 rainbow trout released into Lake Scanewa during 2010 were not harvested, and could negatively impact juvenile salmon in the reservoir through predation or competition. We examined 1,236 stomach samples from rainbow trout and found that 2.1 percent (26 fish) of these samples contained juvenile fish. Large trout (greater than 300 millimeters) had a higher incidence of predation than small trout (less than 300 millimeters; 8.50 and 0.06 percent, respectively). A total of 39 fish were found in rainbow trout stomachs and 13 (33 percent) of these were juvenile salmon. These data and uncertainties associated with movement patterns and survival rates of rainbow trout in Lake Scanewa suggest that future evaluations would be helpful to better understand the potential effects of the mitigation trout fishery on juvenile salmon in the reservoir.

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