Evaluation of Downstream Migrant Salmon Production in 2005 from the Cedar River and Bear Creek

This report provides the results of monitoring five salmonid species as downstream migrants in 2005 from the two most heavily spawned tributaries in the Lake Washington Basin: the Cedar River and Bear Creek. Monitoring sockeye fry production in the Cedar River began in 1992 to investigate the causes of low adult sockeye returns. This annual trapping program, which continued through 2005, was expanded in 1999 with the addition of a second downstream migrant trap to estimate the production of juvenile chinook salmon. With this trap, the production of coho, steelhead and cutthroat smolts were also estimated.

In addition to the Cedar River, downstream migrant production is also measured in the Sammamish basin. A trap was operated in the Sammamish River in 1997 and 1998 to estimate sockeye fry production. This monitoring program was moved to Bear Creek in 1999 to concurrently assess chinook and sockeye production. Since 1999, as in the Cedar River, this trapping operation has also estimated the populations of coho, steelhead and cutthroat smolts.

Cedar River

Declining adult sockeye salmon returns in the late 1980s and early 1990s prompted an effort to investigate causes for this decline. To determine which life-stages were experiencing poor survival, an evaluation of fry production was undertaken in the Cedar River beginning in 1992. Assessing the sockeye population, at this location and life-stage, separates freshwater production into river and lake components. This report documents our evaluation during 2005, the fourteenth year of this project. The primary study goal was to estimate the season total migration of naturally produced (wild) Cedar River sockeye fry into Lake Washington. This estimate enables calculation of a survival rate for natural spawners from egg deposition to lake entry, and for production components from lake entry to subsequent life stages of smolts and adults.

Beginning in January and continuing through mid-May, a floating inclined-plane screen (fry) trap located at river mile (R.M.) 0.7 in the Cedar River was operated to capture a portion of the sockeye fry migrating into Lake Washington (Figure 1). To estimate the capture efficiency of this trap, dyemarked fry were released upstream of the trap on 45 nights during trapping season. Efficiency tests were stratified into four groups based on flow and the elevation of Lake Washington. The average of each group was used to estimate migration within each corresponding strata.

Over the season, 15.3 million hatchery produced sockeye fry were released into the Cedar River from two locations. A portion of these fry (11.6 million) was released below the fry trap at the Cedar River Trail Park. The remaining 3.6 million fry were released from Landsburg Hatchery on four nights. All hatchery fry were internally marked by slightly manipulating water temperatures in the hatchery, which produced a banding effect on the otoliths (calcified boney structures used for equilibrium). Survival of hatchery fry released at the Cedar River Trail Park was assumed to be 100%. Survival of the fry released at Landsburg was estimated based on a model developed using past year’s data that correlates survival to the river discharge on the day of the release. The average survival rate from release at Landsburg to the trap site was 66%, and an estimated 2 million fry released from Landsburg survived to pass the trap.

Over the 60 nights trapped, 1.8 million wild sockeye fry were captured and this catch was expanded for intervals not fished. Application of the capture efficiency to the expanded catch estimated a total of 37 million wild sockeye fry entered Lake Washington in 2005.

Median migration timing for wild fry in 2005 was nearly three weeks earlier than average and earlier than all 13 broods measured thus far. February temperatures and flows explain most of the variation in median migration dates between years. Earlier out migration corresponded to river temperatures during incubation that were higher than normal. The median migration date for wild fry was March 2, 29 days later than that of the hatchery fry. This difference was roughly a week longer than average. Survival of wild fry from egg deposition to lake entry was estimated at 18.8%. This rate is the ratio of 37 million wild fry to an estimated deposition of 197 million eggs.

In response to the listing of the Puget Sound Chinook Evolutionary Significant Unit (ESU) under the Endangered Species Act as a threatened species, the existing sockeye fry monitoring program was expanded in 1999 to include an assessment of the natural chinook production in the Cedar River. The gear operated each year, starting in January, to assess sockeye fry production also captures chinook fry. To capture the larger, later migrating chinook smolts, a screw trap was installed at R.M. 1.1, and operated through July. Juvenile chinook production was estimated by applying the capture rate estimates to the catch data. From the start of the season in January through mid-April, we used the capture rate data generated with releases of marked sockeye to estimate fry trap efficiencies for chinook migrants. Screw trap efficiency was estimated by releasing groups of fin-marked chinook smolts above the trap.

Age 0+ chinook production from the Cedar River was estimated at 134,603 in 2005. Timing was bimodal with fry emigrating in January through mid-April comprising approximately half of the total migration. Egg-to-migrant survival was estimated at 5.9%.

Over the season, age 0+ chinook increased in size from less than 40 mm in January to 138 mm by mid-June. Over the season, based on actual and projected catches and estimates of capture rates we estimated the migrations of coho, steelhead¹ and cutthroat smolts at 71,247, 1,098 and 3,537, respectively.

Bear Creek

A fry trap was installed on Big Bear Creek 100 yards downstream of the Redmond Way Bridge and operated from February through early April. In April, it was replaced with a screw trap that fished until the middle of July. Using the approach described for the Cedar River, downstream migrant production was estimated for wild sockeye fry, age 0+ chinook, coho, steelhead, and cutthroat smolts.

Throughout the fry trapping season, seven efficiency tests were conducted using sockeye fry. Capture rates ranged from 8.7% to 28.3% and averaged 19.4%. Total sockeye production was estimated at 202,815 fry. Relating this production to the estimated deposition of 2.3 million eggs yielded a survival rate of 8.8%.

Migration of age 0+ chinook during fry trap operation was estimated using the average efficiency measured with sockeye fry. During screw trap operation efficiency tests were conducted with marked chinook smolts, similar to the Cedar River. Efficiency was estimated by using 25 efficiency tests stratified into two groups based on flow. The average of each group was used to estimate corresponding migrations. Total production of age 0+ chinook was estimated at 9,317 in 2005. Migration timing was generally unimodal, with most chinook migrating as smolts in May. Weekly chinook fork lengths averaged less than 40 mm in February, and grew to 102 mm by late June. Egg to migrant survival was estimated at 3.0%.

Coho production was estimated at 43,725 smolts and cutthroat production at 4,280 smolts. During the 2005 trapping season, only two steelhead were caught in the Bear Creek screw trap.