Evaluation of Downstream Migrant Salmon Production in 2002 from the Cedar River and Bear Creek

This report provides the results of monitoring five salmonid species as downstream migrants in 2002 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 2002, was expanded in 1999 with the addition of a second downstream migrant trap to estimate the production of juvenile chinook salmon. With this trap we also estimate the production of coho, steelhead and cutthroat smolts.

Assessment of sockeye fry production began in the Sammamish system in 1997. We placed the trap in the Sammamish River at Bothell where we also operated it during the 1998 season. In 1999, to assess chinook production as well as sockeye, we moved this monitoring program to Bear Creek. 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 1980's and early 1990's prompted the creation of a multi-agency 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 2002, the eleventh year of this project. As in previous years, the primary study goal was to estimate the season total migration of Cedar River wild and hatchery sockeye fry into Lake Washington. These estimates enable calculation of survival rates from egg deposition to lake entry, for hatchery fry from release to the trap, and for both production components from lake entry to subsequent life stages of smolts and adults.

Beginning in January and continuing into June, 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, on 43 nights, dye- marked sockeye fry were released upstream of the trap. Due to the wide range of flows exhibited during releases, we were able to examine the effects of flow on capture rate. Linear regression analysis found that trap efficiency was significantly correlated to flow. Daily trap efficiency was estimated by linear regression using a lower limit of 2.4% at flows greater than 2,000 cfs.

Over the season, 12.5 million hatchery sockeye fry were released into the Cedar River from four locations. A portion of those fry (4.3 million) was released below the fry trap at the Cedar River Trail Park. All hatchery fry were internally marked by slightly manipulating water temperatures in the hatchery. On most nights of and following hatchery releases, fry caught in the trap were randomly sampled for thermal marks to determine the proportion of hatchery fish present.

Over the 101 nights trapped, 2.3 million sockeye fry were captured. From this catch and the capture efficiency data, we estimated a total of 43.7 million wild and hatchery sockeye fry entered Lake Washington in 2002. Based on otolith analysis and the hatchery release figures, we estimated that this total included 31.7 million wild fry and 12.0 million hatchery produced fry. Average survival to the trap of the 8.3 million hatchery fry released upstream was estimated at 92.7%. Survival was a function of migration distance and flow during release. Survival of fry released at the Landsburg Hatchery, located 21 miles upstream, averaged 78.6%. Middle River releases, 13 miles upstream of the trap, averaged 89.2% survival. Fry released at the Riviera site, located one mile above the trap survived at an average rate of 100%. We attribute these relatively high survival rates to low predation rates as a result of higher flows during the 2002 migration.

Migration timing for wild fry was near the average for the ten broods measured thus far. February temperature explains most of the variation in median migration dates between years. Median migration dates for hatchery and wild fry were March 4 and March 25, respectively.

Survival from egg deposition to lake entry of wild fry was estimated at 7.5%. This rate is the ratio of 31.7 million wild fry to an estimated deposition of 420 million eggs. Survival of the 2001 brood was approximately the expected value in response to peak incubation flow. With the peak incubation flow of 1,930 cfs, the relationship between peak incubation flow and egg-to- migrant survival developed over the previous nine broods predicted a survival of 8.4%.

In response to the listing of the Puget Sound Chinook Evolutionary Significant Unit (ESU) under the Endangered Species Act as a threatened species, we expanded the existing sockeye fry monitoring program in 1999 to include an assessment of the natural chinook production in the Cedar River. The gear we operate each year starting in January to assess sockeye fry production also captures chinook fry. To capture the larger, later migrating chinook, which we classify as �"smolts”, we installed a screw trap at R.M. 1.1, and operated it until July.

Juvenile production was estimated through applying capture rate estimates to 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 fry to estimate the migration of chinook fry. 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 119,674 in 2002. Timing was bimodal with fry emigrating in January through March comprising two-thirds (79,799) of the total migration. The smolt migration, which primarily occurred in May and June, was estimated at 39,875. Egg-to-migrant survival was estimated at 6.7%. Over the season, age 0+ chinook increased in size from less than 40 mm in January to over 100 mm by July.

Over the season, based on actual and projected catches and estimates of capture rates we estimated the migrations of coho, steelhead and cutthroat smolts at 60,513, 950, and 3,600, respectively.

Bear Creek

We installed the fry trap on Big Bear Creek 100 yards downstream of the Redmond Way Bridge and operated it from February 1 through April 11. On April 12, we replaced it with a screw trap that fished until the morning of July 16. Using the approach described for the Cedar River, we estimated the downstream migrant production of wild sockeye fry, age 0+ chinook, coho, steelhead, and cutthroat smolts.

The scoop trap was moved twice during the fry trapping season in order for the trap to fish properly. Capture rates were estimated for each position of the trap. The average trap efficiencies for the first two positions were 12.4% and 16.5%. While the trap fished in the third position, using logarithmic regression analysis, flow described most of the variation between trap efficiency tests. This regression was used to predict daily efficiency while the trap fished in the third position. Capture rate of sockeye fry in the screw trap averaged 9.5%.

Capture rates for sockeye fry, chinook and coho smolts were evaluated throughout the season. Applying the respective capture rate estimates to actual catches estimates the production of sockeye fry, age 0+ chinook, and coho smolts at 2.7 million, 21,454, and 58,212.

As in previous years, chinook migration timing was bimodal, with most of the migration occurring in May and June. Chinook size increased from less than 40 mm in February to over 90 mm by July.

For the season, we also estimated the production of wild steelhead and cutthroat smolts at 60 and 2,775, respectively.