Evaluation of Downstream Migrant Salmon Production in 2003 from the Cedar River and Bear Creek

This report provides the results of monitoring five salmonid species as downstream migrants in 2003 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 2003, 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 2003, the twelfth 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 through May, a floating inclined-plane screen 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 33 nights, dye-marked fry were released upstream of the trap. Due to the wide range of flows 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. We used this relationship to estimate daily trap efficiency.

Over the season, 16.0 million hatchery produced sockeye fry were released into the Cedar River from four locations. A quarter of these fry (4.4 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. In order to avoid trapping large numbers of hatchery fry, we chose not to operate the trap during nights that fry were released at the Riviera site. Due to the high flows and the proximity of this release location to the trap, we assumed that all of the fry survived to the trap. Fry caught in the trap during the night of and the nights following Landsburg releases were randomly sampled for thermal marks to determine the proportion of hatchery fish present.

Over the 84 nights trapped, 1.9 million sockeye fry were captured. From this catch and the capture efficiency data, we estimated a total of 42.3 million wild and hatchery sockeye fry entered Lake Washington in 2003. Based on otolith analysis and the hatchery release figures, we estimated that this total included 27.9 million wild fry and 14.4 million hatchery produced fry. Average survival to the trap of the 11.5 million hatchery fry released upstream of the trap was estimated at 86%. Fry released at the Landsburg Hatchery, located 21 miles upstream, survived at an average rate of 79%. Middle River releases, 13 miles upstream of the trap, averaged 80% survival. Fry released at the Riviera site, located one mile above the trap, survived at an average rate of 105%.

Migration timing for wild fry was nearly two weeks earlier than the average for the eleven broods measured thus far. February temperatures and flows explain most of the variation in median migration dates between years. Median migration date for hatchery fry was February 21, and that of wild fry was March 8.

Survival from egg deposition to lake entry of wild fry was estimated at 6.2%. This rate is the ratio of 27.9 million wild fry to an estimated deposition of 448 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, 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 chinook 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. 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 235,397 in 2003. Timing was bimodal with fry emigrating in January through April 15 comprising over three-fourths (194,135) of the total migration. The smolt migration, April 16 through July, was estimated at 41,262. Egg-tomigrant survival was estimated at 18.6%. Over the season, age 0+ chinook increased in size from less than 40 mm in January to over 100 mm by mid-June.

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 74,507, 525 and 900, 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 6 through April 8. On April 9, we replaced it with a screw trap that fished until the morning of July 8. Using the approach described for the Cedar River, we estimated downstream migrant production of sockeye fry, chinook, coho, steelhead, and cutthroat smolts.

Throughout the fry trapping season, 40 efficiency tests were conducted using sockeye fry. Capture rates ranged from 6.8% to 31% and averaged 18.8%. Total sockeye production was estimated at 2.0 million fry. This estimate is the result of applying the average capture rate to the expanded catches and estimating migration before and after trap operation using linear extrapola tion.

Migration of chinook during fry trap operation was estimated using the average efficiency measured with sockeye fry. During screw trap operation, 21 tests were conducted with chinook smolts, and capture rate averaged 49.1%. Total production of age 0+ chinook was estimated at 17,313 in 2003. Migration timing was bimodal, however most chinook migrated as smolts in May and June. Chinook fork lengths were less than 40 mm in February, and grew slightly larger than 90 mm by late June.

Coho production was estimated at 48,561 smolts and cutthroat production at 3,708 smolts. During the 2003 trapping season, no steelhead were caught in the Bear Creek screw trap.