Evaluation of Downstream Migrant Salmon Production in 2004 from the Cedar River and Bear Creek

This report provides the results of monitoring five salmonid species as downstream migrants in 2004 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 2004, 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 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 2004, the thirteenth 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 for natural spawners from egg deposition to lake entry, for hatchery produced fry from release to lake entry, 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 54 nights, dye-marked fry were released upstream of the trap. Linear regression analysis found trap efficiency to be significantly correlated with flow while the level of Lake Washington was below capacity and river discharge was adequate. Daily trap efficiency was estimated using the regression from January to April 26. Following April 26, the rise in the lake level and the decreasing river flow reduced trap efficiency. For the interval from April 27 through May 30, we estimated migration with the average trap efficiency (8.6%) of the eight tests conducted during that interval.

Over the season, 9.9 million hatchery produced sockeye fry were released into the Cedar River from two locations. A portion of these fry (7.2 million) was released below the fry trap at the Cedar River Trail Park. The remaining 2.7 million fry were released from Landsburg Hatchery on 12 nights. All hatchery fry were internally marked on the otolith by slightly manipulating water temperatures in the hatchery.

Over the 82 nights trapped, 2.8 million 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 47.9 million wild and hatchery sockeye fry entered Lake Washington in 2004. This total included 38.7 million wild fry and 9.2 million hatchery produced fry. Average survival to the trap of the 2.7 million hatchery fry released upstream was estimated at 74.4%.

Migration timing for wild fry in 2004 was near average for the twelve broods measured thus far. February temperatures and flows explain most of the variation in median migration dates between years. Median migration dates for hatchery and wild fry were February 23 and March 21, respectively. Survival from egg deposition to lake entry of wild fry was estimated at 11.8%. This rate is the ratio of 38.7 million wild fry to an estimated deposition of 327 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 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. 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 120,876 in 2004. 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 8.0%. 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 70,044, 120 and 3,480, 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 5 through April 4. On April 5, we replaced it with a screw trap that fished until the morning of June 27. 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, 12 efficiency tests were conducted using sockeye fry. Capture rates ranged from 8.7% to 20.9% and averaged 16.5%. Linear regression analysis correlating efficiency and flow did not yield a significant relationship. Total sockeye production was estimated at 177,801 fry. Relating this production to the estimated deposition of 2.8 million eggs yielded a survival rate of 6.3%.

Migration of age 0+ chinook during fry trap operation was estimated using the average efficiency measured with sockeye fry. During screw trap operation capture rates averaged 49.2% for the 22 tests that were conducted using chinook smolts. Total production of age 0+ chinook was estimated at 23,647 in 2004. 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 slightly exceeded 90 mm by late June. Egg to migrant survival was estimated at 5.0%.

Coho production was estimated at 21,085 smolts and cutthroat production at 4,540 smolts. During the 2004 trapping season, no steelhead were caught in the Bear Creek screw trap.