Evaluation of Downstream Migrant Salmon Production in 2006 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 2006, the fifteenth 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 wild 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 late 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). Had the trap fished continuously from January 20 through May 27, total catch was estimated at 665,397 sockeye. Trap efficiency was estimated by releasing dye-marked fry upstream of the trap on 43 nights during trapping season. Capture rates ranged from 1.4% to 11%. Total migration for 2006 was estimated at 10.8 million wild sockeye fry. Survival of wild fry from egg deposition to lake entry was estimated at 13.9%. This rate is the ratio of 10.8 million wild fry to an estimated deposition of 78 million eggs.

Over the season, 6.6 million hatchery produced sockeye fry were released into the Cedar River from three locations. A portion of these fry (2.0 million) was released below the fry trap at the Cedar River Trail Park. Survival of hatchery fry released at the Cedar River Trail Park was assumed to be 100%. The remaining 4.6 million fry were released at two different sites upstream of the trap, 2.8 million released at R.M. 13.5 and 1.8 million released at R.M. 24. Survival of the fry released above the trap was estimated using four different approaches and ranged from 12.7% to 104%. We estimated 2.2 million survived to the trap. With the addition of hatchery sockeye fry, we estimate a total of 15.1 million sockeye fry entered Lake Washington in 2006.

Median migration timing for wild fry in 2006 was only two days earlier than average. February stream temperatures averaged 6.3° C in 2006, slightly warmer than the 12-year average (6.1° C), which in turn produced a median migration date fairly close to the 12-year average median migration date. The median migration date for wild fry was March 20, 25 days later than that of the hatchery fry. This difference was only one day longer than average.

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 wild 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. 0.9 in mid-April, and operated through July. Total catch was estimated at 2,917 Chinook fry. From the start of the season in January through the end of April, mark-recapture data generated with releases of marked sockeye were used to estimate fry trap efficiencies for Chinook migrants. Abundance was estimated at 94,601 Chinook for the period of January 1 through April 30.

Chinook catch from the screw trap totaled 830 smolts. Screw trap efficiency was estimated by releasing groups of fin-marked or PIT tagged Chinook smolts above the trap. Capture rates ranged from 3.1% to 8.4%. Total migration from May 1 through July 16 was estimated at 18,592 Chinook smolts.

Age 0+ Chinook production from the Cedar River was estimated at 117,559 in 2006. Timing was bimodal with fry emigrating in January through late-April comprising 84% of the total migration. Eggto- migrant survival was estimated at 7.8%. Over the season, age 0+ Chinook increased in size from less than 34 mm in January to 116 mm by mid-June.

Over the season, based on actual catch and estimates of capture rates we estimated the migrations of coho, steelhead¹ and cutthroat smolts at 38,023, 267, and 2,000, 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 mid April. In April, it was replaced with a screw trap that fished until the end of June. Downstream migrant production was estimated for wild sockeye fry, age 0+ Chinook, coho, steelhead, and cutthroat smolts.

Throughout the fry-trapping season, 22 mark groups were released using sockeye fry. Total catch was estimated at 76,007 sockeye fry. Capture rates ranged from 4.0% to 20.6% and total sockeye production was estimated at 548,604 fry. Relating this production to the estimated deposition of 5.2 million eggs yielded a survival rate of 10.5%.

Migration of age 0+ Chinook during fry trap operation was estimated using sockeye fry markrecapture data. Total catch was estimated at 498 Chinook fry.

Total abundance was estimated at 5,764 Chinook fry. During screw trap operation, 8,179 Chinook smolts were caught. Efficiency for the screw trap was estimated by releasing mark groups above the trap. Capture rates ranged from 25.7% to 64.4%. Chinook abundance during screw trap operation was estimated at 16,598 smolts.

Total production of age 0+ Chinook was estimated at 22,362 in 2006. Migration timing was bimodal with roughly 26% emigrating as fry between February and April, the remaining emigrated as smolts between May and June. Weekly Chinook fork lengths averaged less than 37 mm in February, and grew to 100 mm by late May. Egg-to-migrant survival was estimated at 3.9%.

Coho production was estimated at 46,987 smolts and cutthroat production at 7,855 smolts. During the 2006 trapping season, no steelhead were caught in the Bear Creek screw trap.

¹ We are uncertain if the downstream migrant rainbow trout referred to as steelhead follow an anadromous (saltwater rearing) or ad-fluvial (lake rearing) life history strategy. They are referred to as steelhead in this report since they appear identical to smolted juvenile steelhead from other rivers in western Washington.