Evaluation of Downstream Migrant Salmon Production in 2007 from the Cedar River and Bear Creek

This report provides the results of monitoring five salmonid species as downstream migrants in 2007 from two of the more 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 2007, 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 2007, the sixteenth year of this project. The primary study goal was to estimate the season total migration of natural-origin Cedar River sockeye fry into Lake Washington. This estimate enables calculation of a survival rate for the 2006 brood from egg deposition to lake entry, and provides data to calculate other live stage components such as survival from lake entry to smolts and adult return.

Beginning in January and continuing through early 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). Had the trap fished continuously from January 18 through June 7, total catch was estimated at 326,773 sockeye. Trap efficiency was estimated by releasing dye-marked fry upstream of the trap on 60 nights during trapping season. Capture rates ranged from 1.2% to 11.5%. Total migration for 2007 was estimated at 9.25 million natural-origin sockeye fry. Survival of natural-origin fry from egg deposition to lake entry was estimated at 5.9%, the third lowest observed since this study began. This rate is the ratio of 9.25 million natural-origin fry to an estimated deposition of 155.6 million eggs.

Over the season, 11.8 million hatchery-produced sockeye fry were released into the Cedar River from three locations. A portion of these fry (8.4 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 3.4 million fry were released at two different sites upstream of the trap, 2.3 million released at R.M. 13.5 and 1.1 million released at R.M. 21.8. Survival of the fry released above the trap was estimated using three different approaches and ranged from 5.61% to 148%. We Evaluation of Downstream Migrant Salmon Production in 2007 from the Cedar River and Bear Creek 2 estimated 1.9 million survived to the trap. With the addition of hatchery sockeye fry, we estimate a total of 19.6 million sockeye fry entered Lake Washington in 2007.

Median migration timing for natural-origin fry in 2007 was only one day later than average. February stream temperatures averaged 7.0°C in 2007, slightly warmer than the 15-year average (6.2° C), which in turn produced a median migration date fairly close to the 15-year average median migration date. The median migration date for natural-origin fry was March 23, 35 days later than that of the hatchery fry. This difference was only one day earlier 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 natural-origin 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, a screw trap was installed at R.M. 0.9 in mid-April, and operated through July. Total catch was estimated at 2,670 Chinook fry. From the start of the season in January through the middle 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 109,511 Chinook for the period of January 1 through April 17.

Chinook catch from the screw trap totaled 878 parr. Screw trap efficiency was estimated by releasing groups of fin-marked or PIT tagged Chinook parr above the trap. Capture rates ranged from 3.0% to 12.3%. Total migration from April 18 through July 20, was estimated at 14,225 Chinook parr.

Age 0+ Chinook production from the Cedar River was estimated at 123,736 in 2007. Timing was bimodal with fry emigrating in January through mid-April comprising 88% of the total migration. Eggto- migrant survival was estimated at 4.7%. Over the season, age 0+ Chinook increased in size from 34 mm in January to 125 mm by the end of the season.

Over the season, natural-origin coho migration was estimated at 33,994 smolts. Estimates of production were not made for steelhead and cutthroat in 2007 due to low catches (1 steelhead and 4 cutthroat smolts).

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 July 11. Downstream migrant production was estimated for natural-origin sockeye fry, age 0+ Chinook, coho and cutthroat smolts. Steelhead production was not assessed due to insufficient catch.

Throughout the fry-trapping season, 36 mark groups were released using sockeye fry. Total catch was estimated at 377,314 sockeye fry. Capture rates ranged from 1.5% to 12.5% and total sockeye production was estimated at 5,983,651 fry, more than twice the previously observed high production. Relating this production to the estimated deposition of 33.9 million eggs yielded a survival rate of 17.7%, the second highest survival estimated since trapping began in 1998.

Migration of age 0+ Chinook during fry trap operation was estimated using sockeye fry markrecapture data. Total catch was estimated at 166 Chinook fry. Total abundance was estimated at 4,054 Chinook fry. During screw trap operation, 5,276 Chinook parr were caught. Efficiency for the Evaluation of Downstream Migrant Salmon Production in 2007 from the Cedar River and Bear Creek 4 screw trap was estimated by releasing mark groups above the trap. Capture rates ranged from 28.6% to 52.3%. Chinook abundance during screw trap operation was estimated at 12,816 parr.

Total production of age 0+ Chinook was estimated at 16,870 in 2007. Migration timing was bimodal with roughly 24% emigrating as fry between February and April; the remaining emigrated as parr between May and June. Weekly Chinook fork lengths averaged 38.0 mm in February, and grew to average over 80 mm by mid-May. Egg-to-migrant survival was estimated at 2.9%.

Coho production was estimated at 25,143 smolts and cutthroat production at 3,869 smolts. During the 2007 trapping season, only one steelhead was caught in the Bear Creek screw trap.