Evaluation of Juvenile Salmon Production in 2011 from the Cedar River and Bear Creek

This report describes the emigration of five salmonid species from two heavily spawned tributaries in the Lake Washington watershed: Cedar River and Bear Creek. Cedar River flows into the southern end of Lake Washington; Bear Creek flows into the Sammamish River, which flows into the north end of Lake Washington. In each basin, the abundance of juvenile migrants is the measure of freshwater production above the trapping location.

In 1992, the Washington Department of Fish and Wildlife (WDFW) initiated an evaluation of sockeye fry migrants in the Cedar River to investigate the causes of low adult sockeye returns. In 1999, the Cedar River juvenile monitoring study was expanded in scope in order to include juvenile migrant Chinook salmon. This new scope extended the trapping season to a six month period and, as a consequence, also allowed coho production estimates to be derived, and steelhead and cutthroat trout movement to be assessed.

In 1997, WDFW initiated an evaluation of sockeye fry migrants in the Sammamish basin. In 1997 and 1998, a juvenile trap was operated in the Sammamish River during the downstream sockeye migration. In 1999, this monitoring study was moved to Bear Creek in order to simultaneously evaluate Chinook and sockeye production. Since 1999, the Bear Creek juvenile monitoring study has also provided production estimates to be derived for coho, and described the movement of steelhead and cutthroat trout.

The primary study goal of this program in 2011 was to estimate the number of juvenile sockeye and Chinook of natural-origin migrating from the Cedar River and Bear Creek into Lake Washington and the Sammamish River, respectively. This estimate was used to calculate survival of the 2010 brood from egg deposition to lake/river entry and to describe the migration timing of each species.

Cedar River

An inclined-plane trap was operated at RM 0.8, just downstream of the South Boeing Bridge in Renton between January 30 and May 25, 2011. A rotary screw trap was operated at R.M 1.6, just under the I-405 Bridge between April 27 and July 16, 2011. The abundance of natural-origin juvenile migrants was estimated for sockeye fry, sub yearling Chinook, and coho smolts. The number of cutthroat and steelhead migrants was not assessed in 2011 due to insufficient catch.

Production of natural-origin sockeye fry in the Cedar River was estimated to be 4.5 million ± 1.0 million (±95% C.I.). This estimate was based on a total catch of 122,133 between January 30 and May 25 and trap efficiencies ranging from 0.72% to 7.88%. Survival of sockeye fry from egg deposition to lake entry was 4.39%, based on an estimated deposition of 102.9 million eggs. Over the season, 8.78 million hatchery-origin sockeye fry were released into the Cedar River at two different locations. A portion of these (6 million) were released below the inclined-plane trap at the Cedar River Trail Park where in-river survival is assumed to be 100%. The remaining 2.7 million fry were released at R.M. 13.5. Estimates of hatchery fry survival above the trap ranged from 0% to 49.9%. An estimated 12.4 million combined natural and hatchery-origin sockeye fry entered Lake Washington from the Cedar River in 2011.

Median migration date for natural-origin sockeye fry was March 25, 2011, four days later than the long-term average and thirty-five days later than that of the hatchery fry releases. The timing of sockeye outmigration was correlated with February stream temperatures (R²=0.58) and the 2011 daily average February temperatures (5.8°C) was cooler than the 19-year average of 6.4°C.

Production of natural-origin Chinook was estimated to be 187,806 ± 63,560 (±95% C.I.) sub yearlings, based on operation of both the inclined-plane and screw traps. Between January 1 and May 9, 2011, 177,803 ± 63,481(±95% C.I.) natural-origin Chinook were estimated to have passed the inclined-plane trap. This estimate was based on a total catch of 5,239 and trap efficiencies ranging from 0.72% to 7.88%. Between May 10 and July 31, 2011, 10,003 ± 3,099 (±95% C.I.) natural-origin Chinook were estimated to have passed the screw trap. This estimate is based on a total catch of 3,567 natural-origin juvenile Chinook in the screw trap and a trap efficiency of 7.84%. Egg-to-migrant survival of the 2010 brood year Chinook was estimated to be 15.7%, the third highest estimated since trapping began.

Weekly average lengths of sub yearling Chinook increased from 37.7-mm fork length (FL) in January to 95.4-mm FL by the end of the season. Migration timing was bi-modal. The small fry emigrated between January and early-May and comprised 82% of all sub yearlings. The large parr emigrated between early-May and July and comprised 18% of the total migration.

A total of 52,458 ± 7,813 (±95% CI) natural-origin coho were estimated to have migrated passed the screw trap in 2011 during the period the trap was operating. Steelhead/rainbow and cutthroat trout production were not estimated in 2011 due to low catches (12 steelhead/rainbow and 47 cutthroat).

Bear Creek

An inclined-plane trap was operated 100 yards downstream of the Redmond Way Bridge between January 23 and April 22, 2011. On April 26, a rotary screw trap replaced the inclinedplane trap and was fished until July 16, 2011. The abundance of natural-origin juvenile migrants was estimated for sockeye fry, sub yearling Chinook, coho, and cutthroat trout. No steelhead/rainbow trout were caught in the Bear Creek traps during the 2011 trapping season.

Sockeye fry migration in 2011 was estimated to be 8,160,976 ± 1,063,587 (±95% C.I.). This estimate was based on a total catch of 492,773 sockeye fry and trap efficiencies ranging from 3.5% to 10.4%. An egg-to-migrant survival rate of 42.4% was based on an egg deposition of 19.2 million and was the highest estimate of survival since trapping began in 1998.

Production of natural-origin Chinook was estimated to be 18,175 ± 1,687 (±95% C.I.) sub yearlings. This estimate was based on catch in the inclined-plane and screw traps. A total of 1,651 ± 390 (±95% C.I.) Chinook were estimated to have migrated passed the inclined-plane trap between January 23 and April 26. This estimate was based on a total catch of 49 Chinook and efficiencies ranging from 3.5% and 10.4%. A total of 16,524 ± 1,641 (±95% C.I.) Chinook were estimated to have migrated passed the screw trap between April 27 and July 16. This estimate is based on a total catch of 4,434 Chinook and screw trap efficiencies ranging from 13.0% to 56.3%. Egg-to-migrant survival of the 2010 brood year natural-origin Chinook was estimated to be 6.7%, the second highest survival measured.

Weekly average lengths of sub yearling Chinook migrants averaged 39.5-mm FL in February and increased to an average of 86.3-mm FL near the end of the season. Migration timing of sub yearling Chinook was bimodal. Small fry emigrated between February and April and comprised 5.1% of the total migration. Large parr migrants emigrated between May and July and represented 94.9% of total production in Bear Creek during 2011.

A total of 34,513 ± 8,813 (±95% C.I.) natural-origin coho and 4,569 ± 1,403 (±95% C.I.) cutthroat trout were estimated to have migrated from Bear Creek in 2011.