Evaluation of Juvenile Salmon Production in 2012 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 watershed, 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 watershed. 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 ancillary data on movement patterns of steelhead and cutthroat trout.

The primary study goal of this program in 2012 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 2011 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 22 and May 10, 2012. A rotary screw trap was operated at R.M 1.6, just under the I-405 Bridge between April 18 and July 14. 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 2012 due to insufficient catch.

Production of natural-origin sockeye fry in the Cedar River was estimated to be 14.8 million ± 1.3 million (±95% C.I.). This estimate was based on a total catch of 241,886 between January 22 and May 10 and trap efficiencies ranging from 0.5% to 11.5%. Survival of sockeye fry from egg deposition to lake entry was 37.6%, based on an estimated deposition of 39.2 million eggs. Over the season, 8.11 million hatchery-origin sockeye fry were released into the Cedar River at three different locations. A portion of these (2.77 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 sockeye (5.34 million) were released above the trap: 3.15 million fry were released at R.M. 13.5, and 2.19 million fry released at R.M. 21.8. Total hatchery abundance from upstream releases is estimated at 3.2 million fry from a total of 5.3 million fry released. Hatchery fry survival from individual releases ranged from an estimated 15.8% to 105.4%, with an overall survival estimate of 65.0%. Over 1.6 million mixed natural and hatchery origin sockeye were estimated to have migrated on February 21. This night was kept separate due to our inability to form separate hatchery and natural-origin estimates and is not included in either the hatchery or natural-origin abundance or survival estimates. An estimated 22.3 million combined natural and hatcheryorigin sockeye fry entered Lake Washington from the Cedar River in 2012.

Median migration date for natural-origin sockeye fry was March 22, 2012, 1 day later than the long-term average and fourteen days later than that of the hatchery fry releases. The timing of sockeye out-migration was somewhat correlated with February stream temperatures (R²=0.41) and the 2012 daily average February temperatures (6.1° C) was cooler than the 21-year average of 6.3° C.

Production of natural-origin Chinook was estimated to be 902,514 ± 165,973 (±95% C.I.) sub yearlings, based on operation of both the inclined-plane and screw traps. Between January 1 and May 10, 2012, 863,595 ± 165,775 (±95% C.I.) natural-origin Chinook were estimated to have passed the inclined-plane trap. This estimate was based on a total catch of 16,993 and trap efficiencies ranging from 0.5% to 11.48%. Between May 10 and July 31, 2012, 38,918 ± 8,118 (±95% C.I.) natural-origin Chinook were estimated to have passed the screw trap. This estimate is based on a total catch of 2,692 natural-origin juvenile Chinook in the screw trap and trap efficiencies of 5.1% and 38.8%. Egg-to-migrant survival of the 2011 brood year Chinook was estimated to be 61.8%, the highest estimated since trapping began.

Weekly average lengths of sub yearling Chinook increased from 39.0-mm fork length (FL) in January to 95.9-mm FL by July. Migration timing was bi-modal. The small fry emigrated between January and early-May and comprised 93% of all sub yearlings. The large parr emigrated between early-May and July and comprised 7% of the total migration.

A total of 48,168 ± 9,675 (±95% CI) natural-origin coho were estimated to have migrated passed the screw trap in 2012 during the period the trap was operating. Steelhead/rainbow and cutthroat trout production were not estimated in 2012 due to low catches (4 steelhead/rainbow and 103 cutthroat).

Bear Creek

A rotary screw trap was operated 100 yards downstream of the Redmond Way Bridge the entire season, from January 24 and July 14, 2012. 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 trap during the 2012 trapping season.

Sockeye fry migration in 2012 was estimated to be 266,899 ± 62,030 (±95% C.I.). This estimate was based on a total catch of 24,494 sockeye fry and trap efficiencies ranging from 2.4% to 16.9%. An egg-to-migrant survival rate of 17.7% was based on an egg deposition of 1.51 million and was the fifth highest estimate of survival since trapping began in 1998.

Production of natural-origin Chinook was estimated to be 22,197 ± 2,304 (±95% C.I.) sub yearlings. This estimate was based on a total catch of 6,229 Chinook and efficiencies ranging from 2.4% and 55.0%. Egg-to-migrant survival of the 2011 brood year natural-origin Chinook was estimated to be 9.0%, the second highest survival measured since 2000.

Weekly average lengths of sub yearling Chinook migrants averaged 38.0-mm FL in February and increased to an average of 82.1-mm FL by July. Migration timing of sub yearling Chinook was bimodal. Small fry emigrated between February and April and comprised 18% of the total migration. Large parr migrants emigrated between May and July and represented 82% of total production in Bear Creek during 2012.

A total of 16,059 ± 1,325 (±95% C.I.) natural-origin coho were estimated to have migrated fron Bear Creek in 2012 and 16,284 ± 6,822 (±95% C.I.) cutthroat trout were estimated to have moved past the trap in Bear Creek in 2012.