Evaluation of Juvenile Salmon Production in 2010 from the Cedar River and Bear Creek

This report describes juvenile migrations of five salmonid species emigrating 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 production estimates to be derived for coho, steelhead, and cutthroat trout.

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, steelhead, and cutthroat trout.

The primary study goal of this program in 2010 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 2009 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 17 and May 2, 2010. A rotary screw trap was operated at R.M 1.6, just under the I-405 Bridge between April 15 and July 4, 2010. 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 2010 due to insufficient catch.

Production of natural-origin sockeye fry in the Cedar River was estimated to be 12.5 million ± 799,799 (95% C.I.). This estimate was based on a total catch of 804,648 between January 17 and May 2 and trap efficiencies ranged from 3.3% to 14.7%. Survival of sockeye fry from egg deposition to lake entry was calculated at 56.6%, based on an estimated deposition of 22.1 million eggs. Over the season, 4.5 million hatchery-origin sockeye fry were released into the Cedar River below the inclined-plane trap. If survival of the released hatchery fry is assumed to be 100%, an estimated 17.1 million combined natural and hatchery-origin sockeye fry entered Lake Washington from the Cedar River in 2010.

Median migration date for natural-origin sockeye fry was March 7, 2010, 14 days earlier than the long-term average and three days earlier than that of the hatchery fry release. The timing of sockeye outmigration was correlated with February stream temperatures (R2=0.58) and the 2010 daily average temperatures (7.7°C) were warmer than the 19-year average of 6.4°C.

Production of natural-origin Chinook was estimated to be 152,390 ± 13,058 (95% C.I.) sub yearlings. This estimate was based on operation of both the inclined-plane and screw traps. Between January 1 and April 14, 2010, 115,474 ± 13,058 (95% C.I.) natural-origin Chinook were estimated to have passed the inclined-plane trap. This estimate was based on a total catch of 7,522 and trap efficiencies ranging from 3.3% to 14.7%. Between April 15 and July 4, 2010, 36,916 ± 5,374 (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 trap efficiencies ranging of 8.9% and 11.2%. Egg-to-migrant survival of the 2009 brood year Chinook was estimated to be 11.9%, the fourth highest estimated since trapping began..

Weekly average lengths of sub yearling Chinook increased from 38.2 mm fork length (FL) in January to 95.6 mm FL by the end of the season. Migration timing was bi-modal. The small fry emigrated between January and mid-April and comprised 76% of all sub yearlings. The large parr emigrated between mid-April and July and comprised 24% of the total migration.

A total of 83,060 ± 13,011 (95% CI) natural-origin coho were estimated to have migrated passed the screw trap in 2010. This total included 1,091 coho estimated to have migrated before screw trapping began, 81,720 coho estimated during the trapped period, and 249 coho estimated to have migrated after trapping concluded. Steelhead/rainbow and cutthroat trout production were not estimated in 2010 due to low catches (8 steelhead/rainbow and 73 cutthroat).

Bear Creek

An inclined-plane trap was operated 100 yards downstream of the Redmond Way Bridge between January 31 and April 16, 2010. On April 17, a rotary screw trap replaced the inclinedplane trap and was fished until July 4, 2010. The abundance of natural-origin juvenile migrants was estimated for sockeye fry, sub yearling Chinook, coho, and cutthroat trout. No steelhead/rainbow were caught in the Bear Creek traps during the 2010 trapping season.

Sockeye fry migration in 2010 was estimated to be 129,903 ± 19,443 (95% C.I.). This estimate was based on a total catch of 8,881 sockeye fry and trap efficiencies ranging from 6.7% to 8.1%. An egg-to-migrant survival rate of 4.7% was based on an egg deposition of 2.8 million and was the lowest estimate of survival since trapping began in 1998.

Production of natural-origin Chinook was estimated to be 9,185 ± 2,408 (95% C.I.) sub yearlings. This estimate was based on catch in the inclined-plane and screw traps. A total of 1,554 ± 415 (95% C.I.) Chinook were estimated to have migrated passed the inclined-plane trap between January 31 and April 16. This estimate was based on a total catch of 104 Chinook and efficiencies ranging of 6.7% and 8.1%. A total of 7,631 ± 2,372 (95% C.I.) Chinook were estimated to have migrated passed the screw trap between April 17 and July 4. This estimate is based on a total catch of 1,316 Chinook and screw trap efficiencies ranging from 5.8% to 52.9%. Egg-to-migrant survival of the 2009 brood year natural-origin Chinook was estimated to be 4.3%, the second highest survival measured.

Weekly average lengths of sub yearling Chinook migrants averaged 38.0 mm FL in February and increased to an average of 82.3 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 16.7% of the total migration. Large parr migrants emigrated between May and July and represented 83.3% of total production in Bear Creek during 2010.

A total of 13,100 ± 1,673 (95% C.I.) natural-origin coho and 5,209 ± 769 (95% C.I.) cutthroat trout were estimated to have migrated from Bear Creek in 2010.