Evaluation of Downstream Migrant Salmon Production in 2008 from the Cedar River and Bear Creek

This report describes downstream 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, and Bear Creek flows into the Sammamish River, which flows into the north end of Lake Washington. Juvenile migration abundances are a measure of salmonid production above the trapping location in each basin. In 1992, the Washington Department of Fish and Wildlife (WDFW) initiated monitoring of sockeye fry production in the Cedar River to investigate causes of low adult sockeye returns. In 1999, this annual trapping program was expanded in scope (a second trap) and length in order to estimate production of juvenile Chinook salmon. Production estimates of coho, steelhead, and cutthroat smolts were also made possible by the expanded trapping program.

Juvenile production of sockeye in the Sammamish basin has been evaluated since 1997. In 1997 and 1998, sockeye fry production was estimated from a trap operated in the Sammamish River. In 1999, this monitoring program was moved to Bear Creek in order to concurrently assess Chinook and sockeye production. Since 1999, the Bear Creek study has also provided outmigration estimates of coho, steelhead and cutthroat smolts.

Cedar River

This report documents production and survival of 2007 brood year sockeye as they emigrate from the Cedar River to Lake Washington. These results contribute to a 17-year dataset for the Cedar River. The primary study goal was to estimate total migration of natural-origin Cedar River sockeye fry into Lake Washington in 2008. This estimate was used to calculate survival of the 2007 brood from egg deposition to lake entry. In addition, this estimate provides early life history data useful for calculating survival among other life stages, including juvenile survival within Lake Washington (lake entry to smolt) and marine survival (smolt to returning adults).

A floating incline-plane screen trap, located at river mile (R.M.) 0.7 in the Cedar River, was operated between January 13 and May 17 and captured a portion of sockeye fry migrating from the Cedar River into Lake Washington (Figure 2). Total migration was estimated to be 25.1 million ± 979,000 (95% C.I.) natural-origin sockeye fry. This estimate is based on a total catch of 1,344,757 and trap efficiencies ranging from 1.4% to 14.3%. Based on an estimated deposition of 78.5 million eggs, survival of natural-origin fry from egg deposition to lake entry was 31.2%, the highest survival estimated in the past 17 years. Over the season, 2.49 million hatchery-produced sockeye fry were released into the Cedar River below the incline-plane trap at the Cedar River Trail Park. If survival of hatchery fry released at the Cedar River Trail Park is assumed to be 100%, an estimated 27.6 million sockeye fry entered Lake Washington from the Cedar River in 2008.

Median migration date for natural-origin fry in 2008 (March 16) was six days earlier than the median natural-origin outmigration, and 10 days later than that of hatchery fry. Sockeye outmigration timing is correlated with February stream temperatures. In 2008, temperatures were slightly cooler (5.9?C) than the 17-year average (6.2?C).

Chinook outmigration was evaluated with two different traps. An incline-plane trap, used to assess sockeye fry production, captured the smaller, early-migrating Chinook. A screw trap, operated April 14 through July 19, captured the larger, late-migrating Chinook. A total of 705,583 ± 76,106 (95% C.I.) Chinook are estimated to have passed the incline-plane trap between January 1 and May 17. This estimate is based on a total catch of 31,307 and trap efficiencies ranged from 1.4% to 14.3%. A total of 39,311 ±18,156 (95% C.I.) Chinook are estimated to have passed the screw trap between May 28 and July 19. This estimate is based on a total catch of 741 juvenile Chinook in the screw trap. Screw trap efficiencies ranged from 2.6% to 7.4%. Between May 17 and 28, neither trap operated due to extremely high water. Migration during the outage was estimated to range between 22,068 to 27,726 Chinook.

Between 766,962 and 772,620 0+ Chinook are estimated to have outmigrated from the Cedar River in 2008. Migration timing was bi-modal. Fry emigrated between January and mid-May and comprised between 89.5% and 90.1% of the total migration. Parr emigrated between mid- April and July. Egg-to-migrant survival was estimated at 19%. Age 0+ Chinook increased in size from 34 mm fork length (FL) in January to 121 mm FL by the end of the season.

A total of 13,222 natural-origin coho are estimated to have migrated passed the screw trap in 2008. This total includes 10,404 ? 9,909 (95% CI) coho estimated during the trapped period and 2,962 coho estimated during the trap outage. Steelhead and cutthroat production were not estimated for in 2008 due to low catches (1 steelhead and 26 cutthroat smolts).

Bear Creek

An incline-plane trap installed 100 yards downstream of the Redmond Way Bridge, was operated between February 3 and April 14. A screw trap replaced the incline-plane trap April 14, and fished until July 9. 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.

A total sockeye fry migration in 2008 is estimated at 251,285 ±58,794 (95% C.I.). This estimate is based on a total catch of 21,802 sockeye fry and trap efficiencies ranging from 6.2% to 12.5%. Juvenile production, applied to deposition of an estimated 18.6 million eggs from the 2007 adult return, yielded a survival rate of 13.5%, the third highest survival since trapping began in 1998.

Chinook outmigration was evaluated from catch in incline-plane and screw traps. A total of 1,172 ± 80 (95% C.I.) Chinook are estimated to have migrated passed the incline-plane trap between February 3 and April 14. This estimate is based on a total catch of 110 Chinook and trap efficiencies of sockeye fry, which were used as a surrogate for Chinook fry migrants. A total of 11,598 ± 2,136 (95% C.I.) Chinook are estimated to have migrated passed the screw trap between April 15 and July 9. This estimate is based on a total catch of 2,772 Chinook and screw trap efficiencies ranging from 18.0% to 41.2%.

A total of 12,770 age 0+ Chinook are estimated to have outmigrated from Big Bear Creek in 2008. Migration timing was bimodal. Fry emigrated between February and April and comprised 9.2% of the total migration. Parr emigrated between May and July. Egg-to-migrant survival was estimated to be 1.0%. Sizes of outmigrating Chinook averaged 37.0 mm FL in February and did not increase to an average of 70.0 mm FL until late May. A total of 12,208 ±2,401 (95% C.I.) natural-origin coho and 2,751 ±1,091 (95% C.I.) cutthroat are estimated to have outmigrated from Big Bear Creek in 2008. Only one steelhead was caught in the Bear Creek screw trap during the 2008 trapping season.