2003 Skagit River Wild 0+ Chinook Production Evaluation Annual Report

Skagit River chinook returns (spring and summer/fall combined) have steadily declined over the last fifty years. In 1999, Puget Sound chinook salmon were listed as threatened under the Endangered Species Act. To address this poor stock status, resource managers formed the multi-agency Skagit River Chinook work group in 1995. A major goal of this work group is to determine the factors which limit chinook production. In addition to assessing habitat and adult returns, monitoring juvenile production was initiated as it directly measures freshwater survival. Evaluating the biological attributes of outmigration timing and size contributes to our understanding of chinook freshwater life history. This information is useful for flow management, habitat protection and restoration, and designing hatchery programs to minimize adverse interactions.

In 1990, WDFW initiated downstream migrant trapping in the Skagit River system at Burlington. Although this project was originally directed at assessing coho smolt production (April through June), we identified and enumerated all fish captured. In 1991, through a fisheries settlement agreement with state, federal and tribal agencies, Seattle City Light (operators of seve ral dams on the Skagit River) created the Skagit Non-Flow Plan Coordinating Committee (NCC). Beginning in 1997, this program provided funding to expand our downstream migrant trapping project in the Skagit River to also estimate chinook production (January through July). This report documents our investigations in Spring 2003, the fourteenth year that we have measured downstream migrants from the Skagit River.

We operated two trap types â€" a floating inclined-plane screen trap (scoop trap) and a screw trap â€" from January 15 through July 30. The traps were fished every night and every third day unless flows and associated debris loads were excessive. To calibrate trap efficiency, we marked and released five chinook groups (four wild and one hatchery) above the trap. Recovery rates for the four wild groups released in February through April were lower (1.7%) than the single hatchery group released on May 8 (3.6%). Combined, these groups’ average is nearly identical to the long-term trap efficiency rate of 2.0%. This is the mean capture rate of 25 zero-age chinook calibration groups that we released upstream of the mainstem traps from 1998 through 2003.

Over the season we captured 51,316 and 34,498 chinook in the scoop and screw traps, respectively. The months of January, February, March, and April accounted for 84% of the season total migration, with about 50% of the chinook outmigrants passing the mainstem traps by March 15. Expanding catches for the intervals not fished estimates an additional 15,463 and 10,058 wild 0+ chinook would have been captured in the scoop and screw traps, respectively. Combining these projected catches with the actual catches estimates 111,335 wild 0+ chinook would have been caught in the two traps had we fished continuously from January 15 through July 30. Expansion of the projected season catch in both traps by the average trap efficiency rate yields a system production estimate of approximately 5.5- million zero-age wild chinook. Average survival-to- migration is estimated at 10.8%. This estimate is based on a potential deposition of 51.1 million eggs (9,295 females and an average fecundity of 5,500 eggs/female) for the 2002 brood.

Over the previous thirteen seasons, flow during egg incubation has explained most of the inter-annual variation in our estimates of egg-to-migrant survival rates. The production in 2003 is somewhat lower than predicted by this relationship, which may indicate other factors at work. One explanation for this lower-than-predicted survival may be the effects of the high spawning population in 2002. This return, estimated at 20,656 adults, is the highest from which we have estimated production in this system. Continued monitoring of juvenile production including broods with even higher spawning populations and additional flow analyses will further define the constraints to chinook production from the Skagit River.

In addition to wild chinook, we caught a total of 3,644 ad-marked and coded-wire tagged hatchery 0+ chinook in the mainstem traps. We estimate that, had the trap fished continuously, we would have caught an additional 357 fish. The projected total catch of 4,001 hatchery chinook includes 118 fall 0+ chinook (released at Baker River), 1,883 summer 0+ chinook (released at Countyline Ponds) and 2,000 spring 0+ chinook (released at Skagit Hatchery). Application of the average trap efficiency to the projected season catch yields a combined estimate of 197,000 zero-age hatchery chinook. Relating this estimate to the 486,500 hatchery chinook released estimates in- river survival above Mt. Vernon at 40%.