2004 Skagit River Wild 0+ Chinook Production Evaluation Annual Report

Skagit River chinook returns (spring and summer/fall combined) have 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 that 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 several dams on the Skagit River) created the Skagit Non-Flow Plan Coordinating Committee (NCC). Beginning in 1997, this program provided funding to expand our Skagit River downstream migrant trapping project to also estimate chinook production (January through July). This report documents our investigations in Spring 2004, the fifteenth year that we have measured downstream migrants from the Skagit River.

We used two traps, a floating inclined-plane screen trap (scoop trap) and a screw trap, to capture downstream migrants in 2004. The traps were operated from January 23 through July 28, and were fished every night and every third day unless flows and associated debris loads were excessive. Over the season we captured 13,009 and 6,694 wild 0+ chinook in the scoop and screw traps, respectively. Approximately 50% of the chinook outmigrants passing the mainstem traps by April 15. This is the third latest migration timing we have observed thus far, and the latest observed since 1998. Expanding catches for the intervals not fished estimates an additional 6,019 and 4,389 wild 0+ chinook would have been captured in the scoop and screw traps, respectively. Combining these projected catches with the actual catches estimates 30,111 wild 0+ chinook would have been caught in the two traps had we fished continuously from January 23 through July 28.

In addition to wild chinook, we caught a total of 19,474 ad-marked and coded-wire tagged (CWT) hatchery 0+ chinook in the mainstem traps. We estimate that, had the trap fished continuously, we would have caught an additional 7,980 hatchery fish. The projected total catch of 27,454 hatchery chinook includes 6,911 fall 0+ chinook (released at Baker River), 10,669 summer 0+ chinook (released at Countyline Ponds) and 8,445 spring 0+ chinook (released at Skagit Hatchery). Application of the catch rate for ad-marked/CWT fish (3.5%) to the projected season catch yields a combined estimate of 785,000 zero-age hatchery chinook. This estimate exceeds the reported number of hatchery chinook released because it includes unmarked/untagged fish from an unknown number of additional chinook released from Marblemount Hatchery that were too small or in too poor of condition to tag. This estimate indicates a very high (near 100%) survival rate for hatchery chinook released in 2004. Past years results indicates a 50% survival rate to the Burlington traps is more typical for hatchery releases.

To estimate trap efficiency early in the season (prior to May 26), we conducted four mark-recapture experiments using hatchery chinook. Recovery rates for these groups averaged 1.2%. Trap efficiency increased to 3.5% after this date as indicated by the recovery of ad-marked/CWT hatchery release groups. Production was estimated by stratifying the catch data and applying these two rates to projected catches occurring before and after May 26. This estimated system production at approximately 1.8-million zero-age wild chinook. Average survival-to-migration is estimated at 7.0%. This estimate is based on a potential deposition of 25.7 million eggs (4,668 females and an average fecundity of 5,500 eggs/female) for the 2003 brood.

Over the previous fourteen seasons, flow during egg incubation has explained most of the interannual variation in our estimates of egg-to-migrant survival rates. The survival rate measured in 2004 is nearly equal to the predicted value. Other factors may exert a greater influence on survival during lower, more benign, peak incubation flows. Continued monitoring of juvenile production including broods with high spawning populations and additional flow analyses will further define the constraints to chinook production from the Skagit River.