Fish/Shellfish Research and Management - Fish/Shellfish Research
Date Published: May 2006
Number of Pages: 62
Publication Number: FPA 06-06
Author(s): Greg Vokhardt, Dave Seiler, Steve Neuhauser, Lori Kishimoto and Clayton Kinsel
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 2005, the sixteenth 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 2005. The traps were operated from January 21 through July 25, and were fished every night and every third day unless flows and associated debris loads were excessive. To calibrate trap efficiency, we marked and released seven chinook groups (4 hatchery, 3 wild) above the trap. Recovery rates for these calibration groups were higher (3.6%) than the long-term mean capture rate (2.0%) of 29 zero-age chinook calibration groups that we released upstream of the main stem traps from 1998 through 2004.
Over the season we captured 44,737 and 34,470 wild 0+ chinook in the scoop and screw traps, respectively. The months of January, February, March, and April accounted for 90% of the season total migration, with about 50% of the wild 0+ chinook out migrants passing the main stem traps by March 21. This migration timing is earlier than the median migration date we have observed from 1997-2004. Expanding catches for the intervals not fished estimates an additional 13,603 and 15,475 wild 0+ chinook would have been captured in the scoop and screw traps, respectively. Combining these projected catches with the actual catches estimates 108,285 wild 0+ chinook would have been caught in the two traps had we fished continuously from January 21 through July 25. Expanding the projected season catch in both traps by two average flow related efficiency rates yields a system production estimate of approximately 4.6-million zero-age wild chinook. Average survival-tomigration is estimated at 7.38% this estimate is based on a potential deposition of 62.3 million eggs (11,329 females and an average fecundity of 5,500 eggs/female) for the 2004 brood.
Over the previous fifteen 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 2005 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 2004.
This return, an estimated 25,175 adult chinook (Brett Barkdull, pers. comm.), is the highest from which we have estimated production in this system. Another factor that contributed to this lower survival was the unusually low flow conditions that occurred for the duration of the chinook emigration. 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.
In addition to wild chinook, we caught a total of 1,097 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 365 fish. The projected total catch of 1,462-hatchery chinook includes 845 summer 0+ chinook and 190 fall 0+ chinook (both released at Countyline Ponds), and 427 spring 0+ chinook (released at Marblemount Hatchery). Application of two average flow-related efficiency rates yields a combined estimate of 59,469 zero-age hatchery chinook. Relating this estimate to the 605,390 hatchery chinook released (Steve Stout, pers. comm.) estimates in-river survival above Mt. Vernon at 9.8%.