Green River Juvenile Salmonid Production Evaluation: 2013 Annual Report
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Green River Juvenile Salmonid Production Evaluation: 2013 Annual Report

Category: Fish/Shellfish Research and Management

Date Published: July 2014

Number of Pages: 62

Publication Number: FPA 14-08


This report provides the 2013 results from the juvenile salmonid monitoring study conducted on the Green River in central Puget Sound, Washington. The primary objective of this study was to estimate the juvenile abundance of natural-origin Chinook salmon in the Green River. Tissue samples were collected from a significant portion of the juvenile Chinook migrants captured over the season as part of a project to estimate the number of adult Chinook that returned to the Green River in the fall of 2011 via Genetic Mark Recapture methods. Additional objectives were to estimate the number of juvenile migrants and life history characteristics of other salmonid species. Juvenile salmonids were captured in a five-foot screw trap located at river mile 34.5 (55 rkm). Catch was expanded to a total migration estimate using a time-stratified approach that relied on release and recapture of marked fish throughout the outmigration period.

The trap was operated from January 24 through July 15, 2013. During this period, the trap fished 89% of the time. We estimated the freshwater production (juvenile abundance) of Chinook (sub yearling), coho, and steelhead.

(Table 1). Table 1. Catch, freshwater production, fork length (mm), and out-migration timing of natural-origin juvenile salmonids caught in the Green River screw trap in 2013. Data represent freshwater production above the juvenile trap, which is located at river mile 34.5.

Catch Production
(% CV)
Avg Fork Length
(± 1 S.D.)
Migration Date
Chinook – SubYrlg 22,690 492,737 (6.3%) 48.1 (± 14.4) 21-Mar
Chinook – Yrlg 1 ---a --- 2-Mar b
Coho – Yrlg 1,149 50,642 (20.8%) 103.5 (± 16.8) 9-May
Steelhead – Smolt 444 15,339 (28.8%) 169.1 (± 17.7) 11-May
Chum 69,365 ---c ---c 28-Mar b
a Capture rates were not high enough to derive a production estimate or describe migration timing for yearling Chinook.
b These are median catch dates which are not adjusted for trap efficiency and therefore serves as an index of migration timing.
c Unable to distinguish between natural origin and hatchery production

Chinook salmon spawn above and below the juvenile trap. A basin-wide production estimate was derived by applying estimated survival above the trap to spawning below the trap; a screw trap fished in Big Soos Creek estimated production from that tributary. Egg-to-migrant survival of Green River Chinook for the 2013 outmigration (2012 brood) was estimated to be 9.72%, yielding a basin-wide production estimate of 1,008,512 juveniles. Included in this estimate was a preliminary estimate of 468,119 Chinook migrating from Big Soos Creek with a egg-to-migrant survival of 15.3%. This estimate was generated by a screw trap located just above the hatchery and operated by the Muckleshoot Indian Tribe.
Juvenile migrant Chinook in the Green River are predominantly sub yearlings. Outmigration timing of sub yearling Chinook was bimodal. The fry (<45 mm fork length) represented 72% of all sub yearling migrants and peaked in early March, parr migrants (>45 mm fork length) represented 28% of the migration and peaked in early June.