Coweeman River Salmonid Production Evaluation: 2011 Completion Report

In 2007, the Washington Department of Fish and Wildlife initiated research designed to better understand the abundance, productivity, diversity, and fisheries contributions of tule Fall Chinook salmon originating from the Coweeman River. The objectives of this work were to estimate adult escapement and juvenile production by two outmigrant life histories (i.e., fry and subyearling smolt), to describe the juvenile freshwater residency period of returning adults, and to determine the relative contributions of the two outmigrant life histories to escapement and fisheries. Additional objectives were to estimate the juvenile production and outmigrant life history characteristics of other anadromous salmonids that occur in the Coweeman River which include coho salmon, natural and hatchery-origin steelhead, and coastal cutthroat trout. This report provides the results of work conducted in 2011 to meet these objectives and summarizes past results.

A 1.5 m (5-foot) diameter rotary screw trap was operated near river kilometer (rkm) 12.0 from February 3 through August 24, 2011 to capture and mark outmigrating juvenile Chinook and other anadromous salmonids. During this period, the trap was not fully operational for 10 days due to high discharge and heavy debris loads.

In 2011, a total of 309,000 (95% CI = 227,900 – 441,200; CV = 18.1%) subyearling Chinook salmon were estimated to have emigrated from the Coweeman River. Of these, 260,476 ± 86,713 (abundance, ± 95% CI; CV = 17.0%) were fry migrants and 48,469 ± 10,501 (abundance, ± 95% CI; CV = 11.1%) were Chinook subyearling smolt migrants. In 2011, 14,879 ± 3,666 (abundance ± 95% CI; CV = 12.6%) coho, 29,127 ± 16,285 (CV = 28.5%) natural-origin steelhead, 6,976 ± 4,492 (CV = 32.9%) hatchery-origin steelhead, and 2,033 ± 1,341 (CV = 33.7%) coastal cutthroat trout smolts were estimated to have emigrated from the Coweeman River basin.

Since 2007, we have strontium-marked the otoliths of both juvenile outmigrant life histories to estimate the duration of freshwater residency between the Coweeman River and the Columbia River estuary/Pacific Ocean. Preliminary results from adults returning to the Coweeman River in 2009 and 2010 (n = 147) suggest that freshwater residency duration varies from days to months between the Coweeman trap and the saltwater environment.

Juvenile Chinook have been coded-wire tagged and released from the Coweeman River for four years. We used this marking approach to determine the contribution of Coweeman Chinook to fisheries. A total of 28 coded-wire tags have been recovered from the 2006 and 2007 brood years. Based on fishery sampling rates, these recoveries were expanded to a total of 114 tags or 0.25% of the 45,647 tags released in 2007 and 2008. Of the estimated tag recoveries, 46% were recovered in coast-wide fisheries and the remaining 54% were recovered on the spawning grounds. The majority of estimated 53 fishery interceptions occurred in Washington sport (43%) and Canadian troll (47%) fisheries.

Fall Chinook salmon escapement was estimated using two mark-recapture approaches. The pooled-Petersen escapement estimate was 668 ranging from 594 to 742 (95% CI; CV = 5.7%); however, we found evidence to suggest that size and age selectivity occurred which violated one assumption of the estimator. The Jolly-Seber escapement estimate was 459 ranging from 310 to 723 (95% CI; CV = 24.4%). These estimates did not differ statistically (Z-test; p=0.072).