Evaluation of Coweeman River Salmonids in 2012 and 2013: Juvenile Production and Other Activities

Viable salmonid population (VSP) parameters are monitored in the Coweeman River basin as part of a broader monitoring program in the Lower Columbia Evolutionary Significant Unit. The primary goal of this study is to monitor juvenile abundance, diversity, and timing (distribution) that contribute to VSP parameters for ESA-listed species ("tule" fall Chinook, coho, and steelhead), and coastal cutthroat. Additional study of tule fall Chinook includes collection of genetic tissue for a genetic mark-recapture estimate of adult spawner escapement, application of strontium marks to investigate freshwater residency of juvenile Chinook, and application of coded-wire tags (CWT) to understand where wild tule fall Chinook are intercepted in ocean fisheries. Releases of hatchery-origin winter-run steelhead (the number planted vs. the number that emigrate) are also monitored. This report provides the results of work conducted in 2012 and 2013.

To meet the study objectives, a 1.5 m (5-foot) diameter rotary screw trap was operated near river kilometer (rkm) 12.0 from February 8, 2012 to August 24, 2012 (sampled 93% of the time), and February 6 to August 23, 2013 (sampled 100% of the time).

In 2012, the outmigrant abundance estimate ± 95% CI of fall Chinook was 245,008 ± 44,856 (CV = 9.3%). This estimate includes 200,556 ± 44,655 (CV = 11.4%) fry and 44,452 ± 4,245 (CV = 4.9%) subyearling smolts. Of these, 14,940 were marked with strontium and/or CWTs. The estimates for the other species were 14,014 ± 3,790 (coho, CV = 11.4%), 13,488 ± 4,458 (natural-origin steelhead, CV = 16.9%), and 2,658 ± 774 (cutthroat, CV = 14.9%). The estimated number of hatchery steelhead released from the acclimation pond (11,492 ± 3,437, CV = 15.3%) was significantly more than the estimated number that passed the trap (7,738 ± 2,783, CV = 18.3%; Z = 1.66, p = 0.05, one-tailed test).

In 2013, the abundance estimate ± 95% CI of fall Chinook was 138,273 ± 20,779 (CV = 7.7%). This estimate includes 98,698 ± 20,675 (CV = 10.5%) fry and 39,574 ± 4,179 (CV = 5.4%) subyearling smolts. Of these, 7,500 were marked with strontium and/or CWTs. The estimates for the other species were 13,354 ± 2,400 (coho, CV = 9.2%), 17,924 ± 1,901 (naturalorigin steelhead, CV = 5.4%), and 2,841 ± 799 (cutthroat, CV = 14.4%). The estimated number of hatchery steelhead released from the acclimation pond (8,958 ± 1,676, CV = 9.5%) was not significantly different than the estimated number that passed the trap (10,510 ± 1,562, CV = 7.6%; Z = 1.33, p = 0.09, one-tail test). An estimate for smaller, late migrating (mid-June to mid- July) coho was estimated for the first time. The estimate for this group was 2,192 ± 261 (CV = 6.1%).