Estimation of the Effective Number of Breeders for Warm Springs River Spring Chinook Salmon – Broodyears 2015 and 2016
Spring Chinook Salmon spawning in the upper Warm Springs River basin have demonstrated a relatively sudden increase in the ratio of annual adult escapement to total redd count. Prior to 2010 the ratio averaged 4 fish per redd, but since then the average jumped to 12 fish per redd. The recent high fish per redd estimates infer that a large proportion of the escapement failed to successfully spawn, although the alternative explanation that the redd counts were substantially underestimated is also possible. To confirm which explanation is the more likely, a genetic sibship analysis was conducted on a sample of smolts from broodyears 2015 and 2016 to estimate the effective number of breeders (Nb). The Nb estimate for each broodyear was only 21% and 12% of the total escapement, respectively, and that the ratio of Nb divided by 4, the historic fish per redd value, provided an estimate for the expected number of redds that was very similar to the observed number in each broodyear. The results, therefore, do not support undercounting of redds, but instead that there was a high rate of reproductive failure of adults in these broodyears.
Galbreath, P.F., I. Janowitz-Koch, G. Boostrom, and C. Baker. 2018. Estimation of the effective number of breeders of Warm Springs River spring Chinook Salmon – Broodyears 2015 and 2016. Columbia River Inter-Tribal Fish Commission Technical Report 18-04, Portland, Oregon. 7pgs.