Estimating Chinook Escapement in the John Day River Basin Using a Mark-Recapture Approach

The John Day River basin supports three wild populations of spring Chinook salmon that remain depressed relative to historic levels; escapement estimates from 2000 to 2013 have ranged from 1,817 to 7,808 spawners.  Beginning in 2009, mark‐recapture analysis using Passive Integrated Transponder (PIT) tagged fish has provided an independent alternative to estimating spawner abundance as the product of redd count and an out‐of‐basin fish per redd estimate.  Several thousand smolts are PIT tagged annually when emigrating from the John Day River basin, providing a number of returning adults that are “marked” via passive detection at arrays in the Columbia and John Day rivers.  During spawning ground surveys, recovered carcasses are examined for marks.  The percentage of recaptured fish has ranged from 1.1 to 2.3 and standard deviation of the PIT tag based abundance estimate has ranged from 647 to 1,801.  Although data from 2011 and 2012 suggest that this approach is feasible, the 2013 mark‐recovery escapement estimate demonstrates that a minimum number of returning marked fish is necessary to generate an estimate with acceptable confidence.  The concordance of escapement estimates generated by these discrete methodologies increases our confidence in the suitability of both methods for estimating John Day Chinook escapement.