ESTIMATING SALMON SPAWNING ESCAPEMENT USING CAPTURE-RECAPTURE METHODS

We describe a method of estimating the spawning escapement of coho sal mon (Oncorhynchus kisutch) from capture-recapture data. Traditional ca pture-recapture analyses do not directly provide estimates of escapeme nts; however, we show how simple modifications to the Jolly-Seber meth od can estimate the total number of fish returning to a river includin g those that enter and die between sampling occasions. Spawning runs o f Pacific salmon were simulated and their escapements estimated using capture-recapture. The performance of the maximum likelihood estimator s (MLEs), the censored MLEs, the constrained MLEs, and less-biased est imators in estimating the run sizes and providing estimates of precisi on were evaluated. Simulation results indicated that constrained MLEs provided the most appropriate estimates of escapement and that standar d errors be computed using the large-sample variance formulae evaluate d at these estimates. These methods were used to estimate the escapeme nts of coho salmon to a small river on Vancouver Island in 1989 and 19 90.