Estimating the Size of Steelhead Runs by Tagging Juveniles and Monitoring Migrants

Abstract

The number of individuals in a spawning run of anadromous fish can be estimated by tagging juveniles with passive integrated transponders during the freshwater phase and the subsequent monitoring of adult upstream migrants by use of instream tag readers. The method may enable monitoring of steelhead Oncorhynchus mykiss in systems where other methods are intractable. I developed a hierarchical capture–resighting model and applied it to simulated data to develop a relation between the number of marked juveniles and the precision of run size estimates. Precision is primarily controlled by the number of tagged spawners that ultimately return and are detected; only 30–90 tagged spawners are required to obtain relatively precise estimates of run size. For typical marine survival rates, 0.0033–0.0330, this translates to a tagging effort of between 3,400 and 45,000 juveniles/cohort. Estimates are robust to imperfect detection of tagged fish if at least two instream readers independently scan for tags. Reach sampling allows for estimation of run size in large stream systems. Based on my simulations, the number of reaches could be as low as 30–40 under scenarios of high marine survival. Computer code is provided for the estimator using the freely available statistical software R.