Uncertainties in Estimating Fishing Mortality in Unmarked Salmon in Mark-Selective Fisheries Using Double-Index-Tagging Methods

Abstract

Mass marking of hatchery salmon and selective fisheries on these marked fish are being implemented in the northeastern Pacific Ocean. The potential impact of this management practice on current coded wire tag (CWT) programs has become a vital concern. Methods have been developed to maintain the viability of the current CWT programs by using double index tags (DITs). This study uses Monte Carlo simulations to examine uncertainty in estimating fishing mortality of unmarked coho salmon Oncorhynchus kisutch based on the DIT method. Variables used in the simulation include marine survival rate, encounter rate, hook-and-release mortality rate, precision in catch and escapement sampling, and harvest rate in nonselective fisheries. Results show that random variability in these variables creates significant uncertainty in the estimates. Typically, the 90% confidence interval of the estimated fishing mortality rate may take any value from below zero to more than double the true value. The precision of estimated fishing impacts on unmarked salmon cannot be maintained at the level for marked salmon by using the DIT method.