Using lower river genetic stock identification (GSI) and a tributary-specific mark–recapture program to estimate genetic unit escapements of Pacific salmon within large river systems

There are significant logistical and financial challenges to estimating systemwide escapement and Conservation Unit (CU) escapements of Pacific salmon in large river systems using conventional approaches. To address these challenges, we developed an escapement estimation approach that combines lower river genetic stock identification (GSI) with an accurate mark–recapture escapement estimate for a single major spawning tributary (the “reference unit”). We develop estimators of systemwide escapement as well as escapements for CUs and smaller Genetic Units (GUs) within CUs. Based on the delta method, we provide explicit formulas for estimating variances of our escapement estimators. We identify and discuss a set of assumptions that must be met for valid application of our estimators, and apply our methods for estimation of Skeena River Coho Salmon ( Oncorhynchus kisutch) escapements from 2019 to 2024. Our approach allows assessment of trends in abundance of CUs through time as well as identification of trends in abundance of particular GUs that may be primarily responsible for trends in abundance of CUs. This information is critical for effective implementation of Canada’s Wild Salmon Policy.