USE OF THE STRATIFIED PETERSEN ESTIMATOR IN FISHERIES-MANAGEMENT - ESTIMATING THE NUMBER OF PINK-SALMON (ONCORHYNCHUS-GORBUSCHA) SPAWNERS IN THE FRASER-RIVER

The simple-Petersen estimator is a well-known mark-recapture method to estimate animal abundance. Two key assumptions are equal catchability in both samples and complete mixing of tagged and untagged animals. I f these are violated, severe bias can occur. The stratified-Petersen e stimator can be used to account for some of the heterogeneity in catch ability or mixing. In this paper, we first review recent developments in the stratified-Petersen experiment for fisheries audiences and demo nstrate some of the practical problems that can occur that have not be en discussed in the theoretical literature. Second, we present a case study to estimate the gross escapement of Fraser River pink salmon (On corhynchus gorbuscha) in 1991. The motivation for this study is a disc repancy of over 5 million fish between the estimates as derived by the Pacific Salmon Commission (PSC) (7.5 million fish based on a hydroaco ustic method) and the Department of Fisheries and Oceans (DFO), Canada (13.0 million fish based on a mark-recapture method). One hypothesis put forward was that the discrepancy may be due to the use of a pooled -Petersen estimator when there is differential migration over time. Th e stratified-Petersen model suggests that little of this discrepancy c an be explained by differential migration.