Identification of Hatchery and Naturally Spawning Stocks of Columbia Basin Spring Chinook Salmon by Scale Pattern Analyses

Abstract

Scale pattern analysis (SPA) was used to differentiate hatchery and natural-origin stocks of spring-run chinook salmon Oncorhynchus tshawytscha of age 1.2 (= age 4; spent one winter as juveniles in fresh water and two winters in the ocean; one winter of egg incubation is implied) from the Snake, Wenatchee, and Deschutes subbasins of the Columbia River basin. Linear discriminant analyses indicated that hatchery and natural-origin stocks within each subbasin could be identified with a relatively high degree of accuracy. High classification accuracies were also obtained by comparing pooled hatchery stocks from the three Columbia River subbasins with pooled natural-origin stocks from those same subbasins. For a composite mixed-stock analysis, samples of unknown origin were obtained from Bonneville Dam, a site on the lower Columbia River where a mixed population of spring chinook salmon stocks is found. This analysis, which was done with a classification model based on pooled hatchery and natural-origin stocks, estimated that 71% (90% confidence intervals of 61–81%) of age-1.2 spring chinook salmon sampled at Bonneville Dam were of hatchery origin. The classification model used in the mixed-stock analysis was derived from a limited set of representative samples of known stocks. Estimating hatchery and natural-stock composition of mixed-stock populations by using SPA and a limited set of representative known-stock samples is a potentially valuable technique for management of Columbia basin spring chinook salmon. This technique may also prove applicable to management of other mixed-stock Pacific salmon populations.