Accuracy of Using Scales to Age Mixed-Stock Chinook Salmon of Hatchery Origin

Abstract

Despite a long history of using scales to age Pacific salmon, there have been few attempts to validate scale-derived ages. This is particularly true for Chinook salmon Oncorhynchus tshawytscha, a species exhibiting a wide range of life histories across stocks. This has led to continuing questions regarding the accuracy of scale-based age determination for Chinook salmon. This study assessed the accuracy of Chinook salmon scale age data produced by multiple readers from multiple agencies, who aged hatchery fish of known age from mixed-stock, nonterminal fisheries conducted along the Pacific coast of Canada from 1991 to 2003. The test sample consisted of scales from 434 fish from both stream- and ocean-type stocks marked with coded wire tags (i.e., fish origin and total age were known). Sample stocks originated from Oregon, Washington, and British Columbia. Five readers from three federal or state Pacific Northwest fisheries agencies participated in the study. The readers possessed various levels of experience, which was classified as (1) deep or shallow (depth) depending on the number of years the reader was involved in aging Chinook salmon scales and (2) broad or narrow (breadth) depending on the variety of stocks the reader had previously encountered. Accuracy ranged from 84% to 94%, although readers with both deep and broad experience consistently achieved accuracies greater than 90%, while those with a narrower breadth of experience tended to show age bias. Overall, the results suggest that aging Chinook salmon scales from ocean-caught hatchery fish can be accurate and that readers' previous exposure to stocks comprising a wide range of life history types may be at least as important as the number of years of experience in achieving a high level of aging accuracy.