Scale Pattern Analysis of Selected Scale Characteristics and the First Annulus for Distinguishing Wild and Hatchery Steelhead in the Hood River, Oregon

Abstract

Resource managers need an accurate, reliable means of distinguishing hatchery-origin steelhead Oncorhynchus mykiss from wild stocks. We evaluated the effectiveness of scale pattern analysis at distinguishing between wild and hatchery (progeny of wild broodstock) winter steelhead collected at Powerdale Dam in the Hood River basin. The characteristics within the freshwater zone of hatchery (n = 126) and wild (n = 428) scales were quantified to determine whether the improved growth experienced in the hatchery environment would facilitate discrimination between hatchery and wild steelhead. Scale measurements that depended on the location of the first freshwater annulus were the most effective at distinguishing between hatchery and wild steelhead. Linear discriminant function analysis of key scale characteristics correctly classified a maximum of 73.9% of a blind-sampled population (n = 92) of known-stock winter steelhead. On average, estimates of the proportion of wild fish in known-origin test groups differed by 11.7% from the actual proportion of wild fish and had poor precision (mean coefficient of variation (100 · SD/mean), 259.9%). Our results suggest that scale pattern analysis could be an effective tool, but until ponding checks and true annuli can be accurately differentiated managers should not depend on scale patterns to distinguish between wild and hatchery steelhead similar to those from the Hood River.