Development of Steelhead Trout (Salmo gairdneri) Otoliths and Their Use for Age Analysis and for Separating Summer from Winter Races and Wild from Hatchery Stocks

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Abstract
In steelhead trout (Salmo gairdneri) the first pair of otoliths (sagittae) appeared in X-ray photographs of embryos 14 days before hatching and were the first calcified structures visible. Subsequent growth characteristics of otoliths were used in identifying as freshwater and ocean annuli and spawning checks, and indicated seven age-classes and 26 life history patterns among 434 specimens. Diameters of otolith nuclei were smaller (P ≤ 0.01) in summer steelhead (mean = 0.348 mm) than in winter steelhead (mean = 0.436 mm), and diagonally measured otolith growth representing the first year of life was larger (P ≤ 0.01) in hatchery-reared (1.291 mm) than in wild steelhead (0.868 mm for 2/fish and 0.769 mm for 3/fish). Absence of changes in density in the freshwater growth of otoliths from hatchery-reared steelhead and presence of such changes in the otoliths from wild steelhead served as a subjective means of accurately separating the two stocks.

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