Shrinking Steelhead?: Decline in Older, Larger Steelhead from the Clearwater River

Clearwater River steelhead are older and larger than other populations within the Columbia River basin. These life history traits are valuable from stock viability and fishery perspectives; however, they are not as prevalent as in the past.  We examine age and growth data from the Dworshak hatchery stock (38 years including three years from the extirpated founding wild stock) and the extant Fish Creek wild stock (20 years). We test predictions about how these traits would be affected if selection was occurring during a) spawning and freshwater rearing, b) ocean residence, or c) migration from the ocean to spawning. Both stocks decreased in percent 3-ocean adult returns while proportion of 1-ocean adults increased.  In addition, age-specific size has decreased.  Average length of 2-ocean adults of the Dworshak stock decreased by 5 cm and 0.5kg.  The observed changes may be attributable to a variety of factors including ocean conditions and phenotypic selection.  Hatchery practices at Dworshak Hatchery affect that stock but Fish Creek represents a wild group with no hatchery influence.  Determining the mechanisms that contribute to the decline in age and size at age of Clearwater River steelhead will help inform management options in the future.