Effect of Size at Release on Tendency of Trinity River Hatchery Steelhead (Oncorhynchus mykiss) to Return As Half-Pounders

Steelhead (Oncorhynchus mykiss) in northern California and southern Oregon exhibit the half-pounder life history, where immature individuals return to freshwater after 3-5 months in the ocean. The following spring they return to the ocean and complete their maturation. Past studies documented the decline of this life history form in the Trinity River and suggested that the tendency of steelhead to return as half pounders is negatively correlated with size at release of steelhead from the Trinity River Hatchery. The hatchery released small steelhead smolts prior to 1993, larger smolts from 1993 until 2008, and small smolts in 2011 and 2012. Using scale analysis, I estimated the half-pounder frequency among wild and hatchery steelhead from the Trinity River 2011 and 2012 smolt years. Hatchery steelhead from 2011 and 2012 had higher half-pounder tendencies than those from 1993 until 2008. However, wild steelhead from 2011 and 2012 also had higher half-pounder tendencies than the 1993-2008 time period. In conclusion, this study provide additional support for the hypothesis that increased size at release of hatchery steelhead is related to decline in the half-pounder life history, but also suggest that another (probably environmental) factor influences the half-pounder tendency of steelhead in the Trinity River.