Use of Size for Early Detection of Juvenile Hatchery Steelhead Destined to Be Precocious Males

Abstract

At two Washington State hatcheries in August 1999–2002, juvenile steelhead Oncorhynchus mykiss were size-graded and the largest fish (4.8–9.2% of the population) were coded-wire-tagged and mixed with small fish to determine whether the large fish were those that became precocious males. The results showed that the large fish in August at Merwin Hatchery tended to become precocious, although the mean lengths of the precocious and nonprecocious fish were similar at their time of release in April. At Abernathy Fish Technology Center (FTC), the large fish in August did not disproportionately become precocious, and the nonprecocious fish were significantly larger than the precocious fish at release. We suggest that precocious fish at Abernathy FTC may have committed to precocity before August whereas fish at Merwin Hatchery committed to precocity in August. Because only 0.82% of the large fish became precocious, removing the 5–10% largest fish in August at the Merwin Hatchery would be marginally successful in reducing the number of precocious males.