Low Adult Return of Juvenile Steelhead Treated with 17α-Methyltestosterone to Produce Sterility

Abstract

Juvenile hatchery summer steelhead Oncorhynchus mykiss were treated with the hormone 17α‐methyltestosterone by methods developed for rainbow trout O. mykiss in an attempt to obtain sterile returning adults. Our objective was to determine if sterile steelhead would return to a target stream at frequencies high enough to provide recreational fisheries while providing fishery managers with a tool for reducing steelhead interactions with wild fish. From three brood years of treated releases, only one sterile adult steelhead returned to the collection hatchery on the South Santiam River, Oregon. Gonads were absent in this fish at the end of spawning in February. Other returning adults from treatment groups were 80% male and 20% female. Adults from control groups were 49% male and 51% female. Males from treatment groups developed secondary sexual characteristics similar to controls but contained deformed gonads at the end of spawning. Gonads of treated females appeared normal, but only one ripened by the end of spawning. Although sperm from treated males was viable, based on crosses with eggs from control females, our inability to hand‐strip milt from 81% of the treated males suggest that occluded sperm ducts would prevent many from spawning naturally. The mean return frequency of treatment groups was 0.5%, which was 24% of the mean return frequency of control groups. Treatment groups may have had higher mortality after the juveniles were released, or sterile fish may have survived but not returned to the hatchery. Treating juvenile steelhead with 17α‐methyltestosterone by the methods we used was not effective in producing sterile returning adults, although sterile individuals may have remained in the ocean.