Survival and Dispersal of Hatchery Triploid Rainbow Trout in an Idaho River

Abstract

Hatchery rainbow trout Oncorhynchus mykiss of catchable size (hereafter, “catchables”) are commonly released into streams to improve angler catch and harvest rates, but returns are often less than 50% and the fate of unharvested catchables is largely unknown, especially for sterilized hatchery trout. The survival and dispersal of triploid catchables were investigated using snorkel and telemetry techniques to quantify the persistence and dispersal of catchables stocked three times during the summer of 2006 (June, July, and August) into an Idaho river in which a minimum length regulation precluded harvest. Counts of catchables stocked with T-bar tags (n = 900) and catchables emitting active radio transmitter codes (n = 54) steadily declined throughout the three observation periods. Dispersal of T-bar-tagged catchables 30 d poststocking was generally downstream of the stocking point (median dispersal = 100 m). The median values of the maximum known downstream and upstream dispersal distances for catchables with radio transmitters were 5.0 and 1.2 km from the stocking point, respectively. Only four transmitter-implanted catchables were still alive on November 1, when the study ended. On average, 85% of catchables with radio transmitters were presumed dead at 30 d poststocking (average persistence = 14.3 d). A similar rate of decline was observed for T-bar-tagged catchables. Managers wishing to maximize return-to-creel rates of sterile catchables in lotic systems might do so by limiting stocking events to within 3 weeks of expected needs and limiting the stocking locations to within 1 km of areas frequented by anglers.