Cooperative Fish and Wildlife Research Units Program: Montana Fishery
Education, Research and Technical Assistance for Managing Our Natural Resources

Yellowstone cutthroat trout Oncorhynchus clarkii bouvierii are genomically extinct throughout much of their historic range because of displacement by and introgression with introduced rainbow trout O. mykiss. However, fluvial Yellowstone cutthroat trout still retain their genetic integrity while coexisting with rainbow trout in the Yellowstone River. We assessed whether spatial or temporal reproductive isolation, or both, occurs between these taxa. Time and place of spawning was determined by radiotelemetry. We implanted 164 trout (98 cutthroat trout, 37 rainbow trout, and 29 cutthroat trout3rainbow trout hybrids) with radio tags before the 2001, 2002, and 2003 spawning seasons in four sections of a 140-km segment of the main-stem Yellowstone River. Of the 164 radio-tagged fish, 73 (44 Yellowstone cutthroat trout, 15 rainbow trout, and 14 hybrids) were assumed to have spawned; 55 (75.3%) used 16 tributaries, 17 (23.3%) used 7 river side channels, and 1 (1.4%) used the main channel of the Yellowstone River for spawning. The majority of fish that spawned (62%) used five spawning areas. Spawning area and spawning reach overlap index values were high among all taxa. In contrast, the mean migration and spawning dates of rainbow trout and hybrids were 5–9 weeks earlier than those of cutthroat trout. Rainbow trout and hybrids began migrating and spawning in April and May when Yellowstone River discharges were lower and water temperatures were colder than during cutthroat trout migration and spawning in June and July. The spawning period overlap index values (rainbow trout and hybrids versus cutthroat trout) were typically less than half the spatial overlap index values. Therefore, the difference in time of spawning is probably the predominant mechanism maintaining reproductive isolation among fluvial trout. Management actions focused on protecting and enhancing later-spawning cutthroat trout in tributaries may enhance their temporal reproductive separation from earlier-spawning rainbow trout and hybrids.