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De Rito, J. N., A. V. Zale, and B. B. Shepard. 2010. Temporal reproductive separation of fluvial Yellowstone cutthroat trout and hybrids in the Yellowstone River. North American Journal of Fisheries Management 30:866-886.


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.


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August 2010

Unit Authors

Montana Cooperative Fishery Research Unit Cooperators

  1. Montana Department of Fish, Wildlife and Parks
  2. Montana State University
  3. U.S. Geological Survey