An evaluation of Yellowstone cutthroat trout production in three tributaries of Yellowstone River, Montana

The range and abundance of Yellowstone cutthroat trout (Oncorhynchus clarki bouveri) has decreased due to competition and hybridization with introduced species, susceptibility to harvest, and the loss of spawning sites, especially due to irrigation withdrawals.Fry production, the effects of dewatering on fry production, and instream flows suitable for fry production were evaluated in three tributaries of the Yellowstone River during 1988, a drought year, and 1989, a year of normal precipitation.Cedar Creek produced the greatest number of cutthroat trout fry (2,000 and 14,000 in 1988 and 1989, respectively), although 13-20% of the potential fry production was lost due to dewatering of redds.A minimum instream flow of 0.035 m^3/s would have prevented this loss.Tom Miner Creek was the second most productive spawning tributary, producing an estimated 700 and 3,400 fry in 1988 and 1989, respectively.Fifteen to 50% of production was lost due to dewatering of redds caused by natural fluctuations in streamflow, not by irrigation withdrawals.High sediment loads also may have limited fry production in this stream.Big Creek produced the fewest fry because irrigation withdrawals caused severe dewatering in the lower 1600 km.A discharge of 0.320 m^3/s would have protected redds and should restore cutthroat trout fry production in Big Creek.