ATTRIBUTES OF YELLOWSTONE CUTTHROAT TROUT REDDS IN A TRIBUTARY OF THESNAKE RIVER, IDAHO

We characterized spawning sites of Yellowstone cutthroat trout Oncorhy nchus clarki bouvieri, described the microhabitat of completed redds, and tested the influence of habitat conditions on the morphology of co mpleted redds in Pine Creek, Idaho. Cutthroat trout spawned in June as flows subsided after peak stream discharge. During spawning, minimum and maximum water temperatures ranged 4-9 degrees C and 16-20 degrees C, respectively. Trout spawned where water depths averaged 20 cm besid e redds and 22 cm upstream of redds. Water velocities averaged 42 cm/s beside redds and 46 cm/s upstream of redds. More than 80% of the redd s were constructed in water 10-30 cm deep with velocities of 25-60 cm/ s. Trout spawned in gravel with particles up to 100 mm in diameter. Mo st of the substrate was smaller than 32 mm; an average of 20% was less than 6.35 mm and 5% was less than 0.85 mm. The geometric mean particl e size of the spawning gravel averaged 16.6 mm and the fredle index av eraged 8. Microhabitat conditions differed within completed redds. Wat er depth decreased and velocity increased from the redd pit downstream to the leading edge of the tailspill and to the tailspill crest. The redd shape may facilitate movement of water through the egg pocket. Re dds averaged 1.58 m long by 0.60 m wide, and the pit covered an averag e of 46% of the redd area. Water depths and velocities in redds were c orrelated with water depths and velocities adjacent to redds. Redd dim ensions were typically not correlated with water depths, velocities, o r particle size distributions, suggesting that other factors such as t he size of spawning fish may influence redd dimensions.