Attributes of Yellowstone Cutthroat Trout Redds in a Tributary of the Snake River, Idaho

Abstract

We characterized spawning sites of Yellowstone cutthroat trout Oncorhynchus clarki bouvieri, described the microhabitat of completed redds, and tested the influence of habitat conditions on the morphology of completed 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°C and 16–20°C, respectively. Trout spawned where water depths averaged 20 cm beside 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 redds 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. Most 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 particle size of the spawning gravel averaged 16.6 mm and the fredle index averaged 8. Microhabitat conditions differed within completed redds. Water 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. Redds averaged 1.58 m long by 0.60 m wide, and the pit covered an average of 46% of the redd area. Water depths and velocities in redds were correlated with water depths and velocities adjacent to redds. Redd dimensions were typically not correlated with water depths, velocities, or particle size distributions, suggesting that other factors such as the size of spawning fish may influence redd dimensions.