Spawning Characteristics of Redband Trout in a Headwater Stream in Montana

Abstract

I investigated the spawning characteristics of redband trout Oncorhynchus mykiss gairdneri (a rainbow trout subspecies) during the spring of 1998 in Basin Creek, a third-order headwater stream located in the Kootenai River drainage in northwestern Montana. I examined the timing of spawning as related to discharge and water temperature and analyzed microhabitat selection of 30 completed redds in a low-gradient (0.5–1.5%) reach. Redband trout spawned as flow declined after peak runoff and as mean daily water temperature exceeded 6.0°C and maximum daily temperature exceeded 7.0°C. Redband trout began spawning on 6 June (mean daily discharge = 2.1 m³/s), 10 d after the peak discharge (8.7 m³/s) occurred. The last redd was completed on 24 June, when discharge was 1.5 m³/s. The mean total redd length was 53 cm (SD = 14; range = 31–91 cm), and the mean total area was 51 cm² (SD = 8; range = 46–76 cm²). Eighty percent of the redds were located in pool tailouts, 13% in runs, and 7% in riffles. Spawning redband trout selected redd sites based on substrate size and water depth but not water velocity. Fish selected substrate sizes of 2–6 mm, water depths of 20–30 cm, and water velocities of 40–70 cm/s. My results suggest that redband trout in a low-gradient, third-order mountain stream found suitable spawning habitat in pool tailouts that contained abundant gravels.