Fine-Scale Characteristics of Fluvial Bull Trout Redds and Adjacent Sites in Rapid River, Idaho, 1993–2007

Abstract

From 1993 to 2007, we used single pass, September surveys to locate and measure fluvial bull trout (Salvelinus confluentus) redds in Rapid River, Idaho. Here we describe substrate sizes, redd dimensions, and water depths, velocities, and temperatures within and adjacent to 337 redds. Most (79%) spawning sites had fewer than 20% surface fines (< 2 mm) and mean, annual water depths and velocities ranged from 14.2–23.0 cm and 11.6–30.5 cm s-1, respectively. Mean, annual completed redd total lengths and surface areas averaged from 1.03–1.47 m and 0.37–1.07 m-2, respectively. Pea gravel (2 to < 8 mm) and gravel (8–64 mm) were dominant substrates (> 60%) in redds. Bull trout altered channel water depths and velocities during redd construction; pits averaged 5 cm deeper, leading tailspill edges 1.2 cm shallower, and tailspill crests 6.2 cm shallower than adjacent, undisturbed sites. Conversely, pit velocities averaged 2.1 cm s-1 slower, tailspill edge velocities 2.3 cm s-1 faster, and tailspill crest velocities 10.1 cm s-1 faster than adjacent sites. Mean, annual pit water temperatures ranged from 4.5 to 7.7 °C. Water depths and water velocities over undisturbed sites adjacent to bull trout redds were significantly correlated with water depths and water velocities inside completed redds. Improving our understanding of fluvial bull trout redds will increase the accuracy of redd counts, especially in streams with sympatric, fall spawning salmonids. Data describing fine-scale characteristics of redds and adjacent sites will assist efforts to conserve and restore critical bull trout spawning habitats.