Bull trout (Salvelinus confluentus) are challenging to detect as a result of the species cryptic behavior and coloration, relatively low densities in complex habitats, and affinity for cold, high clarity, low conductivity waters. Bull trout are also closely associated with the stream bed, frequently conceal in substrate, and this concealment behavior is poorly understood. Consequently, population assessments are problematic and biologists and managers often lack quantitative information to accurately describe bull trout distributions, estimate abundance, and assess status and trends; particularly for stream-dwelling populations. During controlled laboratory trials, we recorded concealment, resting, and swimming behavior of juvenile wild bull trout in response to: (1) constant and fluctuating water temperature, (2) presence or absence of light, and (3) substrate size. Light level had the strongest influence on wild fish concealment and more fish concealed as light levels increased from darkness to daylight. Wild fish were 14.5 times less likely to conceal in constant darkness and 4.1 times more likely to conceal in 12 h light x 12 h darkness compared to constant light. Wild fish were 6.2 times less likely to conceal in small (26-51 mm) substrate compared to larger (52-102 mm) substrate. As water temperature increased, fewer wild fish concealed. Knowledge of wild bull trout concealment will improve field sampling protocols and increase detection efficiencies. These data also enhance knowledge of bull trout niche requirements which informs conservation and restoration efforts.

This experiment was carried out in 1991 at the University of Idaho by Theodore C. Bjornn and Russel F. Thurow. Age 1 and older wild bull trout (Salvelinus confluentus) were collected from Trestle Creek, Idaho during the August and brought to the laboratory where they were placed into 16 aquaria (2-6 fish per aquarium) and allowed to acclimate for two weeks. All fish remained in their aquaria and were tested serially in multiple trials. Lighting for the trials was provided by florescent fixtures regulated on a daily cycle. Water temperature for the trials was regulated with supplemental chillers to provide both constant and diurnal cycles.

The response variable for all trials was the number of trout observed in one of three activities within the aquaria: concealed in the substrate; resting on the substrate; or swimming (defined as maintaining position in the water column above the substrate, henceforth swimming). During trials, at approximately hourly intervals in the morning (0600-0700) and in the afternoon (1500-1700), an observer carefully inspected each aquarium and recorded the number of fish engaged in each activity. Between 9 and 12 observations were recorded for each aquarium within a 24 h period. Identical observations were made in light and darkness by inspecting each aquarium, except in darkness the observer used a flashlight to momentarily locate fish. Observations were made by individually inspecting each of 16 aquaria at the assigned time.

We conducted a series of ten, one to three-day trials between September 3 and September 23. The treatments consisted of aquaria substrate: 26-51 or 52-102 mm; light regime, 12h light/dark with a dawn, 24h light, 24h dark with light defined as 2,239 lux; and thermal regime, constant 7 ^oC, constant 15^oC, fluctuating 4-12 ^oC, and fluctuating 7-15 ^oC.