DOI: 10.5061/dryad.mw6m905tm
title: Bull trout concealment experimental data

description: 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.
description: 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 <sup>o</sup>C, constant
 15<sup>o</sup>C, fluctuating 4-12 <sup>o</sup>C,
 and fluctuating 7-15 <sup>o</sup>C.