Bull trout concealment experimental data

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.