Estimating occupancy and detection probability of juvenile bull trout using backpack electrofishing gear in a west-central Alberta watershed

摘要

Occupancy modeling is well suited to quantitative assessment of bull trout (Salvelinus confluentus) distribution at multiple scales. We used models to estimate occupancy of juvenile bull trout (= 150 mm fork length) in a west-central Alberta watershed. Based on a backpack electrofishing survey of 92 sites, we assessed the relative importance of stream habitat characteristics on detection probability (p) and potential for false absences to bias occupancy estimates. Median distance to first bull trout detection was 16 m (range 0-289 m). Models including ambient water conductivity as a covariate of detection probability were most supported with an 85 mu S.cm(-1) increase resulting in over a tenfold increase in detection. Conditional detection probability using backpack electrofishing gear approached 95% in the first 200 m of effort in streams with a conductivity around 200 mu S.cm(-1). The potential for false absences in our study area was relatively low. Modeled site ((theta) over cap = 0.53; SE = 0.13) and patch-scale ((psi) over cap = 0.47; SE = 0.12) occupancy closely corresponded to naive (i.e., assuming p = 1) estimates (0.47 and 0.43, respectively). Our results highlight the potential efficiencies of an occupancy modeling approach when assessing fish distribution, but careful consideration of model assumptions is necessary.