Using Multimethod Occupancy Estimation Models to Quantify Gear Differences in Detection Probabilities: Is Backpack Electrofishing Missing Occurrences for a Species of Concern?

The Bridle Shiner Notropis bifrenatus is a small, rare minnow species native to northeastern streams and lakes. It is declining over most of its range and currently is listed as a species of concern in Connecticut. Surveys conducted with seine nets in the 1960s found Bridle Shiners at 56 locations statewide. In contrast, surveys conducted in the 1990s using electrofishing detected Bridle Shiners at 8 locations. Different sampling techniques made it difficult to assess what portion of the observed decline might be a sampling artifact, confounding efforts to assess the actual conservation status. We sampled 18 habitat patches in three Connecticut watersheds in 2012 to determine if seining for Bridle Shiners yielded a higher detection probability than backpack electrofishing. A multimethod occupancy estimation modeling approach, using the program PRESENCE, quantified the probability of correctly detecting Bridle Shiners by gear and as detection covaried with habitat features. Backpack electrofishing detection probability was lower and approximately half that of seining. The abundance of Bridle Shiners in the sample patch was the most supported covariate to detection and particularly aided detection for electrofishing. High mean water velocity improved the detection probability of backpack electrofishing and reduced that of seining. It is possible that the 1990s sampling underestimated the number of populations of Bridle Shiners, and a repeat survey of all historic locations using a seine is recommended.