Comparison of Methods to Verify Upstream Passage by Trout at Remediated Culverts in Four Rocky Mountain Streams

Abstract

The removal or remediation of thousands of culverts at road-stream crossings to restore connectivity is a major conservation investment in aquatic systems in North America. Effectiveness monitoring is necessary to confirm that passage has been restored for the species of interest and to justify project costs. We compared the performance of (1) recapture of batch-marked fish by backpack electrofishing, (2) recapture of PIT-tagged fish by electrofishing, (3) detection of PIT-tagged fish by a mobile antenna, and (4) detection of PIT-tagged fish at stationary antennas for verifying upstream passage of native Westslope Cutthroat Trout Oncorhynchus clarkii lewisi (WCT) and nonnative Brook Trout Salvelinus fontinalis at remediated culverts in four Rocky Mountain streams. Generally, detection probability at stationary antennas was higher (range = 0.74-0.97) than capture by electrofishing (range = 0.24-0.77) or detection by the mobile antenna (range = 0.47-0.66). All four methods confirmed upstream passage by trout that were originally marked or tagged below the culvert, although overall recapture rates were low (≤20%). During summer and early fall, the continuously sampling stationary antennas detected more than twice as many PIT-tagged trout moving upstream through the culvert than either the mobile antenna or the electrofisher. Upstream movement by PIT-tagged trout was first detected by stationary antennas 1-10 d after tagging. For all methods, upstream passage was most frequently detected for fish that were marked or tagged in the 100-m reach adjacent to the culvert. The relative cost of the four mark-recapture methods to evaluate upstream passage of age-1 and older WCT was compared with the cost of "sibsplit," a genetic method based on pedigree analysis, which was used previously to evaluate passage of age-0 WCT in the study streams. Stationary antennas, the mobile antenna, and sib-split were comparatively expensive for a singleyear study because of PIT equipment and laboratory costs, respectively, and electrofishing was less than half the cost.