Fish assemblage response to habitat restoration in Elk Springs Creek, Montana: implications for arctic grayling (Thymallus arcticus) restoration

Abstract

The abundance and distribution of Arctic Grayling Thymallus arcticus in Montana have declined substantially during the past century as a result of habitat degradation and loss. Biologists tasked with conserving Arctic Grayling populations in the Centennial Valley of southwestern Montana implemented two habitat restoration projects to reclaim historical Arctic Grayling migration corridors and spawning habitats in Elk Springs Creek. I used before-after and before-after control-impact (BACI) study designs to evaluate the effects of these habitat restoration projects on physical habitat, water quality, and Arctic Grayling in Elk Springs and Picnic creeks. Because Arctic Grayling were rare in Elk Springs and Picnic creeks, I also examined the effects of restoration on two additional species (Brook Trout Salvelinus fontinalis and White Suckers Catostomus commersonii) with habitat requirements and life history characteristics similar to those of Arctic Grayling. I used electrofishing to monitor the abundance, biomass, and size distribution of each species before the restoration in 2016, and after the restoration during 2017 and 2018. A PIT-tag detection network monitored the seasonal movements of Arctic Grayling, Brook Trout, and White Suckers from spring 2016 through autumn 2018. In situ data loggers measured summer stream temperatures and dissolved oxygen concentration in expected fish migration corridors both before and after restoration. The abundances and biomasses of Arctic Grayling and White Suckers were similar before and after restoration. However, Brook Trout abundance and biomass increased significantly in the restored (impacted) reaches relative to the unrestored (control) reaches two years after habitat restoration. The size-class distributions of Arctic Grayling and Brook Trout broadened after restoration. Movements of Arctic Grayling, Brook Trout, and White Suckers among unique habitat segments in Elk Springs and Picnic creeks increased after restoration, but pre-restoration movement data was sparse and limited inference. Following channel restoration, summer stream temperatures decreased, and dissolved oxygen concentration increased and equilibrated. Physical habitat improved (i.e., fine sediments decreased, and depth, percentage of pools, and gravels increased) in restored historical Arctic Grayling spawning areas. I thereby showed that channel reconnection and spawning habitat restoration can substantially improve water quality and physical habitat. However, the restoration measures implemented in Elk Springs Creek affected my target species disproportionately.