Relocation of Western Pearlshell before and After Stream Restoration in Tincup Creek, Idaho

Abstract

Freshwater mussels can be negatively affected by heavy machinery during stream restoration projects, requiring mussels to be relocated from the project area to unaffected areas. We assessed recapture and survival of Western Pearlshell (Margaritifera falcata) relocated in Tincup Creek, Idaho before and after a stream restoration project. From 2018 to 2020, we searched 4,350 m of Tincup Creek before restoration and salvaged 1,213 Western Pearlshell. Mussels were measured, marked with shellfish tags, and relocated among 10 sites in previously restored reaches elsewhere in Tincup Creek. At the time of salvage, mussels ranged from 19 to 84 mm with 83% of the mussels ≥50 mm, and most mussels were found in run habitats (63%). We surveyed all sites for tagged mussels 1 to 3 yr after relocation. We recaptured tagged mussels at seven of the 10 sites, and the recapture rate was positively related to the number of relocated mussels and mussel size. Tag retention was high but varied among relocation years. Estimated survival after 3 yr was 69.9–87.4% at two sites, and detection probability was 60.3–62.9%. Estimated survival after 1 yr was 55.8–91.3% at four other sites. Survival was low at three sites, likely due to low numbers of relocated mussels or scarcity of suitable habitat, and survival decreased dramatically at one site (from 91.3% to 28.6%) in 2 consecutive years, likely due to beaver activity. Our results suggest that stream restoration created habitat suitable for Western Pearlshell, and relocation was a successful strategy for avoiding direct mortality associated with restoration activities.