First documentation of Least Chub in the Snake River drainage in Idaho.

The Least Chub (Iotichthys phlegethontis) is a small minnow endemic to the Bonneville Basin in Utah, where the current distribution of extant populations is limited to a few small spring complexes. We report on the first documentation of Least Chub in the upper Snake River drainage in Idaho. In June 2021, October 2021, and June 2022, we surveyed for small-bodied fishes in off-channel habitat of lower Henrys Fork in Cartier Slough Wildlife Management Area west of Rexburg, Idaho. Small-bodied fishes were sampled at 17 sites in off-channel habitat; 5 baited minnow traps (galvanized steel) were set for approximately 24 h at each site. We captured Least Chub in side-channel habitats during all sampling occasions; we captured 214 Least Chub in June 2021, 34 Least Chub in October 2021, and 41 Least Chub in June 2022. Least Chub were captured at 13 of the 17 sites: 6 sites in June 2021, 6 sites in October 2021, and 12 sites in June 2022. Least Chub ranged in size from 35 mm to 56 mm total length. In both June sampling occasions, female and male Least Chub were mature; females were expressing eggs and males had red/orange breeding coloration laterally. The habitat at sites where Least Chub were captured consisted of muddy or sandy substrate, slow-moving or stagnant water, submerged and emergent macrophytes, and filamentous algae. Additional surveys should be completed to determine the extent of the distribution of Least Chub in Henrys Fork and Snake River. Genetic analyses should also be completed to determine the relationship of this population of Least Chub to populations of Least Chub in the Bonneville Basin. [ABSTRACT FROM AUTHOR]