Salmonid whirling disease, first detected in the U.S. in 1956, has spread widely, causing wild salmonid population declines of up to 90% in some areas. Colorado River Cutthroat Trout (CRCT), threatened by habitat loss, invasive species, and disease now inhabit only 13% of their historic Wyoming range. Despite restoration efforts in LaBarge Creek, Wyoming, CRCT population numbers remain critically low.
This study investigated the potential role of whirling disease in the population dynamics by assessing its spatial extent and infection load in CRCT in LaBarge Creek. Electrofishing efforts from 2023 yielded 87 CRCT from 13 different sites in the drainage that were to be analyzed with the pepsin-trypsin digestion (PTD) assay. 24 fish were from 5 sites along the mainstem of LaBarge Creek, and the remaining 63 fish were from 8 different tributaries. A sentinel caging study was conducted in 2024 that exposed susceptible young fish to the stream for 21 days at 6 sites along the mainstem, and 19 sites spread across 9 tributaries.
PTD analysis of wild collected CRCT showed that overall infection prevalence in the drainage was 46%. Fish were found to be infected at all mainstem sites, and at sites in 5 of the 8 tributaries sampled. Preliminary results from the caging study indicated obvious development of clinical signs of whirling disease, including black tail, skeletal deformities, and whirling behavior, in fish at 5 of the 6 mainstem sites and in fish from 3 of the 9 tributaries. Histology, qPCR, and PTD will confirm the infection status of all caged fish.
Taken together, our preliminary results suggest that whirling disease is widespread throughout the drainage and may be affecting the establishment of a sustainable population of CRCT in the LaBarge Creek restoration area.