Juvenile Trout Study - Whirling Disease |
Whirling disease is caused by the parasite Myxobolous cerebralis, which was accidentally introduced from Europe, where it has been known to exist since 1893. It was first discovered in the United States in 1958. Whirling disease may have been introduced to Idaho soon as 1966 as a result of stocking infected trout from across state lines from infected hatcheries in Nevada (Johnson 2003), but was no observed until 1987. The disease has been responsible for the virtual elimination of susceptible trout species in streams from Montana to Colorado. Currently, M. cerebralis is established in 21 river systems throughout Idaho, but overall, salmonid populations do not appear to be impacted. The impact of the parasite can highly vary from stream to stream, but severe infections my lead to recruitment collapses of age-0. Even in highly infected systems, sentinel fish exposures have shown the prevalence of infection can vary greatly over space and time. For example, in portions of the Colorado, Gunnison, Rio Grande and South Platte rivers, whirling disease is severe enough to eliminate almost the entire cohort of wild rainbow trout fry every year. In contrast, the Big Thomson River near Estes Park tested positive for M. cerebralis in 1994, yet recruitment of rainbow trout is excellent. Only young-of-the-year fish are severely affected by the disease, as the parasite attacks cartilaginous skeletal structures before ossification is complete. External signs of infection may include, but are not limited to black tails, spinal curvature, cranial depression, and erratic swimming. Hedrick (1999) reported that rainbow trout are by far the most susceptible to the disease, with cutthroat trout showing no clinical signs of the disease when exposed after three months post-hatch. Vincent (2002) also reported that rainbow trout were the most susceptible of the salmonids tested and classified them as having “very high susceptibility.” Brook trout were classified in the same category as rainbow trout, but Yellowstone cutthroat trout were ranked two categories lower in the “moderate susceptibility” class.
More information can be found at the Montana Whirling Disease Task Force web site. |
The Idaho Department of Fish and Game discovered whirling disease in the Teton Valley in 1997. Test sites included Fox and Teton Creeks. At the time, infection severity was reported as severe, and biologists hypothesized that whirling disease may be responsible for recent declines in trout populations in the valley. The whirling disease investigations conducted under the Juvenile
Trout Study are intended to further investigate the severity and distribution of whirling disease in the valley. Understanding the distribution and severity of whirling disease infection will add clues to answer questions about trout declines, as well as help guide future recovery efforts. Sentinel cage test fish are one of the most reliable methods for establishing the presence or absence of M. cerebralis (Hoffman 1990). Testing will be conducted in late June with up to 13 sentinel cages, depending on availability from Idaho Department of Fish and Game. Cages will be distributed among Teton Creek, Fox Creek, Elliot Creek, Trail Creek, and Fish Creek. Cages will be evenly spaced along streams in an effort to quantify the spatial variation in infectivity. Remaining cages will be used to replicate sites sampled in 2003 in order to compare yearly fluctuations in results.
