Distribution of Myxobolus cerebralis in salmonid fishes in Montana

Over an approximately 2-year period, 20,974 fish (trout and other salmonid species) from 230 separate waters(creeks, rivers, lakes, reservoirs, ponds, hatcheries, and irrigation ditches) within 21 of the 22 major drainages in Montana were examined for Myxobolus cerebralis. Nine of the major river dr ainages have waters containing infected fish: Beaverhead, Big Hole, Blackfo ot, Clark Fork above the Bitterroot River, Flathead below the south fork of the Flathead River, Jefferson, Madison, Missouri above the Marias River, a nd Yellowstone above the Bighorn River. The Beaverhead, Clark Fork above th e Bitterroot River, Jefferson, Madison, and Missouri above the Marias River have the greatest number of waters containing fish infected with M. cerebr alis. Comparisons of infection levels (number of pooled samples that contai n fish infected with M cerebralis) between species among these drainages sh ow significantly lower levels of infection in brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss in the Missouri above the Marias River dr ainage and significantly higher levels of infection of rainbow trout in the Jefferson. Comparisons of differences in infection levels between drainage s among species show that, in the Beaverhead, Clark Fork above the Bitterro ot River, and Madison, infection levels in brown trout are significantly hi gher than in rainbow trout. This is partially attributed to losses of juven ile rainbow trout because of M cerebralis infection, leading to biased samp les. Histopathologic studies showed lesions were consistently less severe i n brown trout than other species and occurred in a different location (gill arches versus ventral calvarium). In six of the nine affected drainages (B eaverhead, Blackfoot, Clark Fork above the Bitterroot River, Flathead below the South Fork, Jefferson, and Madison), infected fish were found at or ne ar the time that intensive sampling was initiated in the spring of 1995. In the three remaining affected drainages (Missouri above the Marias River, Y ellowstone above the Bighorn River, and the Big Hole), infected fish were n ot identified until at least 15 months after the initiation of widespread t esting. This indicates that in the first six drainages listed above, the in fection was well established prior to 1995 but spread into the last three d rainages in the ensuing months. Methods of transmission and the sources of infection are unknown, although the absence of infected fish in state, priv ate, and federal hatcheries in Montana indicates hatchery fish from these s ources are not likely to be responsible.