Ceratomyxa shasta: Longevity, Distribution, Timing, and Abundance of the Infective Stage In Central Oregon

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Abstract
Characteristics of the infective stage of Ceratomyxa shasta were studied in the Deschutes River of central Oregon from 1972 through 1981. The infective stage remained viable < 10 d in Deschutes River water maintained in aerated, river-temperature-controlled aquaria. Live-cage exposures of susceptible rainbow trout (Salmo gairdneri) indicated that infectious C. shasta emanate from the bottoms of two reservoirs. Epilimnions of both reservoirs did not contain the infective stage during thermally stratified periods. Infectious C. shasta originating in one reservoir passed down the Deschutes and Columbia rivers. During spring and summer, infectious C. shasta in the other reservoir did not reach the outlet, located 56 m above the bottom. In the Deschutes River, the infective period began before March 31 in 1978, between April 3 and 10 in 1979, and between April 16 and 23 in 1981 when river temperatures were between 6.9 and 8.6 °C. Consecutive exposures of trout to the same water in aquaria indicated that all infectious C. shasta were removed or inactivated during initial exposures. Progressively doubling numbers of test fish exposed in different aquaria showed that one C. shasta is capable of causing the death of one susceptible fingerling rainbow trout. Peak abundance of C. shasta in Deschutes River water during 1979 was estimated at 148 infectious units/m3 on June 4. Abundance decreased significantly between 1978 and 1981, presumably because of elimination of ceratomyxosis-caused deaths of hatchery rainbow trout and associated spore deposition in one reservoir. Both the formation of reservoirs and liberation of susceptible host fish can lead to an increased number of C. shasta, and result in serious C. shasta-induced losses in native salmonid populations.