175 North American Journal of Aquaculture 61:175–183, 1999  Copyright by the American Fisheries Society 1999 Evaluation of Chronic Gas Supersaturation on Growth, Morbidity, and Mortality of Fingerling Rainbow Trout Infected with Myxobolus cerebralis GEORGE J. SCHISLER* AND ERIC P. BERGERSEN Colorado Cooperative Fish and Wildlife Research Unit, 1 Colorado State University, 201 Wagar Building, Fort Collins, Colorado 80523, USA PETER G. WALKER Colorado Division of Wildlife, Post Office Box 128, Brush, Colorado 80723, USA Abstract.—The effect of environmental stressors on trout infected with Myxobolus cerebralis, the causative agent of whirling disease, is of great interest because the parasite has spread to most areas of the United States that support wild trout populations. Gas supersaturation is an environ- mental stressor found in at least two rivers in Colorado where M. cerebralis exists, and year-class losses of rainbow trout Oncorhynchus mykiss have occurred. Both of these stress factors may also occur in fish culture operations. A controlled laboratory experiment was conducted to test the hypothesis that chronic low levels of gas supersaturation affect growth, morbidity, and mortality of fingerling rainbow trout infected with M. cerebralis. Fingerling rainbow trout, exposed and unexposed to M. cerebralis, were held in gas saturations of 110, 107.5, 105, and 100–102.5% for 22 weeks. Significant effects on growth, morbidity, and survival were found due to M. cerebralis infection. Gas saturation levels tested did not significantly affect growth, morbidity, or survival of fish in this experiment. The presence of Myxobolus cerebralis has been implicated in rainbow trout year-class losses in the Colorado, Cache la Poudre, Rio Grande, Gunnison and South Platte rivers in Colorado (Walker and Nehring 1995; Nehring 1998). Severe population level effects due to M. cerebralis have been rarely reported in other areas where the parasite is known to exist. One possible explanation for this phe- nomenon is that environmental stressors are acting as exacerbating factors, causing fish to succumb to the effects of M. cerebralis infection. Average gas supersaturation levels of 110% have been re- corded in the Colorado River (Schisler and Ber- gersen, in press) and the Gunnison River (Nehring 1998), with occasional peaks of supersaturation of up to 115%. Young-of-the-year rainbow trout On- corhynchus mykiss and brown trout Salmo trutta sampled from the upper Colorado River by Walker and Nehring (1995) displayed signs of disease nor- mally observed in juvenile salmonids exposed to chronic low levels of supersaturation, including exophthalmia, spinal flexures, and discoloration. * Corresponding author: georges@neota.cnr.colostate.edu 1 Cooperators are the U.S. Geological Survey, the Colorado Division of Wildlife, and Colorado State Uni- versity. Received June 22, 1998; accepted December 9, 1998 Gas bubbles were also observed in the gill fila- ments and in the kidneys of some fish. Gas satu- ration even a few percentage points above 100% has been known to be detrimental to newly hatched fry (Dennison and Marchyshyn 1973). Other signs of gas bubble trauma (GBT) reported between 100% and 110% supersaturation include overin- flation of the swim bladder in small fish (Shrimp- ton et al. 1989), ocular lesions, immunosuppres- sion, reduced growth, bubbles in the intestinal tract (NMFS 1995), loss of swimming ability (Schiewe 1974), and even death (Bouck 1976). The reported threshhold between chronic and acute GBT occurs somewhere between 108% and 110% (Jensen et al. 1986). However, the U.S. Environmental Pro- tection Agency suggested 110% as a safe upper limit for aquatic life (USEPA 1976). This criterion is supported by numerous studies indicating that acute gas saturations below 110% do not signifi- cantly affect fish survival (Dawley and Ebel 1975; Nebeker et al. 198; Jensen 1988). The majority of studies have investigated the effect of acute high levels of supersaturation, usually greater than 115%. The effects of chronic low levels of super- saturation on salmonids have been largely ignored. Fish culture operations are often faced with gas supersaturation problems, and M. cerebralis is now present in many fish culture facilities. The com-