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Effects of Multiple Stressors on Morbidity and Mortality of Fingerling Rainbow Trout Infected with Myxobolus Cerebralis

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Effects of Multiple Stressors on Morbidity and Mortality of Fingerling Rainbow Trout Infected with Myxobolus Cerebralis Transactions of the American Fisheries Society 129:859±865, 2000 q Copyright by the American Fisheries Society 2000 Effects of Multiple Stressors on Morbidity and Mortality of Fingerling Rainbow Trout Infected with Myxobolus cerebralis GEORGE J. SCHISLER* AND ERIC P. B ERGERSEN Colorado Cooperative Fish and Wildlife Research Unit,1 201 Wagar Building, Colorado State University, Fort Collins, Colorado 80523, USA PETER G. WALKER Colorado Division of Wildlife, Post Of®ce Box 128, Brush, Colorado 80723, USA Abstract.ÐMyxobolus cerebralis, the causative agent lation level effects occur when M. cerebralis is of salmonid whirling disease, has been implicated in present. Three stressors may be acting on wild ®sh year-class losses of rainbow trout Oncorhynchus mykiss populations in the upper Colorado River in addi- in several rivers in Colorado. The hypothesis that other factors, such as elevated water temperature, bacterial tion to M. cerebralis: elevated water temperature, pathogens, and gas supersaturation, are contributing to bacterial pathogens, and gas supersaturation. these year-class losses was tested in a laboratory setting. Elevated water temperature has been shown to Fingerling rainbow trout were exposed to all combina- increase the virulence and maturation rate of M. tions of these stressors for 6 months. Mortality and mor- cerebralis. Halliday (1976) states that the optimum bidity were evaluated for each of the test groups using temperature range for growth of the parasite is analysis of variance (ANOVA). Mortality was signi®- cantly affected by exposure to M. cerebralis (P 5 between 158C and 178C. At higher temperatures 0.0002) and elevated water temperature (P 5 0.0002). (20±238C) the pathogen reportedly begins to lose Morbidity was signi®cantly affected by exposure to M. its virulence (Schaperclaus 1931). The triactino- cerebralis (P 5 0.0001). A signi®cant linear increase (P myxon stage is very short lived at high tempera- 5 0.0020) in mortality was observed with M. cerebralis tures, surviving only 2±3 d at temperatures ex- infection and addition of all combinations of one, two, and three stress factors. ceeding 198C (Markiw 1992b). Average water tem- peratures in the Colorado River during the summer months range from 138Cto178C and appear to be Myxobolus cerebralis, the causative agent of sal- ideal for M. cerebralis growth. We hypothesized monid whirling disease, has been implicated in that these temperatures may result in higher mor- partial or complete year-class losses of rainbow tality among ®ngerling trout infected with the par- trout Oncorhynchus mykiss in the upper Colorado, asite and could help explain the loss of year-classes Gunnison, Rio Grande, South Platte, Dolores, Fry- in some Colorado rivers. ing Pan, Roaring Fork, and Cache la Poudre rivers Gas supersaturation is caused by excessive dis- in Colorado (Nehring et al. 1998). Until recently, solved gas pressure in water and can result in del- the pathogen has not been considered a threat to eterious effects to young ®sh. Average gas super- wild salmonid populations. Myxobolus cerebralis saturations of up to 110% occur in some reaches exists in at least 21 states (Bergersen and Anderson of the Colorado (Schisler and Bergersen 1999) and 1997), but population level effects in these states Gunnison rivers of Colorado (R. B. Nehring and have been rarely reported. The inconsistent re- K. G. Thompson, Colorado Division of Wildlife, sponse of wild rainbow trout populations to M. unpublished, 1996). Saturations of these levels cerebralis exposure raises the possibility that other have been reported to cause a variety of physio- factors may be contributing to year-class losses of logical problems for young ®sh ranging from re- rainbow trout in environments positive for M. cer- duced growth to death (Dennison and Marchyshyn ebralis. 1973; Schiewe 1974; Bouck 1976; Shrimpton et The presence of an additional stressor or stress- al. 1989; U.S. National Marine Fisheries Service ors is one explanation for situations where popu- 1995). Other studies have indicated that gas sat- urations of up to 110% do not have a serious effect * Corresponding author: [email protected] on survival of ®sh (Dawley and Ebel 1975; Ne- 1 Cooperators are the Colorado State University, Col- beker et al. 1980; Jensen 1988). orado Division of Wildlife, and U.S. Geological Survey. Flavobacterium psychrophilum is a widespread Received December 7, 1998; accepted September 20, 1999 ®sh pathogen that has been known to cause heavy 859 860 SCHISLER ET AL. mortality among salmonid fry (Lorenzen and Kar- per Colorado River. The remaining 24 aquaria as 1992; Lorenzen 1994). Several ®sh examined were maintained at 12.58C. during routine ®sh health evaluations on the upper Gas saturations were elevated to 110% in half Colorado River exhibited signs of disease that may of the aquaria for each treatment group. The re- be related to this pathogen (Walker and Nehring maining 24 aquaria were maintained at 100% 1995). We hypothesized that F. psychrophilum may (nominal) saturation. Saturations used in the ex- exacerbate the effects of M. cerebralis and that its periment were chosen to test the upper bound of presence may also contribute to the loss of year- gas saturations found in Colorado rivers. Nominal classes of rainbow trout in the Colorado River. saturations were achieved by rigorously aerating The focus of this study was to test all four of water in 300-L headboxes before delivery to the the individual factors simultaneously to identify aquaria. Supersaturation was achieved by forcing the relative contributions of each to mortality and atmospheric gasses together with water in a VMG to determine if the addition of more than one stress Industries In-line Oxygenator at a rate of 18 L air factor increases mortality of ®ngerling rainbow and water/min. This produced saturations of ap- trout exposed to M. cerebralis. This information proximately 118%. The water was then aerated in would help us determine if any or all of these 300-L headboxes to reduce supersaturation to factors are contributing to the loss of rainbow trout 110% before it was gravity-fed into the aquaria. year-classes in some Colorado rivers. Water temperatures and gas saturations were close- ly monitored with Common Sensing TBO satu- Methods rometers throughout the entire experiment to en- sure consistent treatment and control levels. Certi®ed disease-free rainbow trout eggs were Half of the aquaria in each treatment group were incubated in a 38-L aquarium at 12.58C with a ¯ow exposed to F. psychrophilum at a dosage of 9.9 3 of 3 L well water/min (pH 7.2, CaCO3 450 mg/ 107 colony-forming units per aquarium 12 weeks L). Treatment and control ®sh were placed in sep- after the beginning of the experiment. The culture arate aquaria 2 days before swim-up as 18-d-old used was isolated from the Colorado State Fish sac fry. All sac fry in the treatment group were Hatchery, Bellvue, and is thought to have been the exposed to an average of 485 triactinomyxons, the cause of a particularly serious epizootic occurring infective stage of M. cerebralis, for 3 h. This ex- at that facility earlier in 1997. To prevent possible posure level was chosen by using data from year- infection in treatment groups designated as F. ling rainbow trout disease testing in the Colorado psychrophilum-negative, Terramycin-343 (75.6% River. Of surviving yearling rainbow trout sam- oxytetracycline) was added to the food of these ®sh pled, 83% were infected, and myxospore loadings at 1-month intervals at a rate of 2.5 g active in- averaged 71,250 per ®sh (Walker and Nehring gredient/45 kg ®sh daily for 10 d. Fish were fed 1995). Markiw (1992a) found that infectivity rates daily using guidelines in Piper et al. (1982). and spore loadings of this magnitude could be pro- Dead ®sh were removed daily from each aquar- duced by exposing ®sh to anywhere from 100 to ium for the duration of the experiment. At the end 1,000 infective units. of the experiment, signs of disease were recorded A battery of 48 aquaria were set up to allow for for all ®sh remaining in each aquarium. Defor- a four-factor experimental design conducted in mities and clinical signs of disease including triplicate. Each factor occurred at two levels, with blacktail, spinal deformities, cranial deformities, each level occurring with each level of the other deformed mandibles, deformed opercules, and ex- three factors, for a complete 24-factorial treatment ophthalmia were recorded. Four-factor analysis- design. Flow rate to each aquarium was set at 0.5 of-variance (ANOVA) tests were conducted to de- L well water/min. Twenty ®sh exposed to M. cer- termine if differences in mortality and morbidity ebralis were placed in each of 24 aquaria. Fish existed due to the treatment effects used in the were distributed ®ve at a time to ensure their ran- experiment. All two-, three-, and four-way inter- dom allocation to treatment groups. Twenty un- actions were tested in the ANOVA. Arcsine square exposed ®sh were placed in each of the remaining root transformations were used in the analyses to 24 aquaria in a similar manner. stabilize variances. However, results of the Water temperatures were adjusted to 178Cin12 ANOVA with and without transformations were of the M. cerebralis-positive aquaria and in 12 of very similar, so results are reported from the the M. cerebralis-negative aquaria to test the effect ANOVA without transformations to simplify in- of summer water temperatures observed in the up- terpretation. Linear regression was used to test if NOTES 861 TABLE 1.ÐMortality and morbidity of ®ngerling rain- mortality was higher (P 5 0.0002) among ®sh ex- bow trout exposed to gas supersaturation of 110.0% (S), posed to M.
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