The Effects of Myxobolus cerebralis on the Physiological Performance of Whirling Disease Resistant and Susceptible Strains of Rainbow Trout

Abstract

The development of rainbow trout Oncorhynchus mykiss strains that are resistant to whirling disease has shown promise as a management tool for populations in areas where Myxobolus cerebralis is present. However, the physiological effects of the disease on characteristics necessary for fish survival in natural river conditions have not been tested in many of these strains. Five rainbow trout strains were evaluated for their swimming ability and growth characteristics in relation to M. cerebralis exposure: the resistant German rainbow trout (GR) strain (Hofer strain), the susceptible Colorado River rainbow trout (CRR) strain, and three intermediate (hybrid) strains (F1 = GR x CRR; F2 = F1 x F1; B2 = backcross of F1 x CRR). Three broad response patterns among strain and exposure were evident in our study. First, exposure metrics, growth performance, and swimming ability differed among strains. Second, exposure to the parasite did not necessarily produce differences in growth or swimming ability. Exposure to M. cerebralis did not affect batch weight for any strain, and critical swimming velocity did not differ between exposed and unexposed families. Third, although exposure did not necessarily affect growth or swimming ability, individuals that exhibited clinical deformities did show reduced growth and swimming performance; fish with clinical deformities were significantly smaller and had lower critical swimming velocities than exposed fish without clinical deformities. Research and management have focused on GR x CRR hybrid strains; however, given the performance of the GR strain in our study, it should not be discounted as a potential broodstock. Additional field trials comparing the GR and F1 strains should be conducted before wholesale adoption of the GR strain to reestablish rainbow trout populations in Colorado.

Genetic basis of differences in myxospore count between whirling disease-resistant and -susceptible strains of rainbow trout

“…Average mortality was low, 2.6% in unexposed control fish and 6.6% in exposed fish (Fetherman et al 2011).…”

“…A full-sibling design was chosen to complete both the exposure experiment and to meet other experimental goals for testing physiological responses to parasite exposure (Fetherman et al 2011). A group of 80 full-sibling families was exposed to Myxobolus cerebralis and comprised 10 GR families, 10 CRR families, 20 F1 families, 20 F2 families, and 20 B2 families.…”

“…Mean individual weights (g) at exposure were 0.67 for GR, 0.49 for CRR, 0.71 for F1, 0.51 for F2, and 0.55 for B2 (Fetherman et al 2011). The GR, CRR, F2, and B2 did not differ in mean weight at exposure; however, F1 hybrids were significantly larger than the F2, B2, and CRR fish (see Fetherman et al 2011 for statistical details).Triactinomyxons (TAMs), the infectious waterborne stage of Myxobolus cerebralis, were produced using Mt. Whitney Tubifex tubifex (origin, Mount Whitney Fish Hatchery, Lone Pine, California, USA).…”

“…After exposure, fish were reared for approximately 6 mo to ensure the full development of myxospores. During this time, developing signs of disease and mortalities were recorded daily, and the effects of Myxobolus cerebralis exposure on growth and swimming performance were also evaluated (Fetherman et al 2011). …”

Survival and Reproduction of Myxobolus cerebralis-Resistant Rainbow Trout Introduced to the Colorado River and Increased Resistance of Age-0 Progeny

“…Despite the resistance seen in the GR strain, its survival and viability in the wild was uncertain due to the strain’s history of domestication [9]. Therefore, the GR strain was experimentally crossed with the Colorado River Rainbow (CRR) [9], [12], [13], a wild rainbow trout strain that had been widely stocked in Colorado and was used to establish many naturally reproducing wild rainbow trout fisheries prior to the introduction of M. cerebralis [14].…”