Critical Dissolved Oxygen Tolerances of Whirling Disease‐Resistant Rainbow Trout

Abstract

A low concentration of dissolved oxygen (DO) is commonly the limiting factor in fish culture systems. Hypoxia tolerance in Rainbow Trout Oncorhynchus mykiss can be affected by both history of domestication and growth rate. As such, selecting strains for specific characteristics such as growth rate or disease resistance could potentially affect DO tolerance, making culture difficult. Here we used two experiments to examine the differences in tolerance to lower DO concentrations among four Rainbow Trout strains and crosses selected for resistance to whirling disease Myxobolus cerebralis. The first experiment examined differences in critical DO concentrations of fry (≥73 mm total length [TL]) when exposed to rapid decreases in DO at 30, 60, 90, and 120 d postswim‐up. In addition, since formalin is a common chemical used in aquaculture to treat for external parasites, the effect of exposure to formalin on tolerance to low DO was evaluated. The second experiment evaluated critical DO concentrations among four strains and crosses exposed to a prolonged decrease in DO at age 7 months (averaging 178 mm TL). Formalin exposure had an effect on low‐DO tolerance, with DO concentrations that resulted in a loss of equilibrium decreasing with an increase in formalin concentration. In addition, low‐DO tolerances were diminished with an increase in fish size, with larger fish losing equilibrium at higher DO concentrations. Differences in DO concentrations resulting in loss of equilibrium and mortality were evident among the strains and crosses in the second experiment. This experiment demonstrated that DO concentrations must be below 2.0 mg/L before loss of equilibrium is observed. However, if fish are soon returned to well‐oxygenated water, losses can be minimized. Additionally, other hatchery practices that compromise hypoxia tolerance may increase mortality more quickly following low‐DO exposure, and care should be taken to correct low‐DO issues shortly after loss of equilibrium is observed.

Concentration, Life Stage, Feeding, Density, Flow, and Strain Effects on Formalin Sensitivity in Rainbow Trout (Oncorhynchus mykiss)

“…The GR × HL 50:50 may have exhibited lower mortality as a result of heterosis. Similar effects have been observed with regard to increased tolerance of the GR × HL 50:50 to low dissolved-oxygen concentrations over both the GR and HL strains [24]. Additionally, the GR × HL 50:50, and other 50:50 crosses between the GR and wild-strain fish, have shown increased resistance to M. cerebralis over the wild strain [27,[37][38][39], as well as advantages in swimming performance and post-stocking survival over the GR strain [40,41].…”

“…Current management efforts are focused on stocking rainbow trout that are genetically resistant to the parasite [23]. However, M. cerebralis-resistant rainbow trout are a relatively new addition to the state aquaculture system [24]. Therefore, four M. cerebralis-resistant rainbow trout strains were evaluated for their formalin sensitivity in these experiments.…”

“…A first-generation cross of the GR and HL strains (hereafter the GR × HL 50:50 strain) and a second-generation backcross of the GR × HL 50:50 and the GR (hereafter the GR × HL 75:25 strain) were also included in these experiments. These strains, previously evaluated for their critical dissolved oxygen tolerances [24], are maintained as brood stock in Colorado's state hatchery system, and produced and stocked for recreational purposes statewide.…”

Are resistances to acute hyperthermia or hypoxia stress similar and consistent between early and late ages in rainbow trout using isogenic lines?

“…In Atlantic salmon (Salmo salar), significant differences in acute hypoxia resistance were found between families (Anttila et al, 2013). Even though this is not formal evidence, significant between-strains differences were reported for acute hypoxia resistance in rainbow trout (Scott et al, 2014;Fetherman et al, 2016), suggesting genetic basis. Acute hyperthermia resistance being heritable in rainbow trout, and most likely resistance to acute hypoxia, selective breeding could positively increase the average resistance of a population over generations for both traits.…”

Effect of Water Management Technology Used in Trout Culture on Water Quality in Fish Ponds

“…Fish tolerance to a low concentration of oxygen is short, species-specific, and fish sizedependent. Trout have high demands for oxygen, that is, at least 5-8 mg O 2 mg•dm −3 water [35,36]. Oxygen concentration decrease below 3 mg•dm −3 may lead to fish immunity suppression and this to their increased susceptibility to infections and parasite invasions.…”