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.

Received February 17, 2016; accepted May 31, 2016

Acknowledgments

This work was sponsored in part by Colorado Parks and Wildlife and the Colorado Cooperative Fish and Wildlife Research Unit at Colorado State University, and funding was provided in part by the Federal Aid in Sport Fish Restoration program, Project F-394R. We thank J. Anderson, T. Davis, B. Neuschwanger, and G. Schisler for technical assistance and rearing of the Rainbow Trout strains and crosses used in this experiment. We also thank S. Brinkman for his help in rearing fish, providing technical assistance and advice, and providing the lab space and equipment for conducting the DO and formalin interaction experiment. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Colorado State University IACUC protocol 13-4000A.