Resistance and Tolerance of Young Speckled Trout (Salvelinus fontinalis) to Oxygen Lack, with Special Reference to Low Oxygen Acclimation

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Abstract
Samples of trout were exposed to flows of water containing constant low concentrations of oxygen (produced by equilibrating aerated water with nitrogen). At 9°–10 °C. minimum tolerance levels were defined by determining the lowest oxygen level permitting survival of 5%, 50% and 95% of the fish for a period of 5,000 minutes. Median resistance times for samples of fish exposed to lethal levels of oxygen were also determined. Within their first year, most trout reared in aerated water could withstand oxygen concentrations down to 1.9 mg.O2/1. The 50% tolerance level was 1.75 mg.O2/1. Most fish died at 1.5 mg.O2/1. Acclimation to low (but not lethal) oxygen levels occurred; for a drop of 1.0 mg.O2/1. in environmental oxygen concentration, the tolerance level decreased by approximately 0.09 mg.O2/1. and resistance times at given lethal levels increased up to five-fold. By acclimations trout could exist at 1.05 mg.O2/1. without direct lethal effects occurring. Between 20 and 33 hours were required for the fish to acclimate to a change of 1.0 mg. per liter in the oxygen concentration of the environment. Acclimation to low levels of oxygen was accompanied by an apparent increase in the ability of the animals to remove oxygen at low environmental oxygen concentrations, suggesting a physiological mechanism similar to that involved in acclimation of mammals to low oxygen. The tolerance levels of small fish were similar to those of large fish at similar acclimation levels, but small fish died more quickly than large fish when exposed to lethal concentrations.