FAO.org

Meaningful minimum levels of 02 concentration at which fish can live are not easily determined. Endurance limits that have been determined in the laboratory or field by various experimental methods can be much lower or higher than the true thresholds of tolerance (incipient lethal levels) under natural conditions. Wide disagreement of "threshold" levels of 02 reported by different
investigators for the same species of fish doubtless are due largely to differences of experimental methods employed, some of which are obviously quite unreliable. Differences in resistance to O2 deficiency between different species or populations of fish and between individuals from the same population undoubtedly are great. Nevertheless, reports of fully developed freshwater fish being
killed within a day or two by reduction of O2 concentration to levels above 3. 0 mg/ l in water of otherwise favorable quality are unusual and should all be regarded with some suspicion. They cannot now be accepted as convincing evidence that reduced concentrations not below 3. 0 mg/l are intolerable for some fish under ordinary conditions in nature. Salmonids are among the most susceptible fishes, but some other kinds of fish, including certain sturgeons, have not proved clearly more resistant than salmonids in comparable tests, and some warrnwater forms may be much more susceptible at some early life-history
stages.

Low levels of 02 endured by fish for 24 hours even at moderately high temperatures are not necessarily tolerated thereafter (i.e. for longer periods) by most of the surviving individuals. It is not possible to specify a maximum exposure period within which death of fish ascribable to acute anoxia will almost always occur if it will occur at all. Pertinent information is very limited and contradictory. True thresholds of tolerance (incipient lethal levels) may or may not be demonstrable by experiments of 7-day duration at moderate temperatures. Young fish tend to be less resistant to reduction of O2 concentration than older and larger individuals, but the reported patterns of variation of resistance with age, especially during the first month of life, are extremely variable. Patterns of variation of the resistance of fish to OZ deficiency with water temperature also are highly variable, and no regular pattern of its seasonal variation independent of temperature has been conclusively demonstrated for any species. The lowest 02 levels endured by fish in comparable tests may increase regularly with any rise of temperature over a wide temperature range. They may also be constant over a wide range of temperatures, but probably always increase markedly at high temperatures not far below the limits of thermal tolerance of the fish. High concentrations of free CO. likely to be encountered under aerobic conditions in waters polluted with organic wastes have little or no effect on the resistance to 02 deficiency of fish that are accustomed to COZ concentrations not much lower. When exposure is sudden, even moderately elevated levels of free COZ together with reduced but normally tolerable levels of 0Z can be rapidly fatal to some fish, because the dissolved 02 requirement of the fish increases with increase of free CO2. However, fish become very rapidly adjusted to high COZ concentrations that they can tolerate, and this adjustment can be expected usually to occur before the fish are subjected to critically low levels of 02 in nature. The observed effects of free CO2 on the dissolved O2 requirements of fishes definitely are not ascribable to the decreases of pH that are normally associated with increases of free COZ but do not have the same effects. Increases of the resistance to further reduction of dissolved 02 of fish subjected for some time to nonlethal low levels have been convincingly demonstrated. The acclimation can be nearly complete in about ten days or sooner, but perhaps is much slower or does not occur at very low temperatures and under other unfavorable circumstances. After complete acclimation of fish to the lowest tolerable levels of 02, their tolerance thresholds can be about half the threshold levels evaluated after acclimation to air-saturation levels of OZ. Large differences intolerance of 02 deficiency between fish of the same species native or acclimatized to different geographic regions have been reported. They have been related to differences of 02 concentrations to which the fish are exposed in their natural habitats. It is not known, however, how permanent these differences of tolerance are and to what extent they are genetic.