Oxygen is essential to all aquatic animal life. Without it respiration is impossible. Because the distribution of oxygen throughout a body of water can vary widely, it is important to find the specific areas which hold optimal amounts to support fish life. Oxygen concentration is a prime factor in locating where specific fish species are found.

It’s self-evident that air includes an abundance of oxygen; however, water holds a much smaller amount. For example, one liter of air contains about 210 cubic centimeters of oxygen while one liter of water has only about 9 cubic centimeters. In air the lack of oxygen is rarely a problem, but in water its effect is more apparent. Water harbors a tenuous margin of safety because its oxygen content is small and varies widely. Insufficient oxygen supplies will result in animal death.

Two physical factors affecting water’s oxygen content are altitude (atmospheric pressure) and temperature. Oxygen solubility in water increases with decreases in both temperature and altitude. Conversely, oxygen solubility decreases with rises in temperature and altitude.

Atmospheric pressure is inversely proportional to altitude. That is, a rise in altitude lowers the atmospheric pressure, and a decrease in altitude raises the atmospheric pressure. On the other hand, atmospheric pressure is directly proportional to oxygen water solubility. That is, a rise in atmospheric pressure increases the water’s oxygen content while a decrease in atmospheric pressure lowers the water’s oxygen content.

Oxygen diffusion occurs at the water’s surface, and its mixing throughout happens during a lake’s spring and fall turnover times. The oxygen diffusion process is very slow. Both wind and wave actions mix oxygen at the surface, while plant photosynthesis generates oxygen when plants are present. In deep water, where light cannot penetrate, oxygen cannot be produced by plants. Hence, a lake’s hypolimnion only receives its limited oxygen by spring and fall mixing turnovers. This is why a lake’s greatest depths can become so oxygen poor. The stratification of water layers seals the hypolimnion from the surface air supply and this seal is only interrupted at spring and fall.

An increase in water temperature lowers the amount of oxygen it can contain. During hot conditions a lake’s shallows may become too warm to hold sufficient oxygen, and the animal life forms must migrate to cooler regions which have more oxygen. Inlet streams and underwater springs can supply the needed oxygenated


water. The deeper shaded areas just above the thermocline can be cool enough to hold sufficient oxygen. Fish concentrate in these limited areas when such conditions prevail.

Since a lake depends upon wave action, photosynthesis, and currents to mix and produce its oxygen, areas of adequate and inadequate oxygenated water exist simultaneously. This results in areas where fish can and cannot live. It is a precarious balancing act which includes oxygen content, temperature, light intensity, protective cover, and available food supply that dictates just where fish can be found.

The altitude’s influence on an alpine lake can be dramatic. For example, in my youth I spent considerable time fishing alpine lakes above ten thousand feet in altitude. During the dog days of summer my success dramatically fell. I was puzzled because these alpine lakes contained colder water than their valley counterparts. The alpine lakes’ depths were void of fish, and my success was limited to the lakes’ shallows. Earlier in the season the angling was good in the alpine lakes’ depths. Now in midsummer the fishing was poor. The valley lakes fished best in their depths while the alpine lakes fished best in their shallows. I neglected to take into account the difference in altitude and atmospheric pressure between the alpine and valley lakes. The high altitude lakes’ oxygen was less because there was less oxygen available when the spring and fall turnovers occurred. This limited oxygen supply was quickly depleted in the alpine lakes’ depths, making it so fish couldn’t live there. The lower elevation lakes took in higher oxygen content during the spring and fall turnover times. This confined the alpine lakes’ fish to the shallows while the valley lakes’ fish were in the lakes’ depths. I once thought that all big fish lived in the depths and only small ones were in the shallows. This notion is untrue.

The current in a river mixes oxygen much better than limited currents and waves in a lake. Also, this thorough mixing causes little variation in both, oxygen and temperature differences. During high temperature times, the whitewater river sections mix higher oxygen content. In times of hot spells fish may migrate to the rapids for survival.

Look for them in the pocket areas downstream from rapids. Also areas close to adjacent inlet streams and underwater springs can harbor more favorable conditions. For example, in warm thermal rivers such as Yellowstone Park’s Firehole River, fish migrate to the mouths of cooler tributary streams during warm seasons.

In conclusion, the oxygen content of water is a prime factor in determining the location of fish. An adequate oxygen supply is essential to sustain fish life.

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