Fish, like humans, need oxygen to survive. Unlike humans, or even aquatic mammals like whales and dolphins, fish don’t pull oxygen from the air. Instead, they get their oxygen from the water they swim in.
While oxygen levels in the air can vary (most notably with altitude), the amount we breathe is almost entirely constant.
For fish, however, the amount of oxygen adsorbed in the water can go from highs to lows in a few minutes. As an angler, understanding oxygen levels, and how it affects fish behavior, can lead to more success on the water.
First of all, let’s talk about the gills. Everyone knows a fish has gills, but not everyone knows how they work. Designed to have a maximum amount of surface space, the gills have tiny filaments that take the oxygen from the water. For highly active fish like trout and bass, there are a lot of filaments, providing the fish with enough oxygen to hunt, swim, hide, and strike.
In general, when there is more oxygen dissolved in the water, fish will be much more active. Fish in less-oxygenated water will often be sluggish, conserving their energy for times when it is absolutely needed, like escaping predators. Lower oxygen levels will also lead to slower and less aggressive feeding, as fish are likely to strike only at prey that presents a good chance of being captured, while passing on prey that is quick or protected by structure.
Water and oxygen interact basically through contact. As water passes through the air, or vice-versa, the water absorbs a small amount of oxygen, creating a better environment for the fish. It’s pretty simple really: when water is moving, there is (in general) more oxygen. Therefore, turbid water below dams, areas near tributaries and wind-swept shorelines are likely to have more oxygen.
Some scientist and biologist speculate that oxygen level is the single most important variable to creating a habitable environment for fish. It makes sense; if there is little oxygen, fewer fish can survive. If you are fishing on a river, the most active spot will often be just below a dam, where water has been freshly oxygenated and species of all types are filling their gills with energy. Choppy waters are usually more oxygenated, at least near the surface, and rain can often inject a large amount of oxygen to a lake or river. (For deep-sea fishing, the oxygen contribution of rain is minimal at best.)
Like trees on land, aquatic plants will give off oxygen as well as cover and food, providing an excellent place for fish to congregate.
Always remember that oxygen matters to fish as much as it matters to you. With this knowledge, you can become a better angler, one breath at a time.
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