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The amount of dissolved oxygen in water can have a significant effect on fish activity and well being and consequently from a fishing point of view its useful to understand why and when there may be more or less oxygen in water.

Oxygen is absorbed into water in only two basic ways. Through diffusion and as a result of photosynthesis.

Photosynthesis is a chemical process in plants and phytoplankton that depends on the interaction of proteins called chlorophylis that consume carbon dioxide and sunlight to produce "energy" in the form of carbon and oxygen as a waste product.

Whilst the science is complex it's worth noting that photosynthesis is vital for all aerobic life on Earth and is responsible for the production of more energy than human civilisation and is responsible for maintaining the availability of oxygen in our atmosphere.

Most of the time the process of photosynthesis runs pretty smoothly with plants and phytoplankton producing oxygen during the daylight hours as a by-product of photosynthesis and consuming some of that oxygen at night and when overcast.

Occasionally however the process gets out of kilter and more oxygen is consumed that in produced resulting on divesting results on fish stocks. The most common occurrence that creates that occurrence is and algae bloom. These generally happen when there are dramatic increases in nutrients in water, for example from farm run off, which cause the phytoplankton to multiply (bloom). Initially there is an increase in the production of oxygen but then as resources are used up the bloom cant be sustained and dies. The dead phytoplankton float to the surface and generally present as blue green or red alga. This floating bloom of course reduces light penetration increasing phytoplankton requirements and bacterial decomposition of the dead phytoplankton also uses oxygen.

The second way that oxygen enters water is through diffusion and that process has many variables including:

1. The turbulence of the water. This includes turbulence created by wave and wind action and also turbulence of rivers as they move past obstructions creating runs, riffles, water falls etc.
2. Atmospheric pressure has a substantial influence and at low elevations (higher atmospheric pressure) more oxygen is available to be diffused into water than at high elevations (low atmospheric pressures).
3. Water temperature also effects diffusion of oxygen into water and as water warms its ability to diffuse and hold oxygen diminishes.
4. Salinity is also a factor and all other things being equal the ability of water to diffuse and hold oxygen demishing as salinity increases.
5. Availability of oxygen in the atmosphere and that of course ultimately depends on photosynthesis.

Fish apparently need least 5 ppm (5 mg/l) of dissolved oxygen in water to survive and below 5 ppm will ultimately lead to fish kills. Trout and other fish species of course operate better with higher levels of dissolved oxygen and around 8 ppm seems to be around their optimum.

Depletion of dissolved oxygen can and does have a dramatic effect on the well being and of course survival of fish and should have a bearing in the selection of water that you fish.

Some useful tips:

1. On hot days favour oxygen rich water such as turbulent areas resulting from broken water on rivers and lakes.
2. Avoid areas where algae blooms are obvious.
3. Areas with weed will generally be more oxygen rich than barren areas and consequently will carry a more complex population of things that trout eat and consequently trout.
4. On hot days avoid large areas of shallow water even when there are terrestrials on the water in favour of similar water that is adjacent to deeper (cooler and more oxygenated water) close to drop off's.

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