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Changing oxygen levels

Changing oxygen levels
In densely vegetated waters there is a daily cycle of oxygen concentration that can affect both fish behaviour and feeding.
Plants are known as primary producers, and they grow using a process called photosynthesis whereby energy from the sun is used to generate food.
Oxygen is a by-product of photosynthesis, so on bright, sunny days when plants are photosynthesising actively, oxygen concentrations rise steadily throughout the day, and are often close to saturation by mid-afternoon.
At night it is a different story. Photosynthesis stops, and so does the release of oxygen into the water. Plants, along with fish and millions of micro-organisms, use up oxygen during respiration, meaning that the oxygen saturation in the water falls through the night, reaching a minimum just after dawn.
Fish are used to this daily fluctuation in oxygen concentration, and sometimes change their behaviour as a result. In affected waters, fish are often found close to the surface overnight and at dawn, but they don’t always feed strongly. If you have ever had one of those frustrating nights when there seemed to be lots of activity but no takes, only for the bites to start as the activity seemed to be subsiding, fluctuating oxygen concentrations were probably the cause.