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Why do fish need oxygen?

During particularly dry summers, we sometimes need to add oxygen to our canals, rivers and reservoirs because there isn't enough in the water to keep fish alive. But why do they need it, and which fish need it most?

Aerator adding oxygen to the water at Selby Canal

To understand why fish need oxygen, we first need to understand why we, as humans, need it and also where it comes from.

Where does oxygen come from?

Most of us know that trees and other plants make oxygen in a process called photosynthesis and this oxygen passes into the air through the plant's leaves. For this to happen, you need sunlight, so no oxygen is produced at night. The actual oxygen molecules we breathe in through our lungs come from two atoms of oxygen joining together when two molecules of water are broken down.

Blue green bacteria

It's not just plants that can carry out photosynthesis. Some bacteria called blue green bacteria carry out this process too, as do seaweed and algae, which are not technically plants but do look like them. Surprisingly, plants and other photosynthesisers don't release all the oxygen they make. They need some for their own use.

Why do most living things need oxygen?

Plants and almost all living things need oxygen, not to breathe but for a complicated chemical reaction which happens inside every living cell. This reaction is known as aerobic respiration. Using glucose or fatty acids as the energy source, the energy needed to carry out everything that keeps humans, fish and all other living things alive is released continuously in this process.

Energy without oxygen

There is another sort of respiration that can take place without oxygen. It's called anaerobic respiration and does not produce much energy, between 5% and 8% of the quantity produced from aerobic respiration. So neither humans nor fish can rely on anaerobic respiration for long.

That's why anglers need to get fish back in the water (where they have a supply of oxygen) as soon as they're able after unhooking them. The toughest fish, in terms of surviving without oxygen, is probably the crucian carp, which produces alcohol as a by-product of anaerobic respiration.

Pike, surprisingly sensitive fish

Pike, courtesy of Jack Perks

Salmon and trout are also very susceptible to low oxygen levels and on top of that they have not evolved to stand high temperatures either. Climate change and spells of above average temperatures are a major threat to these species.

Where do fish get oxygen from?

Most fish get their oxygen from water when it enters their bodies through their gills. Although a small amount of oxygen may also be taken in through their skin. The oxygen molecules dissolve in the water, hiding between the water molecules.

How much oxygen is there in water?

The air we breathe contains around 20% oxygen and nearly 80% nitrogen. In comparison, water has tiny amounts of available oxygen dissolved in it. The maximum amount of dissolved oxygen falls as the temperature of the water increases. The oxygen requirements of fish double for every 10 degree Celsius rise in the water temperature.

This is a real problem, as there is less oxygen available in warm water just at the time when fish need it most. That's why hot dry summers are the stuff of nightmares for fisheries managers.

Measuring oxygen in the water

We use an instrument called a dissolved oxygen meter to measure the concentration of oxygen dissolved in the water.

Dissolved oxygen meter

What we do if there is a dissolved oxygen problem

If fish are gasping at the surface (which is different from basking) we need to act quickly. For coarse fish, getting the oxygen levels above 20% saturation is critical. In most cases, strategically located pumps or aerators do the trick. Quite often, it's the circulation of the surface water that pumps create which helps increase oxygen levels in the water.

In really challenging situations, a compound called hydrogen peroxide can be used. It's quite a dangerous chemical to use but it breaks down quickly to produce harmless water plus vital oxygen. However, in a catastrophic pollution incident, even this won't do the trick.

Aeration taking place at Upper Foulridge reservoir

The rise and fall of plankton

The main oxygen producers in many fisheries, especially boated canals, are microscopic algae and blue green bacteria. High levels of nutrient, especially phosphate and nitrate, often lead to what are known as algal blooms.

Blue green bacterial bloom

How you can help

We look after and bring to life around 2,000 miles of waterways, so one of our key roles is to look after the wellbeing of our fish, and not just to service the healthy aspirations of tens of thousands of anglers. Fish play a vital part in the wider ecology of the waterways, as food for iconic species such as kingfishers and herons.

If you spot fish in distress or see pollution that could affect them, here are the emergency numbers to call.

Who to call if you see fish in distress

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Last Edited: 22 December 2020

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