Why do fish need oxygen?

Or more precisely, why have we been adding oxygen to our canals, rivers and reservoirs? It’s another result from this year’s extraordinary dry summer. The weather has seen water levels drop dramatically along with the amount of oxygen in that water.

Aeration at Selby Canal Aeration at Selby Canal

‘Love is like oxygen’ sang The Sweet in their late 1970s hit single. It’s unlikely they had fish on their minds when they went on to add, ‘you get too much you get too high, not enough and you’re gonna die’.

Where does our 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 from the plant 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 botanists tell us are not technically plants, although they do look like them. Surprisingly, plants and other photosynthesizers 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 (a classic schoolchild comment) but for a complicated chemical reaction which happens include every living cell. This reaction, the bane of some biology student’s life, 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. It’s why neither humans nor fish can rely on anaerobic respiration for long. That’s why we need to get fish back in the water as soon as we are able after unhooking them. The toughest fish in this regard is probably the crucian carp, which produces alcohol as a bi-product of anaerobic respiration.

Pike, surprisingly sensitive fish

Pike, courtesy of Jack PerksAt the other end of the coarse fish scale come the pike. In our experience it’s the species most susceptible to even a temporary drop in dissolved oxygen levels in the water. Salmon and trout are also very susceptible to low oxygen level 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 via the gills. Although a small amount oxygen may also pass into a fish through the skin. The oxygen molecules dissolve in the water, put simply hiding between the water molecules. The air we breathe contains around 20% oxygen and nearly 80% nitrogen.

How much oxygen is there in water?

In comparison to air, 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 10C rise in water temperature, a real conundrum as there is less oxygen available in warm water just at the time when fish need more of it. That’s why hot dry summers are the stuff of nightmares for us fisheries managers as we look forward hopefully to the upcoming wet cold autumn months.

Measure the amount of 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 meterThese are not cheap pieces of kit. On top of that, they can be temperamental and need regular servicing to stay reliable when you most need them.

What do we do if there is a dissolved oxygen problem

If fish are gasping at the surface (as compared to basking) we need to act quickly. For coarse fish, getting the levels above 20% saturation is critical. In most of cases, strategically located pumps or aerators should do the trick. Quite often, it’s the circulation of the surface water that pumps create that 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 breaks down quickly to produce harmless water plus the vital oxygen. In catastrophic pollution incident, even this won’t do the trick.

Aeration at Upper Foulridge reservoir Aeration 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 bloomIf the organisms begin to die off rapidly you get what is known as an algal crash. This acts as a food source for bacteria and so you get an increase in the numbers of bacteria in the water. These decomposing bacteria don’t produce their own oxygen, rather they use it up, sometime rapidly depleting the available supplies in the water and leading to a huge decrease in oxygen levels. These algal crashes are often the cause of fish kills on many fisheries.

How you can help

As the charity who look after and brings to life 2,000 miles of waterways, one of our key roles here in the national fisheries and angling team is to look after the wellbeing of our fish. 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 then here are the numbers to call:

Who to call if you see fish in distress

Last date edited: 16 August 2018