Aeration & Oxygen Levels in Fisheries

We’ve all done it.

You’re looking forward to a good day’s fishing on a lovely summer’s day and have prepared well. Then, at the end of the session, you pack up in the irritating knowledge that the float didn’t twitch, the quiver tip didn’t quiver or the floating crust sat untouched for so long it broke up and sank.

At this point, an angler’s instinct is to blame the bait, the tactics or perhaps the fact that the sunshine meant the fish were simply ‘off the feed’. The truth is that on many of these hot, dry and unsuccessful days, the reason could be much more sinister.

Not only are they off their food, the fish could also be suffering badly in the conditions, perhaps even dying. It might have been more sensible and responsible not to have fished at all because, even if you had caught in such circumstances, the chances of releasing a fish unharmed would be drastically lower than normal – especially after a lengthy battle.

The reason is oxygen – or lack of it. When it’s hot and dry, fish may be having a hard time of it because, even if a venue looks immaculate in the summer sun, the warmer water is naturally carrying less oxygen. The growth of algae, silt disturbance and low water levels can all contribute and at the height of a problem, oxygen levels can drop dangerously low. This on its own can provide a very stressful environment but add in the physical exertion of a post hook-up fight and the fish could quite easily be left gasping for oxygen that simply isn’t there.

For fishery managers, artificial aeration of afflicted waters is the obvious option and you might think it’s simply a case of deploying an aerator or pump to improve a situation, but as with many things in the natural world, it’s not quite that easy.

Before even considering what aeration equipment to use, a fishery or club should always be in possession of its own Dissolved Oxygen Meter to keep track of what’s happening. Monitoring can also save money by reducing the necessary running times of aeration and even the cost of a re-stock because of fish deaths caused by oxygen debt.

Environment Agency Fisheries Specialist Kye Jerrom says: “You can spend anything from £50 up to £2,000 for a meter and you really do get what you pay for. One basic piece of advice though is for fisheries to try and avoid the cheaper-end pond and aquarium kits. They’re perfectly fine for domestic ponds but there are better options for commercial or club fisheries.  I’d aim for digital units, which are easy to calibrate, are resilient and ideally measure in percentage oxygen saturation. This is because measuring in percentage saturation makes it easier to interpret and compare results. You can even buy versions with state-of-the-art telemetry and then monitor the results on your phone.

“We suggest that readings are taken twice a day between April and September and ideally at dawn and dusk – when oxygen levels tend to be at their highest and lowest. This will create a pattern and help you understand what’s going on – and help you spot when things are about to go wrong. You should keep a simple chart so you can spot fluctuations over time as well as oxygen reductions/crashes. And do remember that oxygen crashes can happen very quickly.”

In simple terms, oxygen levels between 80% and 100% are considered ‘normal’ and levels of 40% and below are conditions that will affect the stress levels of your fish – this is the time to think about emergency kit or calling the Environment Agency for advice or assistance. If it gets below 10% a fish mortality is almost certain.


So, with dissolved oxygen readings to hand, what equipment should you bring in? It’s important to remember that success for a venue ‘down the road’ with one specific style of aeration doesn’t automatically mean it’s right for you.

If your water is shallow and silty, for example, you don’t want a system that will disturb the sediment because silt can feed algae and the algae will then consume more oxygen during its growth. In short, you could make things even worse. Ask advice from your local Environment Agency Fisheries Team or from a fishery contractor.

It’s also important to think carefully about where in the fishery to deploy aeration, how to deploy it properly and which unit is right for the venue. Also, make sure everyone (who needs to) has access to wherever it is stored and knows how to use it.

What are the choices? There are systems that agitate the surface, others that push oxygen into deeper levels and even chemical based solutions. There are plenty of options to consider (see below) and even one (see video right) to make an aerator for a small fishery yourself.

Surface Aeration

A paddle wheel is an example of a surface option, but as with all methods, there are pros and cons to consider.


It provides an instant hit

It can provide a survivable area in the water layers

It reduces silt disturbance

It creates flow and encourages feeding


It has a limited effective area and produces a 10-20% increase

It is expensive to run

It requires power

Sub-Surface Aeration

As the name suggests, this system pulls oxygen down and into the deeper layers of the water. A better idea you might think but again, there are pros and cons.


Instant hit

Diffuses air into waterbody

Creates flow and encourages feeding


Can disturb silts

Expensive to run

Requires power

Water Pumps and Plunging Aeration

Perhaps the most easily accessible. For plunging aeration, think water fountain.


Easily accessible

Sometimes better than nothing

Easily deployed


Very limited effectiveness

Can disturb silts

Chemical Aeration

Chemical aeration is a process whereby Hydrogen Peroxide is introduced into the water.


Very effective

Quick acting


Not easily accessible

Not easily deployed – generally via contractors

Health and safety considerations


Emergency use only

As with most activity on a fishery, pro-active management and a state of readiness will always be better than scrabbling around in a crisis and, especially in the current economic climate, it’s easy to make the argument that investment is better made on general equipment and facilities that will provide a more regular year-round benefit. But as with Dissolved Oxygen Monitors, the availability of equipment to balance nature’s ills should be considered on the basis of long-term prudence rather than short-term profligacy.

And there’s always the possibility that ongoing cost can be offset by availability of environmentally friendly solar and wind powered units – although these may not be ideal for relying on in an emergency when a constant, uninterrupted, power supply is essential

There may be other options too, and it’s crucial to remember that the buck doesn’t stop with aeration. Measures like floating islands, marginal planting and even bait limits can serve to mitigate oxygen-related incidents on a fishery. The islands are great at increasing shading and lowering temperatures as well as providing vital cover for fish and even a place to spawn. Floating islands also remove nutrients that would otherwise feed algae blooms as does allowing less bait – but there may be other factors raising nutrient levels, field run off, even geese and cattle – try to reduce these where possible. Your local Fisheries team can help.

There is plenty of professional advice online, and moreover, people who can help and direct. Once again, someone else’s recommendation because; ‘it worked at our place’ isn’t necessarily the way forward.  Environment Agency’s Fisheries Service is funded by fishing licence income – every penny from your fishing licence is proudly invested into the service.  This includes the Fisheries Incident services which offers 24/7 availability of fully trained staff who can help answer questions, offer advice, conduct site visits, respond to reported incidents and even offer kit loans – although such loans are always subject to availability and not guaranteed.

In an emergency

If you see any fish in distress or suspect a fish disease outbreak, please tell us immediately by calling our National Incident Hotline on 0800 80 70 60.

Please contact your local fisheries team by calling 03708 506 506 (Mon-Fri, 8am – 6pm) if you would like advice or assistance.

Useful Links:

  1. Dry Weather Advice for Fisheries 2022
  2. Preparing Your Fishery For Summer – Virtual Forum
  3. Environment Agency Summer Advice for Stillwater Fishery Managers
  4. Environment Agency Advice for River Fishery Managers

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