What a dissolved oxygen reading tells you about whether to fish

Every club with a river has had the same conversation in a hot dry August. The water is low, it is warm, and somebody says we should probably stop fishing for a while.

Then somebody else says it was worse in 2018 and we carried on. And the meeting moves on, because nobody has anything to point at except an opinion.

That is the problem worth solving, and it is a measurement problem rather than a fishing one.

Why oxygen is the number

Fish take dissolved oxygen out of the water through their gills. How much is available depends mainly on temperature and on how much the water is being churned about.

Cold water holds a lot. Warm water holds noticeably less. So a hot spell reduces the supply exactly when the fish's demand goes up, because a warmer fish has a faster metabolism. Low flow makes it worse again, because there is less turbulence putting oxygen back in, and a shallow river with no shade warms faster than a deep one.

That is the squeeze. Less oxygen available, more oxygen needed, at the same time.

Roughly what the numbers mean

These are general figures for salmonids, and thresholds vary by species, by life stage and by how long the condition lasts. Treat them as a guide, not a rule, and check what your local fisheries body recommends for your water.

Above about 8 mg/L. Comfortable. Normal for cold, well-oxygenated water.

Around 6 to 8 mg/L. Acceptable. Fish are fine but the margin is narrowing.

Around 5 to 6 mg/L. Stress begins. Fish become lethargic, feeding drops off. This is where catch-and-release stops being harmless, because a fish played to exhaustion needs a burst of oxygen to recover and there is not enough spare.

Below about 5 mg/L. Serious stress. Fish move to whatever cool oxygenated water they can find, usually deep pools or spring seeps, and stack up there.

Below about 3 mg/L. Kills start, and coarse species will generally hold on longer than trout and salmon.

Water temperature is the other half of the picture. Above roughly 19 to 20°C, salmonids are already under real strain, and above about 23°C mortality after release rises sharply even if the oxygen reading looks survivable.

Why catch-and-release makes it worse

This is counterintuitive and worth being clear about.

Playing a fish is an anaerobic sprint. It builds up lactic acid, and clearing that requires oxygen over the following hours. A fish released into water with plenty of oxygen recovers. A fish released into warm, low-oxygen water in August often does not, and it dies some hours later, out of sight, having swum away looking fine.

So in a hot spell, catch-and-release is not the safe option. It is the option that kills fish invisibly. The safe option is to stop fishing, and the second safest is to fish only at dawn when the water is at its coolest.

Clubs that switch to catch-and-release in a heatwave, believing it is the responsible call, are often doing more damage than if they had closed the water.

What a club can actually do

Most clubs make this call on a hunch, or after somebody sees dead fish, which is too late.

The alternative is to measure. A probe in the water reading dissolved oxygen, temperature and level, reporting several times a day, gives the committee something to point at. The conversation stops being "it feels bad" and becomes "we are at 5.2 and falling, and the forecast is for another four dry days."

That changes three things.

The decision gets made earlier, before the fish are already stacked in the last cool pool.

The membership accepts it. Asking people not to fish is unpopular. Asking people not to fish and showing them the reading is a different conversation entirely.

There is a record. Which matters when you are talking to a fisheries officer, applying for funding, or trying to work out whether this year was genuinely worse than three years ago.

What we did about it

Our own club looked at commercial monitoring systems and was quoted around €26,000. The grant application to help fund it was refused.

So we built one. A probe, a sealed enclosure, a microcontroller and a SIM, reporting readings back on a schedule. It has been in the river through a season. It has had problems, including water getting into the enclosure, which is being fixed with a conformal coating on the boards.

We are going to publish the build specification so any club can make their own. It is not a commercial product and we are not going to sell it. It exists because our club needed it and the alternative was twenty-six thousand euro.

If your club has a stretch of water you worry about in August, get in touch and we will send you what we have.

Nothing here is a substitute for advice from your fisheries body. If you suspect a pollution incident rather than a hot spell, report it immediately.

Ask us about the water quality monitor →