Under pressure :CRAIG McGILL opens a can of worms over the effects of the barometer on fish
  |  First Published: December 2011

Being a fishing guide, I carry an ‘emergency pack’ of excuses to cover the slow days. I also get to hear fishing theories repeated ad nauseam.

One of the favourites common to guides and punters alike is the one about the barometer. It goes like this: A falling and low barometer causes bad fishing and a rising and high barometer generally produces good fishing.

There’s a plethora of theories why this happens but the most common I’ve heard is that it affects the fish’s swim bladder. The bush biologists conclude that a low-pressure system causes the swim bladder to expand and make the fish feel ‘full’, hence suppressing the appetite – the human equivalent of stomach stapling.

While I’m certain that shifting barometric pressure does have some effect on saltwater fish and, to a much greater extent, freshwater fish, I’m thoroughly convinced that the common myth as to why is complete and absolute bunkum.

Consider the following.

The average pressure exerted at sea level by our entire atmosphere is 1000hPa (1 atmosphere). That’s the weight of the entire atmosphere from space to the earth’s surface.

You have to go down only 10m in the water to achieve another 1000hPa. So at 10m depth, you have 2000hPa or 2 atmospheres (atmospheric pressure + water pressure).

Variation caused by high- and low-pressure systems moving around the Earth varies between a low of 950hPa and a high of 1050hPa. That’s a 100hPa variation. This represents 10% variation of the entire atmospheric pressure of 1000hPa.

The theory starts to become very flimsy when you realise that a fish would have to move only 50cm up or down to compensate for the greatest variation in barometric pressure that it’s ever likely to encounter.


Every 1m of water represents 100hPa. And the deeper you go, the less relevant a 100hPa change becomes – 100hPa in 10m of water represents 10% but 100hPa in 100m is only a 1% change

The tide height alone in most places would contribute to at least twice as much pressure change as would ever be caused by changes in barometric pressure. In some places like the Kimberley, where the tide rises 12m, this would have an effect of producing 12 times atmospheric variation.

On our sounders we regularly encounter kingfish at 10m that will swim to the surface to take bait. This ascent represents the equivalent of 10 times the worst change in atmospheric pressure.

Bass sitting 3m down will bolt to the surface to take a cicada and in the process will experience three times the average pressure swing. If its air bladder expanded during that accent, as the theory proposes, then by the time it got to the cicada it would have lost its appetite.

In plain terms, a fish at a fixed height off the bottom (as they do) in 20m of water would experience twice the worst that atmospheric pressure could dish up, simply as a result of a 2m tide.

Furthermore, the fish would only have to move up 50cm up or down to completely negate any effect felt by a full swing of a barometric pressure up or down to 1050 or 950 from the average of 1000mb.

And if the same fish swam up just 10m to grab prey, as many fish do, it would experience the equivalent of 10 times the worst atmospheric pressure change.

Fish experience changes in pressure in their everyday life that far outweigh anything atmospheric pressure can dish up. And the deeper it gets, the less it matters.

Clearly atmospheric pressure variation is not an issue for fish.


Here’s an alternate theory;

The lower the pressure, the less able water becomes to assimilate oxygen from the atmosphere.

To add to this, aquatic plants, including planktonic algae, are the main contributors to the dissolved oxygen (DO) content of water, particularly freshwater, as a by-product of photosynthesis.

They take in CO2, split off the C for building and spit out the O2. Photosynthesis slows down dramatically in the low light that usually accompanies a cloudy, low-pressure system.

Additionally, rough conditions created by low-pressure storms cause the water to become cloudy and inhibit light penetration, slowing photosynthesis more.

To further exaggerate the affects, some plants in low light start to consume oxygen, rather than produce it. Even worse, poorly oxygenated water can cause toxic effects such as the release of ammonia and sulphides from sediments.

It’s a runaway effect that can spiral out of control very quickly.


I keep aquarium fish and it is very apparent that oxygen deficiency has a greater effect on them than any other variable, including pH and temperature. Oxygen deficiency will kill in minutes where temperature and pH usually take days if conditions are not rectified.

Many fish will adapt to pH and temp changes to some degree; they will not adapt to lack of oxygen.

They very quickly stop feeding, become lethargic and in worse cases, die.

Most of the big fish kills in the wild are ultimately a result of oxygen depletion.

Bigger fish will be affected by oxygen depletion first. The effects of dissolved oxygen levels on fish should not be underestimated; they are one of the fastest changing variables.

Prolonged or rapid changes in pH will have some effect on fishes’ behaviour.

As CO2 levels increase, the water pH goes down, which means it becomes more acidic. During a sunny day aquatic plants remove CO2 from the water, which helps raise or at least stabilise pH.

But during a cloudy day, when plants are photosynthesising less and using less CO2 the pH will drop.

Even worse, plants will actually produce CO2 during this period, adding to the pH drop. If this goes on for a number of days, pH will drop considerably.

The amount and speed of drop will vary between waterways, depending on the levels of carbonate hardness (KH) in the water. This is a much greater issue in freshwater than in salt because the general hardness of saltwater makes its pH much more stable.

This all adds evidence to why freshwater fish are much more affected by low-pressure weather.


Storms, common to low-pressure fronts particularly in Summer, are known to break thermoclines, mixing oxygen-depleted water from the deep with oxygenated surface water.

The overall effect is usually a general oxygen deficiency. As you might be starting to notice, there’s nothing good about low-pressure systems from a fish’s perspective.

It’s my theory that oxygen deficiency, change in pH and a drop in water temperature are the major contributors to ‘shut-downs’ in fish.

These come about as an indirect result of the generally bad, cold and cloudy, low-light weather associated with low-pressure systems.

Fishes’ metabolisms are directly linked to water temps. Fish will also absorb some direct radiated heat from sunlight on a sunny day, irrespective of water temps – an advantage lost on an overcast day.

So not only does their metabolism slow down because they are cold, but they also feel bad because they have to work harder to get oxygen and their environment has become more acidic.

No wonder they stop feeding and I’m sure I don’t have to explain why they feed up big-time just before the front hits.

This theory also supports why freshwater fish are more affected.

Bodies of freshwater, being of lower volume than the ocean, harbours and bays, cool down a lot quicker and are subject to flushes of cold rainwater, which drops temps and causes water clouding.


These watercourses also rely heavily on aquatic vegetation for oxygen – much more so than the ocean.

Aquatic plants provide a far better distribution of their released oxygen because they do it from the bottom up.

Dissolved atmospheric oxygen, in a still body of water, will tend to stay near the surface. Once water near the surface becomes saturated with oxygen, it won’t take on any more until turbulence or a current moves that water on and replaces it with less oxygenated water.

Plants anchored to the bottom will oxygenate the deeper layers of water in still freshwater lakes that would otherwise receive no surface oxygen.

This is mainly applicable to the lake margins (where coincidentally we find most fish activity) because aquatic plants don’t grow in very deep water where light penetration is insufficient – even in clear water.

This is why people can often pre-empt an impending fish kill with reports of ‘fish gulping at the surface’.

My aquarium fish congregate around the turbulence created by the filter outlet during periods of low oxygen. Water absorbs and distributes oxygen better when it’s moving.

Dissolved oxygen levels in a freshwater body are generally at their highest in the afternoon and at their lowest just before dawn.

If you are struggling to catch a fish in freshwater during a low, go looking for the warmest water you can find near an eddy or other turbulence in the late afternoon.

You need to find oxygenated water. The downwind end of a lake, where it’s choppy, or where a feeder creek joins the main lake or river, could be a good start.

Rapids entering a still pool would be a great source of aeration.

Alternatively, you could just stay in bed.

The up side of all this is that when things return to normal, the fish are really hungry.

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