Weather and Fishing

There are a number of factors affecting our likelihood of catching fish and our catch rate.
Apart from the obvious ones of location, depth, fly selection, leader thickness, and so on, a major one is the weather.

The weather can affect fishing in a number of ways. A few factors have a major effect on the way fish behave. These are:

• Water temperature
• Wind
• Barometric (air) pressure
• Sunlight (or lack of it)

Water Temperature

Feeding.

Most fish species (including Trout) are cold-blooded, and can’t regulate their body temperature. This means that they must adjust their bodies functions to the temperature of the water around them. In colder waters, fish slow down, and need less food to support themselves. In warm waters, they are  more active, and as such need  more food to survive.

In cold water, enzymes that digest the food a fish eats are very slow acting. Therefore, it takes quite a while for a fish’s meal to be completely utilized and the fish are inactive. As the water warms, these enzymes become more active and fish eat more often.

The oxygen and temperature factors will finally reach a point where a fish’s digestive system functions quickly. However warm water does not contain much excess oxygen, which fish need. When it gets too warm and oxygen levels drop, fish become sluggish and inactive.

Breathing

 fish use their gills to transfer oxygen from the water to their bloodstreams, this is obviously critical to their survival. In warmer water there is less dissolved oxygen for the gills to extract, in colder water there is usually plenty.

Mortality

There are temperature limits which affect the fishes behaviour to the point of survival or death.

 This sounds dramatic, however every living organism on the planet has lower and upper temperature limits, outside which, they will not survive.  

In cold water, the fishes metabolism slows down and they cease feeding, prolonged cold weather can cause the fish to die through malnutrition, whilst in extreme cold temperatures water freezes and causes the fish to die as well.

In most of the lakes and rivers in the UK the water rarely completely freezes from surface to the lake or river bed, this is good news for the fish and anglers. However recent weather events have shown that it’s the upper temperatures which are now becoming a cause for concern.

Most fish are comfortable in the temperature range of 4*c to 25*c, however within the upper   range of water temperature, from 19 to 30* C, three progressive stages of stress response are seen as temperature rises. First, there is a reluctance to feed, sudden bursts of activity and rapid ventilation movements. In the second phase the fish become lethargic with short bursts of weak swimming; they may float on their side or back, and ventilation movements increase. In the third phase, movements are restricted to the gills, pectoral fins and eyes. Studies have shown that fish transferred to cooler well oxygenated water from the first two phases usually recovered, but did not do so from phase three.
It would be prudent to cease any catch and release activity during periods of high water temperature to avoid fish mortality due to lack of oxygen necessary for their recover after being caught.

 Thermocline & Structure

During warm months many lakes develop a thermocline. The top layer of water differs significantly from the colder water below. The top layer of water may be 15 feet deep. Water below that depth may drop 10 or 15 degrees.

High oxygen content exists just above where the cold and warm water meet. This can be the key to success. The thermocline will usually be the same depth over the entire lake however strong winds setting up currents within the body of water can cause the thermocline to tilt such that there are areas of warmer water which is inhospitable to fish. It varies from lake to lake. If you can find areas where the thermocline meets weeds, rocks, bars, structure or cover, you will almost be sure to find fish present.

In reservoirs which have “Aerators” they usually provide slightly higher oxygenated water than the surrounding areas, however this depends on the ambient air temperature. If the air temperature is high (above approximately 25*c) then the transfer or oxygen between the air and water will not take place and all that will happen is that the water temperature will become closer to the air temperature.

What this means is each fish species has a minimum water temperature under which it won’t feed, and a maximum water temperature over which it can’t breathe. As a fisherman, you don’t need to know all the numbers. However, knowing why and when water temperatures change can make all the difference when trying to catch fish.

How does the weather influence water temperatures?

Water temperature can change in a number of ways. There are slow, seasonal changes, which are mainly influenced by the amount of sunlight a body of water receives over an extended period of time. This includes bodies of water which are affected by snow melt in their upper reaches. These changes don’t usually have a tremendous impact on water temperature in the short term.

What can drastically change the water temperature, however, is rain or snowfall.

 When it rains, fresh water pours into a body of water, generally changing its temperature. This change can happen quickly, especially if we’re talking about a small body of water. In addition, rainfall changes the clarity of the water.  Lastly, rain can often introduce large quantities of nutrients into the water from run off from nearby fields or conurbations. (There are many pollutants which will affect our rivers and lakes being allowed to flow into them by our water companies, however that is a different discussion and I will not go down that path here). When all these changes occur at the same time, they can significantly change the way resident fish behave.

Wind

In Britain, winds generally blow from a south westerly direction. This happens for two reasons: first, the Earth’s rotation, and second, the jet stream. During the summer, the jet stream shifts to the north, pulling warm gusts of low-pressure air from the south with it. This is why we have those warm summer winds. During autumn and winter, the jet stream shifts to the south, bringing cold fronts and high-pressure air masses from the north.

When masses of cold and warm air mix, storms start to brew. But more on this in a bit.

Winds can influence water bodies and their residents in several ways. The most obvious one, of course, is by making waves. As winds blows along the surface of the water, it creates friction. The stronger the wind blows, the greater the friction, and the greater the waves. The size of the waves is determined by the distance the waves are acted upon by the wind, larger lakes does equal larger waves!

Waves can increase the turbidity of the water, pulling currents and nutrients along with them, quite often lakes will have underwater currents which concentrate the fish into hospitable areas due to temperature, food availability and safety. All these factors influence the way fish will behave. Last but not least, winds cause changes in barometric pressure.

Barometric Pressure

If there’s one thing that can bring about a feeding frenzy in fish, it’s a change in barometric pressure. Sudden weather changes produce rapid shifts in barometric pressure, and this is precisely why these are the best moments to wet your line.

What is barometric pressure?

Barometric pressure, or atmospheric pressure, is the force exerted by the Earth’s atmosphere on a given area. Think of it as “the weight of the air.” Barometric pressure is measured in several ways: inches of mercury (in-Hg), millibars (mb), or pounds per square inch (psi).. At sea level, barometric pressure of 29.92 inches is “normal.” Anything above that is considered high, and anything below that, low.

Again, you don’t need to focus on absolute numbers, because fish aren’t paying much attention either. What you should make note of is that just as the atmosphere pushes down on Earth’s surface, it does the same to its many bodies of water. These waters, in turn, push on the fish that live there.

Why does barometric pressure affect fish?

To survive in water, fish had to develop a series of physical adaptations. When it comes barometric pressure and fish behaviour, two of these adaptations are important. These are the lateral line and the swim bladder.

The lateral line is an organ fish use to navigate and sense the presence of predators or food. It senses the tiniest of reverberations in the water, and as such, it is very sensitive to pressure changes.

The swim bladder, on the other hand, is an organ similar to the stomach, which can inflate with air and allows the fish to achieve buoyancy. As the air pressure changes, so does the pressure on a fish’s swim bladder. It’s something like a natural barometer.

Fish species like Trout have larger swim bladders, and are more sensitive to changes in air pressure. On the other hand, species with smaller bladders like barracuda are much less affected. And then there are species like Sharks and various types of Tuna that don’t have this organ at all.

Barometric Pressure and Fishing

So air pressure affects fish, this we understand. But exactly when and how does this happen? It’s a well-known fact that rising barometric pressure means improving weather and clear skies. Conversely, dropping barometric pressure means that a storm or a cold front is on its way.

Fishing before and after a Storm

As a storm approaches, a mass of warm, low-pressure air gathers above a mass of cold, high-pressure air. When the two air masses meet, they start creating condensation in the form of clouds. During this time, a noticeable, steady drop in air pressure occurs. The pressure continues to drop until the very end of the storm.

Depending on the scale of the storm, this can happen very quickly, or over an extended period of time. For fishermen the latter is a much better option, as it gives them more time to fish while the pressure is dropping.

What follows the storm is typically a cold front, bringing winds, and a mass of cold, high-pressure air. The cold front often clears the skies, and more importantly, brings about a rapid rise in air pressure. In most cases, this means that fishing is a no-go.

Once the air pressure reaches a high point, it finally stabilizes. There’s no set rule on how long this can take, as it usually depends on the storm that just passed. Fish are mostly inactive after the atmospheric pressure levels out. However, around 72 hours into this period of steady barometric pressure, the fish start feeding again.

Fish seem to be much keener on feeding than before the weather change started. Some anglers speculate that this is because there was no fishing for several days, and fish have “forgotten” about the hooks and lures they normally avoid. A more realistic cause for the increased feeding, however, seems to be that the fish haven’t fed in a while and are hungry.

Fishing Tactics for Changing Weather

All the weather factors we mentioned are closely connected, meaning that one doesn’t change without the other. To make the best out of your time fishing, you’re going to need to be mindful of them all.

Timing

Summarising what we talked about above, fish react to changes in barometric pressure in the following way:

• Rapid drops in pressure:   the fishing is great
• Rapid rise in pressure: the fish cease to feed and seek shelter.
• Stable pressure: after 3 days of pressure not changing the fish begin to feed again.

Temperature and Water clarity

In colder weather, fish are sluggish, so your lure/bait presentation should be slow-moving as well. Of course, the reverse is true for warmer conditions.

As we mentioned, wind and rainfall can make waters more cloudy than normal. this can drastically limit visibility underwater, and in turn, change the way fish behave. For the most part, fish are sight hunters. When they can’t see, they’ll rely more on their lateral line to find prey. This is where fast action and vibrating lures can come in handy. The flutter these lures make is the perfect attention-grabber in such situations.

Still, if you’re fishing cold waters, the fish might not be as easily lured to move. In these situations, you’ll need to rely on brightly coloured lures to get them to bite. Reds, yellows, and greens can all produce results. In cloudy waters, however, bright-coloured lures tend to lose their visibility. If this is the case, your best bet is to stick to dense dark colours.

Sunshine

How often have we been enjoying a productive fishing session only for it to suddenly change for the worse because the sun came out from behind the clouds? Fish do not have eyelids so any sunshine will affect their ability to see food items and predators. The fish tend to drop lower in the water column during periods of bright sunshine and rise when the light levels reduce. Hence fishing early morning and late evening can give you a better chance of catching fish.