South wind is bad
and other weather issues (1st part)

If there's something fishermen talk about almost every day during the season, month after month, year in, year out besides the number of trout and the big ones that got away, it's the weather. The following interview with a fly fisherman and a professional weather forecaster attempts to dispel some doubts and homespun explanations that we anglers think up over the years and are not always exact.

The interviewee at the foot of one of his tools of the trade

Andrés, what exactly do you do at the Spanish National Weather Institute?

I work in the Climate Techniques Section. I spend most of my time doing statistical studies with series of weather data and working with geographic information systems for making climate maps.

Is knowledge of meteorology a clear advantage for a fly fisherman?

Knowledge of weather forecasting can help you to better understand some of the phenomena that you observe in the streams and that are weather-conditioned, but I don't think this will help you catch more fish. In fact, if you compare my usual results with those of other fly fishermen, you could conclude that it's even a disadvantage. 

Actually you'll never have two fishing days with exactly the same environmental conditions. This is probably why the rules that predict better or worse fishing depending on whether it's cold or warm, whether it's cloudy or clear, etc. often fail.

Aside from your hobby, do weather experts study the influence of the weather on animals?

There are studies on the influence of the weather and of climate on animals, but they're usually done by biologists and not by weathermen. In any case, it's a field that needs a lot more research.

According to Andrés, weather affects fishing a lot less than most people think

In your opinion, do people tend to under- or overrate the affect of the weather on fishing?

I think they overrate it. At the end of a good or bad fishing day, we anglers look for an explanation. The simplest thing is to blame the weather. This keeps contributing to the myth of its influence on fishing.

What weather factor or event exerts the greatest influence?

In my opinion, the weather factor that most influences the degree of fish activity is the temperature of the air. It, in turn, affects the temperature of the water in the streams.

The direction of the wind

It's very useful to know the day before you go fishing the direction the wind is going to blow, particularly if the winds are going to be moderate or strong. The weather forecast will tell you the wind direction and intensity for each geographical area. Many trout streams flow through sharply-contoured mountain valleys. The slopes of these valleys channel the wind so that it practically always blows either up or down the valley. This also increases the intensity of the wind. For example, if southwest winds are forecasted, they will probably blow upstream in east-west or north-south oriented valleys. In valleys oriented west-east or south-north, however, the wind will probably blow downstream. Although this simple rule holds true most of the time, as with all rules, there has to be exceptions. But it's especially helpful to avoid the streams where you'll have to be casting into the wind if you can choose among various streams. At least, you can go prepared to cope with a strong headwind.

Describe the ideal weather for a good fishing day.

Very cloudy sky with scattered showers, mild temperature and gentle wind.

Do they bite if the wind blows from the south?

I can't see any grounds for this belief. Southern winds normally increase the temperature, and that will be bad on certain days of the year and good on others.

On a cloudy day, a sunbeam will light up a golden head very attractively

How does wind affect trout?

Apparently, duns are less enticing for trout on windy days because wind shortens the drying time of their wings. This makes them more difficult morsels. So, in these cases, the trout prefer to feed on emergers.

Do they bite more when it rains?

Generally they do, as long as it doesn't rain too hard and the water stays clear. There are various possible explanations. Feeling safer since it's harder to see them, trout become unwary. It may be due to the increase in available food that the rain drags into the stream. Maybe the trout see your flies on the surface less clearly and consequently become easier to deceive.

How does the air temperature affect trout?

The activity of trout and of the aquatic organisms that they feed on is conditioned by the water temperature. Although the water temperature depends on many factors, the main one is undoubtedly the temperature of the air.

Extreme temperatures

There are scores of studies that try to determine the ranges of water temperatures that trout prefer. Most of them are based on experiments performed in laboratories to be able to keep all the environmental conditions constant except the water temperature and thus guarantee that the responses of the fish are not due to other factors. Logically, the results of these studies vary. So they must be taken only as approximate values. According to Barton, the optimum range of temperatures for brown trout is 46 – 63 ºF. The highest temperature tolerated over a prolonged period (they can stand higher temperatures for short periods) oscillates between 74.3 and 80.1ºF depending on various factors (prior acclimation to high temperatures, age of the trout, etc.)

After a heavy storm it is important to find the main river, fishing will always be more productive (somebody should tell this guy about it)

What about atmospheric pressure?

There is a very wide-spread belief among fishermen that atmospheric pressure significantly affects trout due to its effect on the swim bladder. It is believed that, after a sharp drop in pressure, trout stay at the bottom because they can't quickly adapt their bladders to the new pressure. Well, we can categorically state that this theory is no more than a myth. Though trout do feel changes in atmospheric pressure in spite of being submerged, the pressure that a fish feels is the sum of the atmospheric pressure plus the pressure of the weight of the water above it. To be exact, the maximum variation in atmospheric pressure that you can expect in 24 hours equals a vertical shift in the water of just 20 centimeters. Therefore it's highly improbable that a trout has any problem at all adapting its swim bladder to changes in atmospheric pressure.

Nevertheless, it's true that brusque changes in pressure are usually accompanied by fast weather changes, which can affect trout behavior. So we can't say that there isn't any relationship at all between atmospheric pressure and trout activity. The influence is indirect though.

Changes in atmospheric pressure are actually very small compared to the changes in pressure that a trout experiences daily at different depths.