Weather and Fish Behavior

As a rule, fish come up to shores where the surf is breaking and the depth they need is available. Many authors writing about fishing have repeatedly tried to analyse in their works the influence that changes in the state of the air, in particular fluctuations in atmospheric pressure, have on the life of aquatic organisms, fish included. The main conclusion usually drawn is that any change in atmospheric pressure is immediately reflected in the physiological state of the fish, suppressing its activity or, on the contrary, making it move about more intensively in search of food. To be fair, it should be noted that such phenomena do occur and are quite often confirmed by fishing practice. It is also assumed that fish have an organ "responsible" for reacting to fluctuations in atmospheric pressure – the swim bladder, which supposedly contracts or inflates in response to changes in external atmospheric pressure, causing the fish a certain discomfort. I think this is not entirely correct. The swim bladder is, of course, an indicator of the extremely complex physical and biological processes that take place during sharp changes in the weather, one of which may be fluctuations in atmospheric pressure. But most likely this organ serves the fish for vertical orientation in the water column, helping it reflexively hold a certain depth and balance its specific gravity against the density of the water. This allows the fish to move through the water with the least expenditure of energy. Now, using simple reasoning, let us try to show that atmospheric pressure, or more precisely its fluctuations, affects fish behaviour not directly but only indirectly. From a physical point of view, the external pressure acting on a fish in the water is made up of the atmospheric pressure above the water surface and the pressure of the water column corresponding to the depth at which the fish is located. Let us then look at how changes in pressure in the air, measured by the height of a mercury column, compare with those in water, bearing in mind that the density of water is almost 13 times less than that of mercury and 800 times greater than that of air. The simplest calculations show that, for example, for a person a drop in atmospheric pressure of 10 mm of mercury is equivalent to moving vertically by 100 metres. In water, the equivalent movement for a fish is just over 10 centimetres, which practically corresponds to the height of its body. And in general, one often sees how, with atmospheric pressure unchanged, fish make vertical movements through the water column that are gigantic by our standards, without any harm to their bodies, and even leap into the air. For instance, a bream "playing" at the surface in the morning rises for this from a depth of up to 10 metres, which in air corresponds to a rapid ascent to a height of almost 12 kilometres – probably no human could withstand that. By now it should be clear that nature has made fish in such a way that even sharp jumps in ambient pressure cannot have a noticeable physiological effect on their bodies. The natural conclusion therefore suggests itself that the positive or negative influence of atmospheric pressure fluctuations on the behaviour of aquatic organisms is determined by some other mechanisms, possibly related to metabolism. It is known that the main weather-related factor affecting people's well-being is the relative oxygen content of the air. Even slight changes in this value can cause major changes in one's state of health. The same is true in the aquatic environment. The concentration of oxygen dissolved in the water has a very strong effect on the metabolism of fish. It is directly related both to water temperature and to atmospheric pressure: the higher the atmospheric pressure, the higher the oxygen concentration in the water, and conversely, the higher the water temperature, the less oxygen can dissolve in it, yet the more of it the fish needs, since its metabolism speeds up. But these are general notions reflecting a fixed, static situation. What happens in dynamics, when the weather begins to change and atmospheric pressure changes along with it? For example, after a period of calm, atmospheric pressure begins to fall. The upper layers of water will be the first to give up oxygen – the fish will go deeper, to layers richer in oxygen. Conversely, when atmospheric pressure rises, the upper layers of water will be the first to become saturated with oxygen, and the fish will rise into them. And it is not only fish that make such movements, but also the various aquatic organisms they feed on. Of course, it should be borne in mind that in real conditions other factors may have a stronger influence on fish behaviour. For instance, strong wind and considerable waves on a body of water lead to rapid oxygen saturation and an evening-out of its concentration throughout the water column, cancelling out the effect of pressure fluctuations. As a rule, fish come up to shores where the surf is breaking and the depth they need is available. A noticeable swirling current in a body of water also makes the influence of atmospheric pressure fluctuations on fish behaviour imperceptible. The fishing literature has repeatedly described the phenomenon of the midday bream feeding frenzy in the hot summer season, when anglers who had sat through the morning dawn went home empty-handed, while the more patient and "heat-resistant" ones were rewarded with a rich catch. The whole secret in this case is that by midday a considerable wind would pick up and start mixing the water, which became enriched with oxygen faster in the shallow near-shore areas. Fish would come here from the depths and start feeding actively. And one often saw anglers sitting in boats over considerable depth watching with envy as their luckier comrades-in-arms on the shore pulled fish after fish out of the water. On the other hand, in winter, when oxygen starvation sets in on many closed, stagnant bodies of water (the "dead season"), no fluctuations in atmospheric pressure cause the bite to pick up. Probably any observant angler can also cite plenty of examples from their own experience where the decisive factor for successful fishing on a given body of water was its oxygen regime, provided, of course, that other conditions were favourable. And one more thing. Many anglers, when getting ready for a trip to the water, go by the readings of their home barometer. If they see that the pressure is dropping sharply, they often cancel the trip. This is not always justified. For example, after hot and windless weather, a drop in pressure, usually accompanied by a strengthening wind, makes many fish bite more actively. Source: MK.RU