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Atmospheric pressure and fish behavior

In recent decades, many authors of articles on fishing have tried to analyze how the state of the air, in particular fluctuations in atmospheric pressure, affects the life of aquatic organisms, including fish.
The main conclusion reached by almost everyone who wants to understand this issue is as follows: any change in atmospheric pressure immediately affects the physiological state of the fish, reducing its activity, or, conversely, forcing it to move more intensively in search of food (in fairness it should be noted that these phenomena are often confirmed by the practice of fishing).
Also, all authors of publications, as one, found the organ “responsible” for such a reaction in the fish – it is its swimming bladder, which, supposedly shrinking or bloating, responds to changes in external atmospheric pressure, introducing a certain discomfort into the life of the fish.

It seems that this is not entirely true. The swim bladder is, of course, an indicator of the most complex physical and biological processes that constantly occur in nature and among which atmospheric pressure fluctuations can be called. But, most likely, this organ serves the fish for vertical orientation in the water column, helping to reflexively stay at a certain depth and equalize the specific gravity of the fish with the density of water.

This allows fish to move in the aquatic environment with the lowest energy costs.

Now, based on simple reasoning, we will try to show that atmospheric pressure, more precisely, its fluctuations, does not directly, but only indirectly, indirectly affect the behavior of fish.

From a physical point of view, the external pressure acting on the fish in the water is the sum of the atmospheric pressure above the surface of the water and the pressure of the water column corresponding to the depth at which the fish is located.
Let’s see how the changes in pressure in the air, measured by the height of the mercury column, and in water are related. In this case, of course, it should be borne in mind that the density of water is almost 13 times lower than the density of mercury and 800 times higher than the density of air. The simplest calculations will show that, for example, for a person a change in atmospheric pressure by 10 millimeters of mercury will be equivalent to a vertical movement of 100 meters. In water, a similar movement for the fish will be a little more than 10 centimeters, which practically corresponds to the height of its body. And in general, one often observes how, at constant atmospheric pressure, a fish, without any damage to its health, makes gigantic – in our understanding – vertical movements and even jumps out into the air. So, bream, which “played” on the surface in the mornings, rises for this from a depth of 10 meters, which in air corresponds to a rapid ascent to a height of almost 12 kilometers.

Therefore, it should be clear that Nature arranged the fish in such a way that even sharp pressure surges can not have a noticeable physiological effect on its body.
A natural conclusion suggests itself: the positive or negative influence of atmospheric pressure fluctuations on the behavior of aquatic organisms is determined by some other mechanisms, possibly related to the metabolism process.

It is known that one of the main abiotic factors affecting people’s well-being is the relative oxygen content in the air. Even minor changes in this indicator can decisively affect health status. This is also the case in the aquatic environment. The metabolic process in fish is greatly affected by the concentration of oxygen dissolved in water. It is directly related to both water temperature and atmospheric pressure. Moreover, the higher the atmospheric pressure, the higher the concentration of oxygen in the water, and vice versa, the higher the temperature of the water, the less oxygen can dissolve in it, but the more it is needed for fish, as the metabolic process accelerates.

But these are general concepts that reflect a fixed, motionless situation. What happens in dynamics when atmospheric pressure begins to change in one direction or another, for example, after a state of rest, atmospheric pressure drops? In the beginning, the upper layers of water will begin to give oxygen and therefore the fish will go deeper. On the contrary, with increasing atmospheric pressure, the upper layers of water, into which the fish will rise, will be saturated with oxygen first of all. It should be borne in mind that such movements are made not only by fish, but also various aquatic organisms – the objects of its nutrition.

Do not forget that in real conditions other factors influence the behavior of fish a lot. Thus, strong winds and waves of water lead to the fact that it is quickly and fairly uniformly saturated with oxygen, which ultimately minimizes the effect of pressure fluctuations. The same can be said about the whirlwind, about the surf.

Repeatedly in the fishing literature the phenomenon of the midday zhora of the bream was described in the hot summer season, when anglers, who served the morning dawn, went home with nothing, and more patient and “heat-resistant” were rewarded with a rich catch.

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