Salmon and sea trout – population decline

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On January 1, the protection period for sea trout and salmon ended, and hundreds of anglers began the season. This year, however, it was quickly noticed that the population of these fish is much smaller than in previous years. Preliminary estimates have shown that there are as many as half as many fish.

Environmental conditions

Salmon and sea trout are two species of fish from the salmonidae family. They are known for their unusual migrations to lay eggs. To successfully reproduce, they need the right environmental and behavioral conditions.

Salmon and sea trout are among the anadromous fish, i.e. they migrate for reproduction. Both species spend most of their lives in the sea, but to reproduce they return to the rivers where they were born. During their upstream migration, salmon and sea trout travel hundreds of kilometers, overcoming all sorts of obstacles to reach their spawning grounds. Their mating season lasts from October to January, and is most intense in the second half of November. Reproduction of salmon and sea trout takes place in the form of spawning, then the fish lay eggs in “nests” that the female prepares in the rocky bottom. To do this, they usually choose places where the water is sufficiently cool – the optimal temperature is between 4°C and 10°C. In addition, it should be clean and free of toxins. Salmon and sea trout are fish that need places with adequate flow, depth and bottom structure.

Causes of population decline

In autumn, a significant number of salmonids enter Polish rivers to spawn. They are most numerous in the Drava, Parsęta, Inn and Slupia rivers. However, the unfavorable weather conditions that prevailed last year caused these fish to come in much less than in previous years. Scanners placed at several points in Slupia revealed that the number had dropped by more than 50%. The reason for this was probably the water temperature, which last September in Slupia was 20°C , and at the end of November was above 10°C.

Another reason may have been the water level. High provides much better conditions for fish to reproduce. Water flow is also important for salmon and sea trout because it causes water to mix, which increases the availability of oxygen and food. Meanwhile, last year the level of Pomeranian rivers was at a very low level, so low that hydroelectric power plants stopped working several times during the summer.

The decline in salmonid populations in Pomeranian rivers is likely the result of the overlap of several unfavorable weather anomalies. Whether this is indeed the case will be shown by future years of research.

Did you know that:

Poland has a decree of the Minister of Agriculture, defining aquatic animal species of economic importance and areas designated for their protection. The document indicates sea trout and eel. Areas to protect sea trout are mainly the rivers of the Pierzymorze region and rivers or their sections in southern Poland, both in the Oder and Vistula river basins.

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