Oxygen regime of cage farms during the ice-cover period on the Volga

 

In addition to the risk of death of fish kept in cages, which is possible in conditions of elevated temperatures or, conversely, in conditions of ice formation, this can have a negative impact on the physiological and immune reactions of the fish being grown, while short-term low values ​​of dissolved oxygen every day will have a lesser effect than the same values ​​maintained for several days or weeks in a row.

 

In cage farms of the Astrakhan region, mainly sturgeon species of fish are grown, which are characterized by a relatively high sensitivity to oxygen deficiency. The content of dissolved oxygen in water bodies of fishery importance is recommended to be at least 6.0 mg / dm3. The degree of water saturation with oxygen in natural water bodies of 60% or less indicates low water quality and a low rate of oxidation of organic and inorganic substances in the ecosystem. In addition to water temperature, the presence of ice, the rate of photosynthetic processes, the oxygen regime can be affected by wind phenomena, the distribution of currents, the depth of the water body, etc.

 

Specialists from the Volga-Caspian branch of the State Research Center of the Russian Federation "All-Russian Research Institute of Fisheries and Oceanography" (VNIRO) conducted hydrochemical studies of water in the winter in order to determine the characteristics of the oxygen regime in the area of ​​cage farms, both in the absence of ice and after the formation of ice cover of water bodies. The oxygen content in the water was studied in the watercourse to the location of the cage farm, directly in the cages, both with and without aquaculture objects, and downstream from the cage farm.

 

At water temperatures slightly above zero and in the absence of ice on the watercourse, the dissolved oxygen content was 8-14 mg / dm3, while at depths of 5 m and more, the saturation of water in the bottom horizon was low 56-65%. In cages with aquaculture objects, a decrease in dissolved oxygen to 7 mg / dm3 was noted with reduced saturation values ​​​​of 51%.

 

During the onset of freeze-up with an ice thickness of 15-20 cm, there were no significant changes in the oxygen regime of the natural water body as a whole - the oxygen content is up to 14 mg / dm3. At the same time, a redistribution of oxygen concentration by water horizons was noted - the lowest values ​​​​were recorded in the surface horizon under the ice, and in the middle layer of water, saturation can reach 65-70%.

 

The conditions of the winter of 2024-2025 were unusual. The onset of low air temperatures in mid-February shifted the onset of freeze-up on large watercourses of the Volga Delta to the end of winter. It should be noted that a long-term decrease in water saturation with oxygen in cages containing aquaculture objects can create risks of deterioration of their physiological state, a decrease in the overall resistance and stress resistance of the body of farmed fish during the wintering period.

 

Press service of VNIRO