Growing of fish and aquatic invertebrates in industrial breeding systems based on Recirculating Aquatic Systems (RAS) in our country becoming more common and is very promising, since it allows to minimize the consumption of water and build a fish-breeding enterprises on water sources of small capacity. However, during the cultivation of fish (and any animals) in captivity a lot of common problems occur – disorders of osmosis regulation function, temperature shock, partial asphyxia, reducing of resistance to disease and fertility, slow growth, metabolism disorders and others. These and other diseases are the result of environmental conditions deterioration. Thus, the regulation of temperature, salt, light modes in the Recirculating Aquatic Systems in accordance with the objectives of manufacture and also the life cycle management of grown fish are possible only by constant monitoring of water quality.

The aim of this study was to evaluate the seasonal changes of water quality in Recirculating Aquatic Systems in order to ascertain their possible impact on the livelihoods of rainbow trout, and also of wastewater quality.

The study was conducted in full-system trout farm – East-Ukrainian Center of Valuable Fish Species Breeding «Mzha». The source of water in trout farm is an artesian well (depth – 40 m, water supply – 10 m³ per hour. The average level of water replacement in RAS was 10 % per day.

Water quality in RAS was examined on the following parameters: temperature, pH, total hardness, turbidity, color, oxygen content, total iron, nitrates, nitrites, chlorides, sulfates, ammonium, fluoride and solids. Water samples for analysis were taken at three points in RAS three times a month in March, October and December 2016. Samples were taken as from pools indoors for young fish and in pools for commodity fish located outside. Such indices as temperature, oxygen content and pH were controlled daily by farm staff.

In addition, during the year farm wastewater that is discharged into the river Mzha has been studied. Content of suspended solids, ammonia nitrogen, nitrates and phosphates were determined in samples of wastewater.

In the result of study of hydrochemical parameters of water in RAS for growing of rainbow trout the minor seasonal fluctuations of indices of used water and wastewater were found. The content of chlorides, sulfates, nitrates, nitrites, ammonium salts in the water during 2016 did not exceed permissible values, therefore, these compounds did not adversely impact on the fish. Regarding the total iron content, it is the only index whose value exceeded normative. In March and December the excess accounted for 21.3 and 15.3 % respectively, but in October the number of total iron was below compared to norm – 0.37 mg/dm³. Increasing of iron content in water is dangerous to fish, because iron hydroxide can settle on their gill lobes and impair breathing and ion exchange. Sensitivity to the diseases increases significantly as well.

The oxygen level in the pools was stable and within the optimum. For commodity fish it fluctuated within 7.6–8.4 mg/dm³, and for young fish was within 7.8–8.4 mg/dm³.

Hydrochemical indices of wastewater indicate that its quality is deteriorating for September and December, but in early spring wastewater quality becomes better. In winter in wastewater we fixed the significant (by 70.2 %) reduction of suspended solids content, increase of ammonia nitrogen content (by 82.4 %) and nitrate content (by 14.4 %) compared to March. Even in case of exceeding of some indices values in comparison with background ones, farm wastewaters are safe for the ecosystem of the river, which they are discharge.

Key words: RAS, rainbow trout, hydrochemical indices of water, wastewater, seasonal changes.