Impact assessment of national grower trout mixed fodders on the condition of rainbow trout (Oncorhynchus mykiss Walbaum, 1792) during its farming in cages

. The paper presents the study results of the various recipes of production trout mixed fodders produced by LLC "RybProm" on the condition of rainbow trout ( Oncorhynchus mykiss Walbaum, 1792) during its cultivation in cages in comparison with the feed produced by Coppens. The physiological condition of the studied fish was evaluated, haematological, biochemical studies and pathological and anatomical autopsy were carried out. It was shown that the feed coefficient in the control group was 1.2, and in the experimental feeds from 1.3 to 1.5. Of the three studied feeds, the feed "Recipe #3" is practically not inferior in feed coefficient and growth rate to the control group. The conducted researches at the cage trout farm showed that the feeds of domestic production correspond to imported analogues. All studied feed recipes are safe for the health of rainbow trout and are sufficiently balanced for its cultivation in cages.


Introduction
Aquaculture is the most important source of fish and seafood for human consumption.Global aquaculture production has been steadily increasing in the 21st century, with an average annual rise of 5.8% between 2001 and 2010, 4.5% between 2011 and 2018, and about 3% in 2020 [1].By 2030, aquaculture is expected to provide two-thirds of the world's fish production [2].
Feed and feeding of fish in fish farming are important for the economic production of healthy, high quality products with high survival rate and fast growth [3].It is possible to obtain high quality products only through complete and rational feeding of fish [4].Therefore, further progress of the industry mainly depends on the developing of high quality fish feeds.For many decades, fish feeding specialists have tried to develop aquaculture feed formulations that would maintain or increase the growth of farmed fish while reducing costs [5].As early as the 1920s, there was a desire to develop more sustainable aquaculture.The concern was to reduce water pollution caused by raw meat and fish and by-products that were commonly used as feed [6].In the late 1950s, Edward Grassl [7][8][9] evaluated the use of dry feeds.He compared the growth of trout fed either wet minced meat or dry pellets and reported similar growth of fish when the amount of pelleted feed fed was 50% less than recommended for raw feed, provided chopped liver was given once a month.The introduction of granulated feeds resulted in a 60% increase in fish farm productivity and a reduction in fish feed costs [10].
For normal growth fish require a certain set of nutrients including proteins, carbohydrates, fats, minerals and vitamins.Fishmeal and fish oil are key sources of protein and lipids used in feed formulations [11].The development of new species-specific feed formulations supports the aquaculture industry as it expands to meet the growing demand for affordable, safe and high quality fish and seafood products.
Rainbow trout are one of the most widely farmed species in many countries around the world [12].Rainbow trout (Oncorhynchus mykiss) belongs to the salmonid family and is one of the commercially farmed species worldwide with a total annual production of more than 916,000 tonnes [13].In our country, trout farming has always been an insignificant part of the total volume of farmed fish.However, in recent years there has been a rapid growth in trout production.Potential opportunities are far from being exhausted.In the future, Russia may equal Denmark and Finland in this indicator.
In connection with the above, the goal of the research was to study the impact of different recipes of productive trout mixed fodders produced by LLC "RybProm" on the condition of rainbow trout (Oncorhynchus mykiss Walbaum, 1792) during its cultivation in cages.

Material and research methods
The research was conducted on a cage trout farm in the period from 18.01.2023 to 20.05.2023 in the Leningrad region, Kirishi.
The research object was a two-year-old rainbow trout (Oncorhynchus mykiss Walbaum, 1792) with an average weight of 0.83-0.86kg, which was fed experimental productive trout mixed fodder produced by LLC "RybProm" with different percentage of fish meal in recipes.Sorrans feed (Supreme Astax) was used as a control.At the beginning of the experiment and during the period of its implementation, control catches were performed in order to determine the average live weight of fish in the cage.Feeding was calculated according to Coppens norms, adjusted according to water temperature and weight of reared fish (Table 1).At least 25 fish were weighed and not fed previously.Feeding rates were adjusted every 2 weeks based on weighing.Each group of fish was kept in a separate cage of 5x5 m with 20 mm mesh.During the experiment, hydrochemical parameters were constantly monitored.Before the beginning of the experiment, blood was taken for haematological and biochemical studies to determine the general condition of the fish.During the experimental feeding period, fish were subjected to ichthyopathological examination every 2 weeks for health monitoring:

Results and discussion
During the experimental period, the water temperature at a depth of 3 m fluctuated from 4 to 18-20 °C .It is necessary to note that even at low negative air temperatures the water temperature had values above 4 °C -this condition allowed to feed fish daily and not to disturb the course of the experiment.The state of the main hydrochemical indicators of water remained unchanged throughout the experiment.In the studied samples the concentration of dissolved oxygen in water did not decrease below 10 mg/l and was within 10-11 mg/l, water pH fluctuated within 6.8-7.2.The kH and gH values ranged between 6-8 gH.No nitrogen, phosphorus, copper and iron compounds could be detected in the water, regardless of seasonal changes, which could negatively affect the health of the hydrobionts and the results of the experiment.
For the period of the experiment, 4 groups of rainbow trout fish were selected and fed with feeds of different formulations viz: The characteristics of the studied feeds are presented in Table 2.The table shows similar characteristics of the main indicators of feed nutrition.In all studied groups was used identical in composition vitamin-mineral premix.The concentration of astaxanthin in the studied feeds was slightly lower than in the control sample, but it did not significantly affect the quality of fish colouring.
Before the start of the experiment, blood was taken for haematological and biochemical studies to determine the general condition of the fish.The results of the studies are given in Tables 3 and 4. By results of haematological studies it was found that the number of erythrocytes and haemoglobin content (except for sample No. 1) are within normal limits.The changes obtained can be primarily connected with the impact of various stress factors (water temperature, oxygen level, mechanical effects) during the transportation of the study objects.
However, significant deviations in the monocyte content may be associated with inflammatory processes in internal organs.In spite of this, the lymphocytic profile of blood is preserved in the studied subjects, which is normal.
The results of blood biochemical tests are presented in Table 4.The average content of total protein, mainly due to globulins, is higher than normal.Moreover, the increased content is observed in all individuals.Increased content of albumin may be due to stress factors.
Attention is attracted by a pronounced increase in AST.This enzyme is contained not only in the heart and liver, but also in large quantities in muscle tissue.Therefore, its sharp increase in all samples more than 2-4 times can be associated with traumatisation, mechanical impact on fish during transportation.
Glucose level in 80 % of fish is decreased, that we connect with possible increased consumption of glucose during transportation and prolonged starvation.
In this way, by analysing the obtained haematological and biochemical data it is impossible to give an unambiguous conclusion about the causes of the detected changes in the blood picture, due to the fact that they were performed after exposure of fish to sharp stress factors.
The following haematological parameters were determined at the end of the experiment (20.05.2023): total haemoglobin (Hem, g/l), number of erythrocytes (Er, mln/μl), average haemoglobin content in erythrocyte (HCE, pg), total serum protein (TSP, g/l), relative leukocyte content (Leuk, thousand/μl).The number of fish for analysis in each sample -5 pcs.Blood microscopy was carried out on smears prepared on the day of blood sampling.As a result of microscopy, leucocyte formulae and percentage of immature erythrocytes were determined.The obtained average values are given in Tables 5 and 6.Differences in haemoglobin mean values are not statistically significant.However, two individuals from cages 2 and 8 have slightly lower haemoglobin than normal.These fish have increased liver index: 1.46 and 1.63 accordingly, also their weight is more than 50 % less than the average weight in these cages.
The mean values of erythrocytes in all variants are similar.The exception was the value of this parameter for the individual from cage 8 (0.60 million/µl), this fish had a low value of both haemoglobin and total protein (below average by about 50%), and a higher proportion of fragmentation of erythrocyte nuclei -more than 12.5%, when the other fishless than 1%.Increased fragmentation of erythrocyte cell nuclei may be a genetically determined process, possibly unrelated to feeding.In other fish, the morphology of blood cells is normal.
The number of immature forms of erythrocytes is slightly higher in fish from cages No. 2 and No. 3, which may be due to their faster growth -the average weight of the studied fish is on average 1.4 times higher than in cages No. 6 and No. 8.In addition, the average value of total serum protein was higher in variant No. 2 (cage No. 2) than in the others.Mean cell hemoglobin (MCH) is a relative value and indicates the functional state of this group of blood cells.A significant increase or decrease in this value is a sign of nutritional disease.This is not observed in this case.
According to leucogram data blood of all fish has typical lymphoid character, a slight increase in the proportion of polymorphonuclear forms of leucocytes may be normal during this period of rearing at the fish farm.
Average values of leucocyte content -relative calculated index.In all variants, the number of leukocytes was less than the conventional norm, which is not a sign of physiological deviation, as this parameter of fish blood has variability.
Therefore, the general blood picture reflects a good physiological condition of the studied fish.No clearly expressed haematological differences were found in fish from these cages.According to blood parameters fish are healthy, general physiological condition is normal.
At visual inspection of rainbow trout it was noted that behavioural reactions are within normal limits, the fish is active -reacts well to movement and noise, intensively consumes food, weakened individuals were not observed.Body surface is clean, without ulcers and neoplasms, colour is light, normally oslysed; gills are scarlet in colour, without traces of hyperplasia and other disorders.Condition of fins and skin without disorders.
Results of pathology findings showed that internal organs within normal limits: liver of dense consistency, filled with blood, brown colour, no foci of fatty degeneration and infiltration were found; spleen with well-defined edges; kidneys smooth, homogeneous.Fish is well-fed, gastrointestinal tract is filled with food, fat accumulation is sufficient.No pathologies of kidneys, heart and GI organs were found.No ecto-and endoparasites were found during the examination.Clinical signs of infectious diseases (ulcers, exudate in internal organs and body cavity, foci of inflammation, haemorrhages, exophthalmia, etc.) are also absent.Muscle tissues of dense consistency, coloured homogeneously (Fig. 2-3).Throughout the experiment, the studied feeds were not inferior to the control group in terms of muscle tissue colouration.If necessary, the astaxanthin concentration can be increased to achieve more intense colouration.
During the experiment, no waste was observed in all 4 cages, which may indicate good hydrochemical parameters of the water body and the quality of the studied feeds.
The fish weighing results at the beginning and end of the experiment are presented in Table 7.By analysing the data presented in the table, it can be concluded that all groups have good growth of fish, but when feeding in the groups "Control" and "Recipe №3" lower feed coefficient is observed -1.2 and 1.3 respectively.In terms of feed coefficient and growth rate, the fish that consumed the feed "Recipe №3" is not much inferior to the control group.Recipe feed №3 contains more protein (43%) relative to the other two experimental feeds (42 and 42.48%), but is within the range of that of the control feed (41-44%).This feed also has a higher fibre content of 2.28%, which is 1.3 and 1.8 times higher than in feeds №. 1 and № 2, respectively, and higher than the limits of this indicator in Coppens feed (1-2%).
All studied recipes of feeds produced by LLC "RybProm" from the point of view of their safety for the health of rainbow trout are not inferior to the feed of the control group and can be considered sufficiently balanced for their use in the cultivation of rainbow trout in cages.
According to the calculation results of the feed coefficient, "Recipe No. 3" should be preferred among the studied feeds, as feeding with this feed will be more economically advantageous, as the fish will reach marketable weight faster.

Fig. 1 .
Fig. 1.Water and air temperature fluctuations from January to May 2023: -air temperature, -water temperature

Table 1 .
Feeding rates for trout to achieve optimum growth rates depending on fish weight and rearing environment, % of body weight.

Table 2 .
Characteristics of the studied feeds (pellet size 6.0 mm).

Table 3 .
Comparative analysis of the results of rainbow trout feeding.

Table 4 .
Results of blood biochemical tests.

Table 7 .
Comparative analysis of rainbow trout feeding results.