Structure Of Respiration In Fish

Published Date: 02 Nov 2017

1 Introduction

In the global fisheries aquaculture production over the past forty years has increased more than by 100 times, and in 2004 exceeded 50 million tons that means more than 55 per cent of a universal catch of fish and other hydrobionts. The Russian Federation has the reservoirs, which meet the requirements of cultivation of hydrobionts, ranking high place in the world. (Bogeruk 2005, 12-25)

Today the aquaculture in Russia is the most developing branch of production of food. In the conditions of constant reduction of netting oceanic fish and other seafood when fish stocks of inland waters are in critical condition, the aquaculture is the only reliable source of increasing fish for food and is one of the guarantees of fish food safety in Russia.

The effectiveness of the fishery development of water resources of several reservoirs of the North-West Federal District of Russia, even under the conditions of a low biological productivity, can be considerably improved by using modern technology. (Ryzhkov et al. 2007, 5)

On the inland waters of the Republic of Karelia, the trout-breeding is one of the priorities of fishery management. Now there is a steady development of commercial fish production, mainly due to the intensive cultivation of rainbow trout in cages. The total amount of trout production increased from 400 tons in 1993 to 6000 tons in 2005. Further development of the trout-breeding in Karelia is determined by "Concept for Fisheries of the Russian Federation up to 2020" (2010), and the "National Programme for Fisheries of the Republic of Karelia 2020" (2010), according to which the production of trout in Karelia in 2015, should be 10 thousand tons.

One type of fishery management in many regions of Russia, and in particular in the North-West Federal District, is the cage fish breeding, in Karelia, mainly - the production of commercial rainbow trout (Parasalmo mykiss Walbaum). This is due to favorable climatic conditions, abundance of natural water resources, good transport connection, energy and the availability of trained personnel. (Savosin 2010, 2-3)

The aqueous medium inhabited by organisms is a part of the biosphere, which can vary significantly due to natural factors and human activities. The most influential scientific, social and economic problem in the modern society is the preservation and sustainable use of the environment. In addition to the traditional problems of biological study, there are some new reservoirs connected with providing the growing requirements for clean water and fish. In the Northern Europe there are freshwater ponds with different efficiency and abiotic conditions. Currently, due to the reduction of stocks and a sharp fall in fishing of valuable fish species, there is an intensification of work on the introduction of hydrobionts and the development of biotechnology cultivation of various organisms. (Savosin 2010, 34-38)

The quality of feed is important for the health of fish in cages. Feed used in modern hatcheries must meet physiological requirements of trout, to be balanced in composition, and contain essential vitamins and minerals. Assimilation of feed granules should be high not only to ensure rapid growth of fish, but also to reduce the level of pollution. Inconsistency of feed for these requirements causes adverse interaction with the environment. The use of fish feed containing oxidized fats, and lacking or excess of vitamins also leads to various pathologies in fish. (Ryzhkov et al. 2007, 15-17)

Along with the problem of food quality, reducing negative effects on the fish promotes observance of optimal modes of feeding. The basis for determining the favorable regime of feeding fish is the temperature conditions of the aquatic environment, as well as age and weight indicators of trout. Insufficient and especially excessive fish feeding impairs their physiological condition and may lead to disease and death.

Consequently, the success of the hatchery trout production depends on the following factors:

The quality of the water environment;

The availability of quality feed;

Optimal feeding model;

Effective use of biotechnology;

The level of health protection for fish.

Water quality is a combination of physical, biological and chemical parameters that affect the growth and prosperity of cultured organisms. The success of a commercial aquaculture activity depends on the optimal environment for accelerated growth at the lowest cost of resources and capital. Water quality affects the general condition of the cultured body as it determines the conditions of health and growth of the cultured body. The water quality is, therefore, an essential factor to consider when planning a high aquaculture production. (Timmons et al. 2002, 27-30)

Although the environment of fish aquaculture is a complex system, consisting of some water quality variables, only few of them play a crucial role. Critical parameters are temperature, suspended solids and dissolved oxygen, ammonia, nitrite, carbon dioxide and alkalinity. However, the dissolved oxygen is the most important and key parameter, requiring continuous monitoring in aquaculture production systems, because fish aerobic metabolism requires dissolved oxygen. (Timmons et al. 2002, 32-33)

2 characteristics of rainbow trout and conditions of cultivation

Rainbow trout, as the object of growing is characterized by plasticity, rapid growth, and high feed conversion. Relatively short period of incubation of hard roe, and also possibility of spawning at any time of the year by creating optimal temperature for the producers have become rather profitable. These characteristics allow us to consider a rainbow trout as the main object of aquaculture in the countries of Europe. (Munro et al. 1987, 11)

The trout is conventionally related to cold water production facilities, although the range of comfortable temperature for its growth is quite broad. Optimal temperatures for rainbow trout are in the range from 9 to 18 °C. Fish feed and growing at water temperatures of 4 to 20 °C is most optimal. At water temperatures below 4 °C and above 20 °C the intensity of its nutrition and growth is reduced. Temperature above 20 °C is not comfortable for trout, but the lethal temperature as a function of temperature acclimation is from 24.9 to 26.3 °C. (Matschak et al. 1998, 12-25)

Rainbow trout is safe at diurnal temperature 5 °C and above, but prefers a particular temperature. Rainbow trout is extremely demanding on the level of dissolved oxygen in the water, the optimal concentration should not be less than 9 mg/l. Trout can tolerate water saturation with pure oxygen to 50 mg/l. Lethal concentration of oxygen in the water for trout is 2.5 mg/l. At high temperatures, the content of dissolved oxygen in water is not less than 9 mg/l. (Munro et al. 1987, 27-43)

During the whole period of cultivation of a rainbow trout - especially during periods of intensive feeding, it is necessary to continuously monitor oxygen concentrations in cages because the concentration of oxygen limits the amount of fish breeding. The concentration of oxygen at the normal growth of fish and feed conversion ratio should be at the water temperature of 5 °C – not less than 5.0 mg/l, at 10 °C – not less than 6.0 mg/l, at 15 °C – not less than 7.0 mg/l and at 20 °C – not less than 8.0 mg/l. (Industry standard 1988, 13-19)

Carbon dioxide content at trout farming in optimal conditions should not exceed 10 mg/l, although trout can survive in water with carbon dioxide concentration up to 50 mg/l with a significant slowdown of growth and increased feed conversion ratio (FCR). FCR is simply the amount of feed required to grow for one kilogram of fish. The maximum permissible value of carbon dioxide in water is 30 mg/l. (Industry standard 1988, 21-27)

At cultivation of rainbow trout, it is more preferable to use water with рН values from 7 to 8. Water with рН within 6.5 – 8.5 is quite satisfactory for trout, but рН values lower 4.5 and higher than 9 are critical for trout. The toxic effect of pH is increased with different content of ions of calcium, sodium and chlorine in water. The presence of iron hydroxide in the water reduces the resistance of trout to low pH values. If the pH value is below 7, the iron concentration above 1.5 mg/l results in the death of trout. In general, the growth rate of a trout in acid waters is lower, than in alkaline, and at constant pH within its optimum values the growth rate is ​​higher than at varying pH values. (Industry standard 1988, 27-31)

Nutrients such as nitrogen and phosphorus have no toxic effect on trout at rather high concentrations, and the maximum allowable concentrations are not determined by the needs of trout, but by requirements for quality of the environment. The admissible concentration of phosphates in water is 0.3 mg/l, and that of nitrites is 0.1 mg/l. (Industry standard 1988, 31-35)

The negative impact of ammonia on fish increases with increasing temperature and pH. By increasing pH of water from 7.0 to 7.3 and temperature by 10 °C, the toxicity of ammonia is doubled. Unionized ammonia (NH3) is harmful to trout also at admissible concentration of 0.05 mg/l but preferably there is none at all. Admissible values of the concentration of ammonia in water at trout cultivation in various oxygen and temperatures is presented in Table 1.

Rather low concentrations of iron compounds are dangerous to trout in water, because iron hydroxide forms a brown cover on the gills, causing suffocation of fish. Especially dangerous for trout is ferrous iron. With a relatively high saturation of oxygen in water it oxidizes and precipitates in the gills. (Munro et al. 1987, 13-15)

TABLE 1. Valid values ​​of ammonia according to hydrochemical indexes (based on Medinor 1995, 24)

Indexes

NH3, g/m3

O2, g/m3

Temperature, °C

Hardness, mmol/l

Standard

0.01-0.07

8±2

18-22

>1.5×10-3

Allowed for 1-2 days

1.0-1.5

18±5

20

>1.0×10-3

Allowed for 3-5 days

0.1-0.2

7±2

20

>1.0×10-3

The hydrogen sulfide is also dangerous for the fish as sulfides, penetrating into the body and it reduces the ability of tissues to absorb oxygen. Lethal concentration of hydrogen sulfide for a trout is 0.86 mg/l. It should be noted that is required about 2.5 mg of O2 for oxidation of 1mg of H2S. Hydrogen sulfide can also bind iron hydroxide and can be utilized by sulfur bacteria. Optimal cultivation of trout requires the lack of hydrogen sulfide in water. (Industry standard 1988, 36-39)

Chlorine in the form of hypochlorous acid and chloramines is toxic for trout, and its toxicity increases with decreasing concentration of dissolved oxygen in water. The maximum content of suspended matter in the water for cultivation of trout is not more than 10 mg/l. It is proved that the concentration up to 100 mg/l, without affecting mortality of rainbow trout, reduces the intensity of the feeding until the complete termination. (Matschak et al. 1998, 12-25)

Phenols have harmful effects on trout, both because of the direct toxicity and rapid oxidizing, leading to a decrease in the concentration of dissolved oxygen in water. In addition, they give the sour taste for fish. The toxicity of phenols increases with decreasing concentration of dissolved oxygen, with decreasing temperature and increasing salinity of water. The maximum concentration of phenol at temperatures higher than 5°C is 0.5 mg/l, and at a temperature of 5 °C it is 0.25 mg/l. (Industry standard 1988, 40-49)

The toxicity of zinc caused by zinc ion and it depends on the water composition. It decreases with increasing hardness, temperature, salinity, suspended solids, and increases with decreasing concentration of dissolved oxygen in water. The maximum concentration of dissolved zinc in water is 0.3 mg/l. (Medinor 1995, 33-38)

Copper toxicity is associated with the divalent ion and increases with decreasing water hardness, temperature and dissolved oxygen and decreases in the presence of humic acids, amino acids and suspended solids. The maximum allowable concentration of copper is 1.0 mg/l. (Medinor 1995, 25-29)

Cadmium is found in low concentrations in the sand and shale soils, from which cadmium is slowly leached into surface waters. The concentration of cadmium in uncontaminated fresh waters is typically 0.01 – 0.5 mg/l, and the maximum concentration that has no negative impact on the rainbow trout is in the range 0.5 – 2.0 mg/l. (Medinor 1995, 33)

In general, requirements of rainbow trout for the chemical composition of the water environment are given in Table 2.

TABLE 2. The water quality requirements for cage rainbow trout farms (based on Industry standard 1988, 12-52)

Parameters

Standard value

Temperature, °C

20

Transparency, m

1.5 – 1.8

pH

6.0 – 8.5

Suspended solids, mg/l

10

Dissolved oxygen (DO), mg/l

≥9.0

Carbon dioxide (CO2), mg/l

10

Hydrogen sulfide (H2S), mg/l

Absence

Ammonia (NH3), mg/l

0.07

Chemical oxygen demand (COD), mg/l

15.0

Biochemical oxygen demand (BOD), mg/l

30.0

Nitrites (NO2−), mg/l

0.05

Nitrates (NO3−), mg/l

1.0

Phosphates (PO43-), mg/l

0.3

Total iron, mg/l

0.5

Among the above parameters in Table 2, the most unstable is dissolved oxygen in the water. When fouling or silty of cages the oxygen for a few hours may be reduced to a critical value (6 – 7 mg/l.) or even lethal concentration (2 – 3 mg/l). The lack of oxygen in the cages can be judged by the behavior of fish that float to the surface of the water. Eliminating such a situation can be achieved by aerators, which intensively pump air or oxygen through the water environment in the cages. (Mikheev 1982, 36-45)

3 adaptive mechanisms in fish

Oxygen as gas has a low solubility in water. In addition, the amount of oxygen in water varies depending on the temperature and salinity in a predictable manner. Less oxygen can be fixed in totally aerated warm salt water than fully aerated cold freshwater. While the oxygen concentration of water sets the absolute availability of oxygen in the water, it is the gradient of the partial pressure of oxygen, which determines how quickly oxygen can move from the water to the fish blood to maintain its metabolic rate. This is because the oxygen moves through the gills of trout.

The rate of diffusion of oxygen through the trout gills is identified by the gills area, the diffusion distance through the gill epithelium, the diffusion coefficient and the difference in partial pressure of oxygen over the gills, according to Fick's law of diffusion. (Crampton et al. 2003, 12-20) Therefore, the partial pressure of oxygen is the most appropriate term to express the oxygen levels in the freshwater. However, the concentration of oxygen depends on temperature and salinity. Oxygen partial pressure and oxygen concentration in the water are linearly related. Other suitable method to indicate the oxygen levels in aquaculture is air saturation as percentage (often just % of saturation), which is also directly proportional to the partial pressure, and is reported in most of the oxygen studies that have built in algorithms for temperature and salinity. (Bergheim et al. 2006, 41-46)

3.1 The absorption of oxygen and carbon dioxide release from by the fish

During the respiration activity the trout take in oxygen and give out carbon dioxide, like other animals. The whole process is done by gills in almost all of fish, although some may also use the skin, and some have lungs as structures used in addition to the gills. When a fish breathes, a pressurized gulp of water flows from the mouth to the gill chamber on each side of the head. The gills themselves are located in gill clefts within the gill chambers, composed of fleshy, skin like filaments transected extensions, and called lamellae. As water flows through the gills, the oxygen in the range is diffused in the blood circulating through the vessels in the filaments and lamellae. At the same time, the carbon dioxide in the blood of fish is diffused into the water and carried out of the body (see Figure 1).

Function of fish gills

For most species of fish, the gills are working in the directed flow of water through the gills epithelial surfaces, where the exchange of gases occurs. Cause of unidirectional flow of water is energetic nature of the system. The energy that is needed to move the water in the respiratory system and out of it will be much more than that used to move air as the water contains low levels of oxygen due of its low solubility. (Groot et al. 1995, 22-24)

how fish breathe

FIGURE 1. Structure of respiration in fish (based on Edmondson 2006)

The blood flowing just under the gill epithelial tissue usually moves in a counter current flow of the water moving over it. This covers most of O2 to be taken with blood because the gradient diffusion kept up by the blood, raising the oxygen as it moves along, but always associated with water entry, which has a higher amount of O2. (Groot et al. 1995, 30-35) The blood receiving the O2 continues to pick up O2 as it moves on because it is fresh water flooding the epithelial lining of gills. By doing so, this water is ventilated the gills of fish and also taking in oxygen and releases carbon dioxide. (Jobling 1995, 25-27)

However, there are two ways in which the fish gills are aired: active ventilation and passive ventilation. In the active ventilation of fish, water is pulled through the mouth (buccal chamber) and pushed through the gills and out of the opercula chamber. At this time, the pressure in the buccal chamber is maintained higher than the pressure in the chamber opercula so as to allow fresh water to be continually flushed over the gills. In passive ventilation, a fish swims with its mouth open, allowing water to wash over the gills. (Bailey et al. 1996, 13-18) This method of ventilation is regular for fast moving fish and this allows the trout to keep enough oxygen going to the gills surface, floating at a high speed. During this time, oxygen is absorbed into the blood while carbon dioxide diffuses from the blood to the water. (Boyd et al. 1998, 254-310)

The pathway made by carbon dioxide explains that the CO2 is transported in the blood in the form of bicarbonate. The bicarbonate moves from the blood passing through the erythrocyte, in which O2 binds to hemoglobin at the respiratory surface, causing the hydrogen ions (H+) to be released. The increase in H+ ions combines with HCO3- to form CO2 and OH-. So more CO2 is formed and can leave the blood through the respiratory surface. Excess H+ binds to OH- forming water and allowing the pH to increase enough to sustain oxygen to hemoglobin. The O2 released from hemoglobin in the tissues makes available to bind with H+, promoting the conversion of CO2 to HCO3-, which helps to pull CO2 from the tissues. (Groot et al. 1995, 69-80) Therefore, CO2 which is transported in and out of red blood cells minimizes changes in pH in other parts of the body due to the proton binding and proton releasing from hemoglobin, as it is deoxygenated and oxidized respectively. However, carbon dioxide is rarely a problem when the concentration of dissolved oxygen is in saturation level. Because of these processes, the oxygen level must be maintained at or slightly higher during the entire culture period. (Bailey et al. 1996, 48-63)

3.2 Effects of oxygen levels on oxygen uptake by rainbow trout

Commonly it is believed that if there is not enough oxygen in the water, then the fish will be gasping at the surface, but this is the last means of breathing. The first sign of too little dissolved oxygen in the water is when fish are unusually lethargic and stop feeding. As oxygen level decreases generation, the fish do not have enough energy to swim and feeding uses more oxygen. Often the fish have trouble at this stage, and often some form of medication is added to water, which can cause the oxygen level to drop even lower, and leads to the deaths. This may lead to the erroneous conclusion that the fish suffer from some forms of the disease. (Yovita 2007, 14-30) From the management’s point of view of any aquatic system, it is always advisable to increase aeration when any fish start behaving abnormally before adding any form of medication in the water. Increased aeration will make the environment more comfortable for the fish, even if the level of dissolved oxygen has been satisfactory. Aeration improvement before adding medication will allow any level of oxygen depletion caused by chemical reaction with medication. (Svobodova et al. 1993, 13-28)

Hypoxia in fish

The aquatic system lacking dissolved oxygen (0 % saturation) is called anaerobic. The system is anoxic with low concentration of dissolved oxygen (DO) in the range from 1 up to 30 %. DO satiation is called hypoxic. Most fish cannot live below 30 % saturation of DO. The "healthy" aquatic environment should rarely experience DO of less than 80 %. In response to low concentrations of dissolved oxygen in the water the fish can react in two ways. The blood flow can be increased by opening up further secondary lamellae in order to increase the effective area of the respiratory tract (it can be difficult to increase significantly the flow of blood through the capillaries). The concentration of red blood cells can be increased by raising the oxygen capacity per unit volume. The last one can be achieved by reducing the amount of blood plasma (e.g. by increasing the flow of urine) in the short term and by releasing additional blood cells from the spleen in the long term. (Svobodova et al. 1993, 13-45)

At the same time, the level of ventilation is increased to bring more water in contact with the gills at the unit of time. However, a restriction to the increased flow is attainable. The space between the secondary lamellae is limited (in trout it is about 20 μm), and water is usually forced to pass tips of the primary lamellae when the respiratory water flow is high, thus, bypassing the respiratory surface. (Boyd et al. 1998, 245-281) These reactions are quite enough to compensate for normal fluctuations in energy demand of fish and dissolved oxygen concentrations in the water. One of the consequences, however, of increased ventilation rate is that there will be an increase in the amount of toxic substances in the water reaching the surface of the gill where they can be absorbed. (Svobodova et al. 1933, 13-45)

3.3 Effects of oxygen level on growth of fish

The successful production of fish depends on good levels of oxygen in the water. Oxygen is required for respiration process in fish to maintain healthy fish and bacteria which decompose fish production waste and to meet the biological oxygen demand within culture system. Dissolved oxygen levels can affect respiration of fish, as well as ammonia and nitrite toxicity. When the oxygen level is near saturation or even near super saturation, it increases growth, reduces feed conversion ratio and increases in volumes of production of fish.

Oxygen plays a decisive role in breathing and metabolism of animals. In fish, the metabolic rate is heavily dependent on oxygen concentration in the environment. If the level of dissolved oxygen is reduced, breathing, and nutrition activities are also reduced. As a result, the growth rate is declining and the diseases increases. The fish are unable to digest the food where the oxygen level is low. (Wedemeyer 1996, 8-18)

Several studies have examined the relationship between the saturation of oxygen and consumption of fish food. When oxygen levels fall below 60 % in the water, trout begins to lose appetite. (Jobling 1995, 37-38)

FIGURE 2. The effect of oxygen level on growth and food conversion ratios (based on Linde gas 2007)

Overall health and physiological conditions are better if dissolved oxygen is closer to saturation. When the level is lower, growth of fish can seriously suffer from increased stress. Tissue hypoxia declines in swimming activities and reduces resistance to disease. Therefore, it is necessary to maintain the dissolved oxygen at saturation level, which will not affect the physiological and metabolic activity to have a high performance in any culture. Moreover, it must be born in mind that the level of oxygen demand depends on the species, fish size and activity of the fish. (Wedemeyer 1996, 22-32)

4 Harmful water quality characteristics for rainbow trout

The following physical and chemical changes in the aquatic environment are the most frequently recorded as the underlying cause of harm to fish in trout farming. In this work the most significant changes in the quality of the water environment are described and the choice has been done based on the chemical analyses conducted in the company.

4.1 Temperature

Fish are poikilothermic animals that mean their body temperature is the same as the water in which they live or from 0.5 to 1 °C below or above the temperature of this water. Cold water fish, such as salmon and whitefish have a specific type of metabolism: their metabolic rate may continue at relatively low temperatures, otherwise at high temperatures, usually above 20 °C they consume less food and become less active. Water temperature also has a strong influence on the initiation and course of the fish diseases. The immune system of most fish species has optimal performance at a water temperature about 15 °C. (Schmidt-Nielsen 1991, 382-412)

In fish natural habitats, the fish can easily bear seasonal temperature changes, for example, drop to 0 °C in winter and up to 20 – 30 °C (depending on species) in summer under central European conditions. However, these changes should not be sharp; thermal shock occurs when the fish are placed in a new environment where the temperature difference is 12 °C warmer or colder (8 °C in the case of trout), compared to original water temperature. In these circumstances fish may die with symptoms of paralysis of the respiration. With young fry problems can arise even when the temperature difference is only 1.5 – 3 °C. If the fish are fed, and then drastically transferred to the water cooler by 8 °C or more, fish digestive processes will slow down or stop. (Svobodova et al. 1993, 13-39) The fed granules remain of undigested or half-digested in the gastrointestinal tract, and the formed gases can cause the fish to lose balance, bloat and eventually die. If the trout are given a high nitrogen feed (for example, high protein granules), a dramatic transfer to much colder water significantly increases levels of ammonia nitrogen in the blood serum, as the slowdown in the rate of metabolism reduces the diffusion of ammonia from the gills. This can lead to ammonia self-poisoning and death. (Finstad et al. 1988, 317-330)

Significant progress has been made in recent years in warm water fishery. Methods for controlling water temperature will be maintained in optimal conditions, so that the fish can take full advantage of their growth potential to achieve maximum weight gain.

4.2 pH of water

The optimal pH range for fish is from 6.5 to 8.5. Alkalinity above pH 9.2 and acidity below pH 4.8 can damage and kill salmonids. So salmonids are more vulnerable to high pH and more resistant to low pH. Low pH water most often occurs in spring, especially when the acidified snow melts, and drainage of peat lands flow into water. Alkaline pH may occur in eutrophic waters, where green plants (green algae, blue-green algae) take significant quantities of CO2 during the day for intense photosynthetic activity. (Bosawowsky et al. 1994, 300-

318)

This process influences the buffering capacity of water, and pH can rise to 9.0 – 10.0 or even higher, if bicarbonate is adsorbed from waters of medium alkalinity. The pH of the water can also be changed when the mineral acids and hydroxides, or other acidic or alkaline substances discharge or fall into the water. (Wagner et al. 1997, 979-990)

As a protection against the effect of low or high pH water, fish can produce a high quantity of mucus on the skin and the inner side of the gill covers. Extremely high or low values of pH cause damage to the tissues of the fish, especially the gills and bleeding may occur in the gills and in the lower part of the body. Excess mucus, often with blood, can be seen in post mortem analysis on the skin and gills. (Bradley et al. 1985, 115-140)

The pH of the water also has a significant influence on the toxic effect of a number of other substances (e.g. ammonia, cyanides, hydrogen sulfide and heavy metals) on fish.

4.3 Dissolved oxygen

Oxygen diffuses into the water from the air, particularly where surface is turbulent. The second source of dissolved oxygen is the photosynthesis of aquatic plants. Oxygen is removed with the help of aerobic decomposition of organic matter by bacteria and respiration of all organisms present in the water as previously mentioned. The concentration of dissolved oxygen in the water can be expressed in mg per liter, or as a percentage of the value of air saturation. Water temperature, air pressure and dissolved salts in the water must be taken into account when the mg per liter is converted to % of saturation or vice versa. (Davies et al. 1994, 1215-1354)

Different species have different requirements for the concentration of dissolved oxygen in the water. Salmonids have higher requirements for oxygen in the water, and their optimal concentration is 8 - 10 mg\l. If the level drops below 3 mg per liter, the fish begin to show signs of choking. (Svobodova et al. 1993, 120-125)

Oxygen requirements for fish also depend on many other factors, including temperature, pH and CO2 from the water level and the metabolic rate of the fish. The main criteria of oxygen demand of fish include temperature and the average individual weight of fish and the total weight of fish per volume of water. Oxygen requirements increase at higher temperatures (e.g. increase in water temperature from 10 to 20 °C the need for oxygen at least doubles). A higher total weight of fish per volume of water can result in increased activity and, therefore, increases fish breathing as a result of the overflow. (Svobodova et al. 1993, 127-132)

As noted earlier, the factor, most often causing a significant reduction in the concentration of oxygen in the water, is the contamination by biodegradable organic substances. These substances are decomposed by bacteria that use oxygen from the water for this process. A number of chemical substances can be oxidized in the absence of bacteria. The organic substances in water in terms of their capacity to consume oxygen out of the water can be measured by chemical oxygen demand (COD) and biochemical oxygen demand (BOD) within five days. For salmonids optimal levels are up to 10 mg O2\l for COD and 5 mg O2\l for BOD. (Svobodova et al. 1993, 145-168)

The lack of oxygen causes choking and fish will die, depending on the oxygen consumption of species and a lesser extent on the rate of their adaptation. The main pathologico-anatomical changes include unusually pale skin color, the accumulation of blood in the gills, adherence of the gill lamellae, slight bleeding in the front part of the eye cavity and the skin of the gill covers. (Yovita 2007, 62-83)

Corrective actions reduce access of degradable materials or the aeration of water. The latter is usually the best option. Aeration can be done by air or oxygen pumps, by spraying water into the air, like fountain system or by increasing the input of aerated water. It should be remembered that these corrective measures are most effective at night when the oxygen deficiency is the highest.

Damage caused to fish by too much dissolved oxygen in water is rare. However, this can happen, for example, when the fish is transported in plastic bags with oxygen filled air space. Critical oxygen level in water ranges from 250 to 300 per cent of the value of air saturation and the fish may suffer from higher values. When this fish is used for stocking they may suffer from secondary infections, fungi and some of them may die. The fish adapted to such a high level of oxygen should be gradually acclimated to normal concentrations. (Svobodova et al. 1993, 120-148)

4.4 Ammonia

Ammonia contamination of water can be organic or inorganic origin. In water or biological fluids ammonia is present in the molecular form (NH3) and the form of ammonia ion (NH4+). The relationship between these two forms depends on the pH and temperature of the water (see Table 3). The walls of the body's cells are relatively impermeable to the ammonia ion (NH4+) but molecular ammonia (NH3) can easily diffuse through the tissue barriers where there is a concentration gradient. Therefore, ammonia is potentially toxic form for fish. (Thurston et al. 1981, 981-993) In addition, under normal conditions there is the acid-base balance in the water-tissue interface. If this balance is changed, the side which the pH is lower will attract additional molecular ammonia. This justifies how molecular ammonia penetrates from water through the epithelium of gills in the blood and how it moves from the blood into the tissues. Ammonia has a harmful effect on the brain that is why nerve symptoms are pronounced in cases of ammonia toxicity to fish. (Svobodova et al. 1993, 152-167)

TABLE 3. The total ammonia content as % in water in different temperature and pH values (based on Svobodova et al. 1993, 162)

pH

T, °C

0

5

10

15

20

25

7.0

0.082

0.12

0.175

0.26

0.37

0.55

7.2

0.13

0.19

0.28

0.41

0.59

0.86

7.4

0.21

0.30

0.44

0.64

0.94

1.36

7.6

0.33

0.48

0.69

1.01

1.47

2.14

7.8

0.52

0.75

1.09

1.60

2.32

3.35

8.0

0.82

1.19

1.73

2.51

3.62

5.21

8.2

1.29

1.87

2.71

3.91

5.62

8.01

8.4

2.02

2.93

4.23

6.06

8.63

12.13

8.6

3.17

4.57

6.54

9.28

13.02

17.95

8.8

4.93

7.05

9.98

13.95

19.17

25.75

9.0

7.60

10.73

14.95

20.45

27.32

35.46

9.2

11.53

16.00

21.79

28.95

37.33

46.55

9.4

17.12

23.19

30.36

39.23

48.56

57.99

9.6

24.66

32.37

41.17

50.58

59.94

68.63

9.8

34.16

43.14

52.59

61.86

70.34

77.62

10.0

45.12

54.59

63.74

71.99

78.98

84.60

10.2

56.58

65.58

73.59

80.29

85.63

89.70

10.4

67.38

75.12

81.54

86.59

90.42

93.24

11.0

89.16

92.32

94.62

96.26

97.41

98.21

Finally, table 3 shows that the concentration of ammonia is dependent on the acidity of the water and the temperature. Therefore, toxicity would be much more in warm alkaline waters than in the cold acid waters.

In addition to water temperature and pH, there are other factors that affect the toxicity of ammonia. Such factors include the dissolved oxygen concentration in water, the lower the concentration of oxygen in the water is, the greater the toxicity of ammonia will be.

To a small extent, the toxicity of ammonia is dependent on the number of free CO2 in water. This is because diffusion of respiratory CO2 lowers the pH of water on the surface of gill, thereby reducing the amount of abnormal ammonia inside. Reduction of pH depends on the quantity of CO2 that is already presented in the water.

It should be noted that all of the standards of acceptable values apply to ammonia as a toxic substance. Other general standards for ammonia are used for control of eutrophication of water and prevent the excessive growth of algae and plants that can cause physical problems and influence the oxygen balance in the environment.

The first signs of ammonia toxicity display a slight anxiety, increased respiration. The fish are close to the surface of the water. In later stages, the fish gasp, anxiety grows with acceleration of movement and breathing becomes irregular followed by the stage of intense activity. Finally, the fish start rapidly responding to external stimuli, losing balance, jumping out of the water, their muscles are reduced in convulsions. Infected fish are lying on their sides, frantically and widely opening their mouths. Then a short recovery period begins and fish returns to regular swimming and seems a little calmer. This phase is replaced by another period of intense activity, in the end, the surface of the body becomes pale and the fish die. (Thurston et al. 1981, 744-812)

Fish poisoned by ammonia have a light skin color and is covered with a thick layer of mucus. In some cases, there is small hemorrhage mostly based on the pectoral fins and in front of the ocular cavity. The gills are seriously overloaded and contain significant amounts of mucus. If fish are exposed to high concentrations of ammonia that cause severe bleeding from the gills. The sticky mucus can be seen on the inner side of the gill covers. (Thurston et al. 1981, 783-785)

In recent years significant losses among cultivated trout were caused by toxic necrosis of the gills. Factors that affect the development of this disease include ammonia poisoning in which the amount of ammonia in the blood considerably increases. As has been noted earlier ammonia is the end product of nitrogen metabolism in fish and most of it goes through the gills in the water. If the rate of diffusion is reduced to one reason or another (oxygen deficiency, high pH water, damage to the gills, etc.), the level of ammonia in the blood will rise, leading to a condition known as self-poisoning which can lead to toxic necrosis of trout gills. (Solbe et al. 1989, 112-128)

4.5 Nitrites and nitrates

Nitrites are usually together with nitrates and ammonia nitrogen found in the surface waters, but their concentrations are low due to their instability. They are easily oxidized to nitrates or reduced to ammonia chemically and biochemically using bacteria. Nitrates are the end product of aerobic decomposition of organic nitrogen compounds. In all surface waters nitrates are present in low concentrations. Practically nitrate is not present in the soil since it is easily washed away into reservoirs, ponds and lakes. The main source of nitrate contamination in the surface waters is the use of nitrogen fertilizers and manures on arable land and the dumping of sewage from the treatment plants. (Brune et al. 1991, 412-425)

Nitrites can be associated with the concentration of ammonia in the water. Ammonia is oxidized to nitrite and nitrate by using two separate bacterium actions regularly in normal aerobic conditions. Nitrite concentration will increase if the second stage of oxidation is inhabited by bactericidal chemicals in the water. (Russo et al. 1991, 45-59)

Toxic effect of nitrite on fish is not fully known. It depends on a number of internal and external factors (e.g. fish species and age, and the overall quality of the water). Value and role of these factors have been studied and analyzed for general comparison. Different authors often come to contradictory conclusions and usually do not provide a definite explanation or mechanisms of nitrite effected harmfully on fish or change the effects of various environmental factors. (Westin 1974, 79-85)

It has long been known that the nitrite ions penetrate into fish through the chloride cells in gills. In the blood of fish nitrites are closely related to hemoglobin structure resulting in reduced transportation of the oxygen capacity of the blood. The increase in methaemoglobin can be seen as brown color of gills and blood. If its amount in the blood does not exceed 50 % of the total hemoglobin fish usually survive. If the fish have more methaemoglobin in the blood (70 – 80 %) they are torpid and with the further increase in the level of methaemoglobin they lose direction and may not respond to stimuli. (Svobodova et al. 1993, 135-138) However, the fish may still be alive because the red blood cells in the blood contain an enzyme that can convert methaemoglobin into special cover. This process can return to a normal level of hemoglobin within 24 - 48 hours if the fish is placed in nitrite-free water. (Westin 1974, 89)

It is known that chloride in gills of fish cannot distinguish form nitrite and both ions move across the gill epithelium. The level of nitrite uptake depends, therefore, on the nitrite-chloride ratio in the water.

Nitrite toxicity may also be influenced by bicarbonate, sodium, potassium, calcium and other ions but their impact is not as great as from the chloride. Among them, potassium is the most significant, whereas, sodium and calcium have a smaller effect. The monovalent ions are also involved in ion fluxes through the epithelium of gills and directly or indirectly affect the uptake of nitrites. The pH value is also considered being important for nitrite toxicity, pH and temperature affect the dissociation of HNO2 into NO2. The uptake of nitrites in blood plasma of fish depends on the diffusion of non-dissociated HNO2 through the gill epithelium. However, experimental results later disproved this theory and found out that within the acidity-alkalinity range encountered in natural waters pH was of little significance in nitrite toxicity. (Solbe et al. 1989, 130-133)

Another factor that affects the toxicity of nitrite is the concentration of dissolved oxygen and temperature. This is because the fish need water with high concentrations of oxygen when oxygen carrying capacity of blood decreases by the formation of methaemoglobin and oxygen demand of fish increases with increasing temperature. Prolonged exposure to sub lethal concentrations of nitrites do not do much damage to the fish. Concentrations 20 – 40 % of the minimum levels with lethal effect on fish can slightly depress growth but no serious damage was ever observed.

To evaluate the safe concentration of nitrite for different places, the ratio of chlorides to nitrite must be measured. These relationships should not be less than 17 for rainbow trout. Instead, nitrate toxicity for fish is exceedingly low, and mortality were only recorded where the concentration exceeded 1000 mg/l. 20 mg/l is considered the maximum allowable concentration of nitrates for rainbow trout. In fish farms where the water contains enough oxygen without risk of denitrification, the concentration of nitrates does not need to be monitored. However, like ammonia, water quality standards for nitrates should be installed to prevent eutrophication and excessive growth of plants and algae which can have a negative effect on the fish. (Solbe et al. 1989, 138-142)

4.6 Hydrogen sulphide

Hydrogen sulphide (H2S) in polluted waters is a result of the decomposition of proteins. It is also available in industrial effluents including those from chemical and metallurgical plants, and pulp mills. Hydrogen sulfide has from high to extremely high toxicity to fish. A dangerous concentration for salmonids is 0.4 mg/l. The toxicity of H2S decreases with increasing of the pH of the water. The concentration of non-dissociated H2S is transformed into less toxic HS ions. Hydrogen sulphide may form from organic rich muds and goes to the overlying waters together with other gases (such as carbon dioxide and methane) formed by the anaerobic decomposition. In aerobic waters H2S quickly oxidized to sulfates. However, it is possible that fish can be exposed to hydrogen sulfide near the surface of such solutions. (Hellawell 1986, 488-495)

4.7 Carbon dioxide

Carbon dioxide in water is dissolved in the gaseous state and only 10 % is in the form of carbonic acid (H2CO3). These two forms of carbon dioxide together constitute what is called free CO2. Carbonate ions and bicarbonate (CO32-and HCO3-- respectively) are fixed carbon dioxide. Their presence is necessary for the buffering capacity of water. The quantity of CO2 in the flowing surface water is usually in the order of a few mg/l, and rarely rises above 20 - 30 mg/l. (Svobodova et al. 1993, 215-220) In the standing surface water CO2 emission level is stratified because of photosynthetic assimilation of phytoplankton. Upper layers usually have less CO2 than the lower layers. If all free CO2 in the surface layers is used for photosynthesis the pH of the water can rise above 8.3 and originally moderate alkaline water rises up to pH 10.0. Ground water from limestone or chalk layers typically contains a few tens of milligrams per liter of free CO2, and it may be relevant, where the water is used for fish farming. (Colt et al. 1991, 372-385)

There are direct and indirect toxic effects of carbon dioxide. Indirect effect associated with CO2 affects fish through its effect on the pH of the water, particularly as described earlier the values rise to toxic levels. In addition, changes in pH affects the toxicity of chemicals that exist in dissociated and non-dissociated forms of which only one is non-toxic, such as H2S and ammonia. (Svobodova et al. 1993, 235-240)

Direct negative impact occurs when there is an excess or deficiency of free CO2. In water with low oxygen content, for example, in case of intensive biodegradation or wherever the fish is stored or transported in a high density or when farm use poorly aerated water free CO2 could reach a dangerous level. In such cases the diffusion of CO2 from the blood of fish into the respiratory water is reduced and then CO2 in the blood rises and acidosis is developed. If the CO2 concentration growth is relatively slow, fish can adapt to acidosis due to the increase of the bicarbonate concentration in the blood. Adapted fish can suffer from alkalinosis in case of return to the water with low content of CO2. (Svobodova et al. 1993, 241-243)

In the water with low oxygen and high CO2 concentration where gaseous exchange in the respiratory surface is restricted, fish increase ventilation rate, the fish become restless, lose balance and can die. 20 mg/l of free CO2 is considered to be the maximum allowable concentration for trout. Sensitivity of fish to the free carbon dioxide decreases with increasing acid capacity of water. (Svobodova et al. 1993, 255-258)

However, more frequent is the lack of free carbon dioxide in the water. Carbon dioxide deficiency occurs when too much free CO2 is used for photosynthetic activity of phytoplankton. Low carbon dioxide concentration below 1 mg/l affect the acid-base balance in the blood and tissues of fish and cause alkalosis. A low partial pressure of free CO2 in the water leads to a high rate of diffusion of CO2 from the body which leads to alkalosis and death. (Alabaster et al. 1980, 135-143)

4.8 Iron

Iron is found in the surface waters in ferric state (Fe3+, mostly insoluble compounds) or ferrous state (Fe2+, soluble compounds). The ratio of these two forms of iron depends on the concentration of oxygen in the water and pH as well as other chemical properties of water. The fish may suffer from iron compounds in poorly oxygenated water with low pH where the iron is present mostly in the form of soluble compounds. Because the surface of the fish gills tends to be alkaline, soluble bivalent iron can be oxidized to insoluble ferric compounds which cover gills lamellar and respiration. (Svobodova et al. 1993, 155-178) At a low temperature of the water and in the presence of iron, iron-depositing bacteria multiply rapidly at the gills and further oxidize iron compounds. Their filamentous colonies cover gills, which at first are colorless but eventually the residue of iron gives them brown color. The precipitated iron compounds and tufts of the iron bacteria reduce the space in the gills and damage the respiratory epithelium, thereby, suffocating fish. (Roch at al. 1984, 58-65)

Lethal concentration of iron for fish is difficult to define because this value depends to a great extent on the physical and chemical properties of the water. The concentration of soluble forms of ionized iron should not exceed 0.1 mg/l for salmonids culture. (Roch at al. 1984, 77-79)

Summary

The factors considered in this part of the work may occur in the natural environment and can be enhanced by human activities. The fish have a limited ability to adapt to the changes in these factors if they happen slowly enough but a rapid change can be harmful. If fish suffer to some extent from these changes, full recovery is possible by returning to normal place with better conditions. If irreparable damage has been inflicted on the tissues of fish, there are likely to be long-term implications for their health.

5 Company profile

The company "Kala ja marjapojat", created in 1992, is engaged in cultivation, processing, distribution and sale of rainbow trout. Nowadays this is the biggest company in Russia in growing and processing of rainbow trout and the head office is located in the town of Kostomuksha, Karelia. Cultivation is carried out on five trout farms located on the clean lakes in Northern Karelia (Kuyto and Nyuk) in the near future the company has plans to create two more farms. At the moment overall output of the rainbow trout is 1200 tons per year. By 2015 it is planned to achieve the volume of 2000 tons per year. The cooperation takes place with Finnish companies as well as with Moscow region, St. Petersburg and other Russian regions.

The production cycle of "Kala ja marjapojat" is based on the traditions and practices of Finnish trout farmers. The primary attention is paid to correct feeding of trout, control of growth, timely sorting and changing cages.

The company always strives for environmental technologies and for improving its products through the introduction of new technologies. The boiler that works on fish oil is located on the main base of "Kala ja marjapojat". It is important to admit that the boiler heats the whole base. This system is used for the purpose of environmentally friendly technologies.

The quality and relevance of products "Kala ja marjapojat" is caused by the following factors:

Sustainable use of planting material from Finnish and domestic suppliers;

Food for the fish is supplied by the leading Finnish manufacturer "Raisioagro";

Continuous monitoring of the environmental conditions and the state of the grown product;

Constant veterinary supervision;

Qualified personnel;

Availability of advanced fleet, refrigerators and freezing chamber with capacity up to 1000 tons;

All products are certified and delivered in high quality disposable packaging;

Recycling in modern manufacturing plants with the equipment conforming European standards.

6 Study area

The deployment area of the studied cages is located at a distance of 50 – 55 km to the south-east of the town of Kostomuksha in the western part of the lake Nyuk, the distance of about 5 km south-east of the mouth of the river Aittayoki. The coordinates of module cages are 64025.9’N and 31021.7’E, 64025.8’N and 31021.8’E. It is possible to reach studied modules only by water. (The state committee for stand 1933)

Lake Nyuk belongs to the basin of the river Chirko-Kem, which is the tributary of the river Kem. The area of the lake is 214 km2, the maximum length is 39 km and the average width is 5.5 km and the widest part of the lake is 22.5 km. (The surface water resources in the USSR 1972, 115-116)

The maximum depth of the lake is 40 m, the average 8.5 m, the volume of the water is 1.8 km3, the height above the sea level is 134.5 m and the catchment area is 3090 km2. On the lake there are 126 islands with a total area of 10.3 km2. (The surface water resources in the USSR 1972, 120-122)

The lake bottom is highly uneven and is replete with numerous depressions and elevations. Aquatic vegetation is unusually poor and occurs only in the shallow waters and the coastal zone. The lake belongs to the waters of the highest category as it is inhabited with animals with high sensitivity to oxygen content. In the past this area was used as permanent fishing in large volumes. Today the lake is used for recreational fishing and rainbow trout farming. Therefore, the settlements on the shores do not pour wastewater into the lake. (The surface water resources in the USSR 1972, 120-131)

The cages for the analysis of data are located in the bay. The bay length is about 7 km and a maximum width is about 2 km (see Appendix 3). The greatest depths are in the southern part of the bay. This place has two modules with cages within the distance of 500 m from each other. The sensor for water quality control is on one of the modules (see Appendix 3).

6.1 Climate

The climate of Karelia is characterized by long winters and short cool summers, considerable cloudiness, high humidity and plenty of rainfalls and snowfalls throughout the year. These climatic features are caused by geographical location the proximity of the Baltic, the White and the Barents seas and the effect of the transfer of air masses from the Atlantic Ocean and the Arctic regions with the preponderance of cyclonic activity. Both winter and summer cyclones bring windy and overcast conditions (175 – 195 cloudy days per year). (Natural Resources and Ecology of Karelian Republic 2007, 48-50)

Invasion of air masses from the Atlantic Ocean in winter follows by warm temperatures and sometimes by strong thaw with heavy snowfalls. Frequent invasion of arctic air at this time causes drastic frosts. Summer cyclones bring a decrease in temperature and rainfall. Mild weather is replaced with cold. (Climate handbook in the USSR 1968, 144-149)

The territory with the cages is located in the severe climate, with late spring (the end of May – the beginning of June) and early autumn (the second half of August). (Natural Resources and Ecology of Karelian Republic 2007, 82) The period with a temperature above 10 °C is at least 75 – 85 days a year. On the territory the longest duration of daylight in summer is more than 20 hours in June and July. (Climate handbook in the USSR 1968, 183-185)

6.2 Air temperature

The average annual temperature in the study area is 0.5 °C. The coldest months are January and February; their average monthly temperature is equal to -12 °C. Air temperature falling to -40 °C is observed once or twice in 10 years. The warmest month is July with an average temperature of 15 °C. The absolute maximum of air temperature is 31 - 32 °C. (Climate handbook in the USSR 1968, 177-180)

TABLE 4. Average monthly, the absolute maximum and minimum air temperatures (based on Salo et al. 2012)

Description

January

February

March

April

May

June

July

August

September

October

November

December

The average monthly

-12

-12.1

-8.3

-1.3

5.3

11.6

15

12.8

7.4

1.2

-4.5

-9

Absolute minimum

-43

-50

-43

-34

-13

-5

0

-3

-9

-24

-38

-45

Absolute maximum

7

6

12

21

29

31

31

29

24

16

9

8

6.3 Rainfalls

The territory under the question belongs to the zone of excessive humidity. This explains the relatively low heat and well-developed cyclonic activity. Average annual precipitation is 586 mm. The annual distribution is given below. (Climate handbook in the USSR 1968, 195-213)

TABLE 5. The annual distribution of rainfall in mm (based on Salo et al. 2012)

Month

January

February

March

April

May

June

July

August

September

October

November

December

Sum of rainfalls

43

36

31

31

40

65

69

70

61

50

48

42

The average date of formation and destruction of the snow layer and a number of days per year with snow layer are shown in Table 6.

TABLE 6. Date of formation of the snow cover and the number of days per year with snow layer (based on Salo et al. 2012)

Number of days with snow layer

Date of occurrence of snow layer

Date of stable snow layer

Date of breakdown of the stable snow layer

Date of disappearance of snow layer

179

19 October

12 November

29 April

14 May

During the winter, the greatest snow depth is 57 cm with an absolute maximum of 94 cm and a minimum depth of 44 cm.

TABLE 7. Snow depth (based on Salo et al. 2012)

Month

November

December

January

February

March

April

Part of the month

1

2

3

1

2

3

1

2

3

1

2

3

1

2

3

1

2

3

Depth in cm

6

13

17

24

28

32

37

42

47

50

53

56

57

57

57

49

31

16

6.4 Wind

According to the wind rose of the study area, in this area there is no significant predominance of any wind direction. In the period between the beginning of May and the end of October the average monthly wind speed is 4 m/s and the average number of calm days is not more than 15 %.

7 Process of fish growing and feeding in "kala ja marjapojat"

Various designs of cages can be used at the trout cultivation farm. The company "Kala ja marjapojat" uses polygonal cages with depth of 7 m and the volume of 1450 m3 for growing commodity trout. The most viable option of support structure can withstand the wind and wave loads. The integral framework is welded from polyethylene pipes with a diameter up to 500 mm.

Optimal place for cultivation of the fish in the cages in spring is the coastal area of lake Nyuk where the water is warmed faster than in the central part. The flowing areas of the lake, where the cages are located, improve oxygen conditions and reduce water pollution. However, the flow rate should not exceed the level of 1.5 the length of fish or 0.5 m/s. (Neville 1979, 84-87)

When cultivating commodity fish in cages it is necessary to consider that the iridescent trout is physostomous fish. Therefore, it needs to rise to the surface to breathe in the air. In this case, the access of the fish to the air is constantly provided in the cages. In addition, to protect the trout from the birds that not only eat but also hurt and damage the fish, the nets are stretched over the cages.

The whole processes consist of the total care of cages, fish feeding, and monitoring oxygen regime and water temperature, controlling the growth of the fish, removing dead fish.

Trout feeding

When using dry granules for feeding the trout it is necessary to consider not only their quality but also the sizes of granules. Granule sizes should match the fish mass. Under or overestimation of the size of the granules will usually lead to the slowdown of the growth rate of the trout and the increase in metabolic cost, loss of food which eventually reduces the effect of the growing rate. (Kindschi et al. 1991, 197-200)

One of the major operations for commercial fish farms is the frequency of feeding during the day. From the start of feeding it is necessary to set up a timetable and stick to it while feeding throughout the growth of the fish. Frequency of feeding of studied fish is twice a day from 10 to 12 am (60 % of daily volume) and 18 to 19 pm (40 % of daily volume).

In the company "Kala ja marjapojat" feeding is done by hand because the feed is distributed more evenly over the whole area of the cages. In this case, visual monitoring of fish behavior and their need for food is possible. Proper and careful hand-feeding reduces the degree of inequality in the distribution of the weight of fish in cages.

Another important parameter to be considered for trout feeding is the dissolved oxygen in the water often given as the saturation level in percent. The fish constantly needs oxygen for biotic processes. As a result of food consumption it is self-sufficient in nutrients and essential biotic energy which is released by the oxidation-reduction processes occurring in the body with the direct participation of oxygen. (Kindschi et al. 1991, 209-218)

During the feeding process oxygen consumption increases dramatically due to the increased activity of the processes associated with digestion, absorption in the gut and activation of metabolism. The more nutrients absorbed by the body of fish the more oxygen is required for this. In the water the level of oxygen saturation changes depending on the temperature of the water (see Table 8). (Beveridge et al. 1996, 97-98)

TABLE 8. Solubility of oxygen in water (mg/l) different temperatures at 100 % saturation (based on Beveridge et al. 1996, 105)

t, °C

O2

t, °C

O2

0

14.6

12

10.8

1

14.2

13

10.6

2

13.8

14

10.4

3

13.5

15

10.2

4

13.1

16

10.0

5

12.8

17

9.7

6

12.5

18

9.5

7

12.2

19

9.4

8

11.9

20

9.2

9

11.6

21

9.0

10

11.3

22

8.8

11

11.1

23

8.7

However, it should be noted that in freshwater absolute saturation of water practically almost never occurs. Commonly, for satisfaction of requirements of the trout a maximum level of water saturation is not required.

When using modern feed granules, potential risk is overeating by fish, leading to significant loss of feed, poor absorption and as a result reduces the rate of trout growth. Hungry trout eats more food than it needs for efficient growth which leads to overeating and reduces the appetite for the next day. Next day it becomes so hungry that again the risk of overeating rises. This situation can be repeated again and again. As a result, the body of fish is weakened so that it will not effectively absorb nutrients resulting in inefficient use of feed granules. The feeding process in "Kala ja marjapojat" is done with feeding tables from the company "Raisioagro" which provides dry food granules (see Appendix 2).

Energy value of food

Efficiency of fish growing is measured by feed conversion ratio. Feed conversion ratio (FCR) is calculated as the ratio of the mass of food consumed by increase of fish weight during this time:

To assess the development of fish in natural environment, the use of this indicator is suitable since the chemical composition of the food and the body of fish is similar. When feeding fish with pellets, the water concentration in the feed varies from 6 to 7. This means that the amount of feed conversion ratio is underestimated equally. Therefore, the feeding rate (FR) should be used in assessing the effectiveness of fish feeding. It shows the ratio of the weight of the fish feed to the general increase in biomass of fish in a given time:

Feeding rate s