Fish survival in a good state of health during transport is influenced by a number of factors, or combination of factors.
The quality of fish transported is a decisive criterion. The fish to be transported must be healthy and in good condition. Weakened individuals should be eliminated from the consignment, particularly when the temperature during shipment is high. When the fish are of poor quality, even a great reduction of fish density in the transport container fails to prevent fish losses. Weak fish are killed at a much higher rate than fish in good condition when the transport time is longer.
A need for adapting the fish to a lower water temperature may also arise before transport. Natural ice is used to cool the water; the ice of carbonic acid should be avoided. As a guide ratio, 25 kg of ice will cool 1 000 litres of water by 2°C. If the water contains fish during the cooling process, the temperature drop should not be faster than 5°C per hour. Direct contact of fish with ice should be prevented at the same time. The total temperature difference should not be greater than 12–15°C, with respect to the species and age of the fish (FRG recommendation, 1979).
The fish to be transported, except for the larval stages should be left to starve for at least a day; if the digestive tract of the fish is not totally cleaned, the possible time of transport is reduced to a half, though the conditions may be the same (Pecha, Berka and Kouril, 1983; Orlov et al., 1974). The fish with full digestive tracts also need more oxygen, are more susceptible to stress, and produce excrements which take up much of the oxygen of the water. However, when fish larvae are transported, their time of survival without food should be taken into consideration. The transport time of the larvae of herbivorous fishes should not last longer than 20 hours and that of many aquarium species should be shorter than 12 hours (Orlov, 1971).
The most important single factor in transporting fish is providing an adequate level of dissolved oxygen. However, an abundance of oxygen within a tank does not necessarily indicate that the fish are in good condition. The ability of fish to use oxygen depends on their tolerance to stress, water temperature, pH, and concentrations of carbon dioxide and metabolic products such as ammonia.
The crucial factors underlying oxygen consumption by fish in relation with oxygen metabolism during transport are fish weight and water temperature. Heavier fish and those transported in warmer water need more oxygen. For instance, if the water temperature increases by 10°C (e.g., from 10 to 20°C), oxygen consumption is about doubled. From the point of view of fish transport, for each 0.5°C rise in temperature, the fish load should be reduced by about 5.6%; conversely, for each 0.5°C decrease in temperature, the load can be increased by about 5.6% (Piper et al., 1982). Oxygen consumption also increases with fish excitement by handling. Excitement increases oxygen demand three to five times and, for instance, salmonid fry need up to several hours to return to the normal level of oxygen metabolism which is, in fact, usually after the end of the transport (Lusk and Krcál, 1974).
In water provided with an unlimited amount of oxygen, a fish at rest will consume a minimum amount of oxygen. In a fish transport system, the fish will require more than the minimum amount since they are not at rest. Furthermore, if they are excited at loading or disturbed during transport they may consume near to the maximum amount.
The amount of oxygen a fish consumes also depends on the amount of oxygen available. At high levels, the fish will consume at a steady rate. When water oxygen levels are low, fish consume lower amounts of oxygen than when oxygen levels are high, despite the degree of activity.
Fish transport systems often contain water with oxygen levels that do not provide enough oxygen required to satisfy the fish bodies. To offset this predicament, the fish will shift its metabolism to use the stored oxygen of the body. This condition is likened to that of a man who is at rest and suddenly performs strenuous activity before a proportionate amount of oxygen is taken in. For the man and the fish, an oxygen “debt” is created which must be repaid when favourable oxygen conditions are experienced.
The first hour after loading is a particularly critical time for fish in respect to their oxygen needs. They are excited and require a large amount of oxygen with a short time for adjustment. Significant differences in oxygen demand exist also within fish families. As asserted, for instance, by Uryn (1971), when water temperature increases (4–14°C) during transport, the fry of Coregonus lavaretus consume 2.4 times more oxygen than the fry of C. albula. Fish size is also important. A large fish consumes less oxygen per unit weight than does a small one. Oxygen levels of water for most warm water fish should be above 5 mg.1-1 for normal conditions. This level should prevent oxygen from becoming a major stress factor.
Some conversion coefficients of oxygen demand are indicated by the FRG recommendation (1979): 25 kg of rainbow trout at an individual weight of 250 g have the same oxygen demand as 20 kg of 12 cm stock trout (1 100 fish), or 17 kg of 8 cm stock trout (3 200 fish), or 12 kg of forced fry at the length of 4 cm (ca. 23 000 fish). Taking the oxygen demand of carp as 1, the converted oxygen demand levels for other fishes are as follows:
According to Shevchenko (1978), the oxygen consumption of Coregonus peled per kg of weight per hour at a temperature of 10°C is 100 mg; in sturgeon this value is 68 mg, in pike 50–60 mg, and in carp weighing 500–600 g it is 45 mg.
During fish transport in closed systems with pressurized oxygen atmosphere, oxygen content in water usually is not a limiting factor because there is enough pressurized oxygen in a closed bag. Oxygen deficit may occur in exceptional cases when the density of fish is too high or the transport is longer than the fish can stand. The dead fish compete with the living ones for oxygen: they increase bacterial multiplication requiring much oxygen, and this multiplication may further produce toxic metabolites. The slime produced by the fish is another substrate for bacterial growth resulting in a decrease of the water oxygen content; this process is intensified when water temperature is higher.
A high oxygen content of water has no unfavourable influence on the fish, e.g., the limit for rainbow trout is 35 mg per litre which is not attainable in practical conditions, as asserted by Heiner (1983); the fish are able to regulate the volume of oxygen entering their bodies. This holds generally with possible exceptions; for the time being there are no data on the effect of a longer exposure to a high oxygen content at a higher temperature in the larval stages of fish unable to keep oxygen content of their blood at an optimum level.
In closed systems, slight shaking of the bag supports the penetration of atmospheric oxygen to water. During long steps when the bags with fish are left without movement, the fish may die though the oxygen reserve in the bag is still high. This applies mainly to dense stocks of salmonids, requiring much oxygen; no such problems are encountered when cyprinids, except their sac fry, are transported, because these fish move the water in the bag by their own movement, thus driving it into sufficient contact with the oxygenated atmosphere. The time of the onset of the threshold oxygen concentrations during salmonid fry transport in closed bags left without movement is suggested in Table 1 (Orlov et al., 1974).
Water quality is a function of the load of fish concentration and the length of time for which the fish are transported. The source of the water used during transport must have been tested before dispatching a mass consignment of fish. The water pH level is a control factor because the proportions of toxic ammonia and CO2 contents are direct functions of pH (Fig. 1).
With increasing transport time, CO2 production through fish respiration shifts water pH towards acidity. Water pH levels about 7–8 are considered as optimum. Rapid changes in pH stress fish, but buffers can be used to stabilize the water pH during fish transport. The organic buffer trishydroxylmethylaminomethane is quite effective in fresh and salt water. It is highly soluble, stable and easily applied. This buffer has been used on 29 species of fish with no deleterious effects. Levels of 1.3–2.6 g/litre are recommended for routine transport of fish (Piper et al., 1982).
The onset of the threshold concentrations of oxygen during salmonid fry transport in closed bags without movement (in hours)
|Individual average weight|
|Total weight of fish|
Figure 1 Proportion of each chemical species of ammonia and carbon dioxide expressed as a percentage at various pH levels (Amend et al., 1982)
Elevated carbon dioxide concentrations are detrimental to fish and can be a limiting factor in fish transport. A product of fish and bacterial respiration, CO2 acidifies transport water. Although this reduces the percentage of un-ionized ammonia in the water, it also reduces the oxygen-carrying capacity of fish blood. Fish may succumb if CO2 levels are high, even though oxygen levels are seemingly adequate. Trout appear to tolerate carbon dioxide at levels less than 15 mg 1-1 in the presence of reasonable oxygen and temperature, but become distressed when carbon dioxide levels approach 25 mg 1-1 (Piper et al., 1982).
Fish transported in tanks are exposed to gradually increasing concentrations of carbon dioxide. Unless aeration is adequate, CO2 levels may exceed 20–30 mg 1-1, in general, for each milliliter of oxygen a fish consumes, it produces approximately 0.9 milliliters of CO2. If the CO2 level increases rapidly, as with heavy fish loads, fish become distressed. However, elevated concentrations of CO2 can be tolerated if the rate of buildup is slow.
Adequate ventilation is a necessity for transport units. Tight covers or lids on the units can result in a buildup of CO2 which will stress the fish. Aeration of the water will reduce concentrations of dissolved CO2, if there is adequate ventilation.
As Pecha, Berka and Kouril (1983) assert, the critical CO2 concentrations in closed systems range about 140 ml/litre for thermophilous fish and about 40 ml/litre for those preferring cold conditions. Kruzhalina, Averina and Vol'nova (1970) give a closer specification of these critical CO2 concentrations in closed fish transport systems and suggest the following levels: 60–70 ml/litre for salmonids, 40 ml/litre for mature sturgeons and 20 ml/litre for sturgeon fry, 140–160 ml/litre for mature herbivourous fishes, 100 ml/litre for herbivorous fish fry and 80 ml/litre for the larvae of the herbivourous species. All these data hold for closed systems; in open systems CO2 is released from water by any system of aeration. When the fish concentration in the container of a closed system decreases, the critical CO2 concentration loses much of its importance.
Another important factor is chlorine concentration in water, although - like carbon dioxide - chlorine is also removed from the water by aeration. The concentration of 0.5 mg/litre is considered as dangerous, though even lower chlorine levels, e.g., 0.2 mg/litre disturb the fish respiration mechanism considerably (Shevchenko, 1978).
Ammonia (NH3) builds up in transport water due to protein metabolism of the fish and bacterial action on the waste. Decreasing metabolic rate of the fish by lowering the water temperature, and thus lessening fish activity, reduces the production of NH3. The production of NH3 by bacterial action can be decreased by shipping fish only after food has been witheld long enough to void the stomach and intestine.
Temperature and time of last feeding are important factors regulating ammonia excretion. For example, trout held in water at 1°C excrete 66% less ammonia than those held in 11°C water, and fish starved for 63 h before shipment produce half as much ammonia as recently fed fish. Fish larger than 10 cm should be starved at least 48 h; those 20 cm and larger should be starved 72 h (Piper et al., 1982).
The amount of un-ionized ammonia increases as water temperatures and pH increase (Table 2).
No maximum permissible values can be given because the toxicity of ammonia is so greatly affected by water temperature and pH. However, critical concentrations of toxic ammonia are scarcely obtained under standard fish transport conditions.
Water temperature is an important factor. When water temperature is low, the pH remains higher and fish metabolism decreases. The generally applicable zones of optimum temperatures for transported fish are 6–8°C for cold-water fishes and 10–12°C for warm-water fishes in summer, 3–5°C for cold-water fishes and 5–6°C for warm-water fishes in spring and autumn, and 1–2°C for all in winter. Naturally, these temperature ranges do not apply to the early stages of fish fry. The early fry of cyprinids cannot be transported at temperatures below 15°C, early fry of salmonids at temperatures higher than 15–20°C, and the temperature of 10°C, is considered as optimum for the early stages of the fry of coregonids (Pecha, Berka and Kouril, 1983; Orlov et al., 1971, 1974; Shevchenko, 1978).
Consideration should also be given to the factor of space. As to fry, the ratio of the volume of the fish transported and the transport water should not exceed 1:3. Heavier individuals, e.g., parent fish can be transported in a fish: water weight ratio of 1:2 to 1:3, but with smaller organisms this ratio decreases to 1:100 to 1:200 (Pecha, Berka and Kouril, 1983). In the FRG recommendation (1979), the following ratios between fish weight and the volume of water in the transport tank (with good aeration of water at a temperature of 8–12°C during shorter transports lasting (1–2 h) are table carp 1:1, stock carp 1:1.5, table rainbow trout 1:3, stock trout 1:4.5, stock pike 1:2, herbivorous fishes 1:2.
Percent un-ionized ammonia in water at 0 to 30°C and pH 6 to 10 (Emerson et al., 1975)
The conditions of fish transport are also influenced by overexertion and fatigue of fish. When fish are placed in transport containers, they usually exert a large amount of muscular activity. When muscles are actively used, there is not enough blood (thus oxygen) to supply their needs. An alternate system shifts into use where energy is provided in the absence of normal amounts of oxygen. Lactic acid accumulates in the muscles and blood and causes the pH of the blood to drop. Oxygen utilization is reduced by the lower pH of the blood. Following a few minutes of strenuous muscular activity, lactic acid accumulation may not be reduced for 24 h. Excitability and recovery from the side effects excitement vary with the species. More oxygen is consumed within the first 15 min. in the transport unit than during any subsequent 15-min period (Dupree and Huner, 1984). For this reason, additional oxygen, as much as twice the flow normally required should be provided during loading and the first hour of hauling. The oxygen flow can be reduced to normal level 6 mg 1-1 after this acclimation period, when the fish have become settled and oxygen consumption has stabilized (Piper et al., 1982).
A lower individual weight of fish means a much lower total weight of the fish that can be kept in a transport container (Fig. 2); this is due to the higher oxygen consumption and greater demand for space (the space factor increases considerably).
Figure 2 The dependence of cyprinid stock density in closed systems on the individual fish weight, in water temperature 20°C and transport time 5 h (Orlov et al., 1974)
Stock density of the fish in container also depends on the length of transport time (Fig. 3). The pattern of this dependence is characterized by hyperbolic curve, not straight line.
Figure 3 The dependence of the stock density of cyprinids in a closed system on shipment time; individual weight 10 g, transport time 5 h, water temperature 20°C (Orlov et al., 1974)
The relation between the fish stock density in the container and water temperature is shown in Fig. 4. Higher temperatures mean a lower total stock weight.
Figure 4 The dependence of cyprinid stock density in a closed system on water temperature; 1 - individual weight 10 g, transport time 15 h; 2 - individual weight 5 g, transport time 25 h (Orlov et al., 1974)
The time of shipment experts its influence mainly on the larval stages of cyprinids; transport longer than 24 h always means some risk, although all conditions are otherwise good (Pecha, Berka and Kouril, 1983).
Salmonid stock densities in the transport container are always lower than the standard densities for cyprinids, owing to a higher oxygen consumption and a lower critical CO2 concentration.
Shipment conditions also influence the composition of fish blood and the parameters of blood serum biochemistry. Increased temperature and a lower fish weight-to-water concentration ratio mean a higher number of erythrocytes and a greater haemoglobin concentration of fish blood. No such changes occur at lower temperatures and a lower fish proportion in relation to water volume (Shevchenko, 1978). Haematological conditions changed by transport were also recorded by Carmichael (1984) in advanced fingerlings (15–24 cm) of largemouth bass. When fish were transported at higher densities, the levels of corticoids and glucose in the plasma increased and were retained when the transport was finished. Although mortality as a direct consequence of transport was low, the secondary effects of stress were responsible for delayed mortality, caused by the consequences of osmoregulatory disfunction and disease. For largemouth bass, the author recommends to let the fish recover for at least 64 h.
It should also be noted that release of fish at the destination can be the most critical stage of the transport process. The fish are under some degree of stress in the transport unit and sudden exposure to water of different characteristies or low quality will further stress the fish, often beyond what they can stand. Poor-quality water may mean freshly pumped ground water with low oxygen or high carbon dioxide content. Different characteristics of water often mean a pH, temperature or gas saturation difference between the transport unit and the receiving water.
Finally, several concluding notes, or technical and organizational considerations, can be quoted from the literature. The majority of authors recommend, irrespective of the guide numbers of stock density during transport, to consider the specific transport conditions in each case and to change the basic guide numbers if such a change appears necessary after a brief test. It is also recommended to use a fish density at which the time of transport can be prolonged at least 1.5 times to prevent the consequences of a possible delay during transport, e.g., failure of a truck, failure to stick to the train or plane schedule, etc. When fish are transported for acclimation, or when endangered species are transported, the stock density should be lower: in such cases the economic aspects are not of primary importance and 100% survival is required. Nevertheless, the economic side of transport can never be neglected; hence, when the transport costs are high and the value of fish of transported comparatively low, the stock density in the transport units can be increased though losses of fish may be expected to be higher.