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The biology of the fish

2. In some respects, fish are very different from birds and mammals because of their adaptations for aquatic life. For example, they obtain the oxygen which they need from water via their gills. The heart and circulatory system are adapted to this means of respiration. The systems of many fish for eliminating waste from the body depend on the presence of much water for dilution and dispersion. On the other hand, in some aspects of their biology, fish are very similar to birds and mammals. The basic functioning of the muscles, liver, hormonal control mechanisms and nervous system is similar in fish and in other vertebrate animals including those which are warm-blooded. Of particular significance in relation to the welfare of animals is that both the adrenal system producing hormones in emergency situations and those senses which are not specific for use in the aquatic environment, function in very similar ways in fish and in mammals.

3. Almost all fish live the whole of their lives in water and show a maximal emergency response when removed from water, even for a very short period. This response includes changes in heart rate, increased production of adrenaline, noradrenaline and cortisol and vigorous muscle contractions which could result in escape and return to water. Some parts of the short-term emergency responses are shown in other disturbing circumstances. In water, low oxygen tension and the presence of toxic substances can also lead to emergency responses, as can attack by predators or other members of their own species. Vigorous avoidance and the associated physiological changes, whether shown in response to a sudden disturbance or to stimuli which the fish have learned are dangerous, often indicate fear in the fish.

4. Physical and social problems which are long-lasting may also result in adrenal and other physiological responses. However, as in mammals, the levels of adrenal hormones do not remain high for long periods. Prolonged problems and frequent activation of adrenal responses can result in immunosuppression, with increased susceptibility to disease, and inhibition of reproduction. There may also be effects, via changes in growth hormone levels, on fish growth. All of the scientific evidence concerning such effects makes it clear that the term stress is certainly relevant to fish and that the means by which stress effects are mediated are very similar to those in mammals (Strange and Schreck 1978, Pickering 1981, 1989a,b, Sumpter et al 1986, Flos et al 1988).

5. Evidence that the term pain is applicable to fish comes from anatomical, physiological and behavioural studies whose results are very similar to those of studies on birds and mammals. The fact that fish are cold-blooded does not prevent them from having a pain system and, indeed, such a system is valuable in preserving life and maximising the biological fitness of individuals. The receptor cells, neuronal pathways and specialised transmitter substances in the pain system are very similar in fish to those in mammals (Mathews and Wickelgren 1978). Localised tissue damage such as cuts and bruises and electric shocks to the skin result in electrophysiological changes in the nerves connected to pain receptors in fish. Behavioural avoidance responses are often shown in circumstances which might be expected to involve pain. Fish can learn to avoid places where they had unpleasant experiences including those in which they received tissue damage likely to involve pain (Ingle 1968, Verheijen and Buwalda 1988). The anatomy of the fish brain is different in some respects from that of the mammalian brain but there are some functional similarities (Echteler and Saidel 1981, Laming 1981, Ehrensing et al 1982, Busch 1992). It is not possible for scientists to determine exactly what any other individual is experiencing and we do not know what fish feel but the evidence available makes it very likely that at least some aspects of pain are felt by fish. In addition to any effect of pain, injury to a fish results in poor welfare where there is impairment of function or increased susceptibility to disease.

6. The needs of fish can be determined by studies of stress and welfare in these animals. Some of such work has involved examining the conditions in which fish will grow and reproduce. Other work has concerned the factors which affect the incidence of fish diseases. A relatively small amount of work has been carried out on other indicators of poor welfare and on studies of preferences in fish. Farmed fish need unpolluted water of an appropriate temperature and oxygen level, an adequate quantity and quality of food, a stocking density which allows for normal movement and social interaction, and good possibilities to avoid perceived danger. The means of meeting the various requirements interact with one another, for example a sufficient rate of water flow past the fish will provide oxygen, remove waste products and mitigate some of the effects of crowding. Fish also have specific needs in relation to handling and management procedures.

7. The Atlantic salmon Salmo salar is a species only relatively recently used in fish farming but the output of salmon farms has soared over the past twenty years from almost nothing to many thousands of tonnes per year. In the wild the species is widely distributed on both sides of the north Atlantic in the cold to temperate zones. It is successfully farmed mainly in Northern European waters. The most significant feature of the salmon is its anadromous life-cycle; the early years are spent in freshwater but, after a physiological transformation, the young fish migrate to sea, completing growth there over one or more years and attaining sexual maturity, or near maturity, before returning to their river of birth to spawn. Other important features of the salmon are its preference for cool water with high oxygen content and its carnivorous habits.

8. The rainbow trout Oncorhynchus mykiss is a native of the Pacific drainages of North America but it has been transported all over the world and is now acclimatised very widely. In the UK it has not been able to adapt fully to natural conditions except in one or two special circumstances. It has been the easiest of salmonid fish to farm and is the basis of trout farming the world over. Like the salmon it is carnivorous and needs fresh, running water to spawn but it can grow in much more varied circumstances than salmon and in fresh, brackish or even sea water. In the wild this species and the brown trout have river, lake and sea going forms. Rainbow trout are farmed throughout most of Europe.

9. The brown trout Salmo trutta is widely distributed in Europe and temperate parts of Asia and Africa and like the rainbow trout it has been acclimatised in many parts of the world outside its normal range. It is like the rainbow trout in its basic life style but under farm conditions it is less easy to rear and grows at about half the rate that is possible in rainbow trout. Its principal commercial use is for re-stocking for angling but some fish are farmed in fresh water or sea water for food.

10. The carp Cyprinus carpio is the species of fish reported farmed in antiquity. It is naturally distributed in much of Europe and Asia and has been extensively introduced outside this range. The carp flourishes in warm conditions but grows little, if at all, below 8C. Its natural habitats are lakes and slow moving rivers. It is omnivorous but needs animal protein in its diet. Its oxygen requirements are much less stringent than are those for salmonid fish and it can take air into its buccal cavity for respiration.