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| Fish Health and Welfare |
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Farm Conditions - avoiding stressors is a key prevention strategy
The needs of aquacultured fish and shellfish can be determined by studies of stress and welfare in these animals. Some of this work has involved examining the conditions in which fish and shellfish will grow and reproduce. Other work has concerned the factors, which affect the incidence of diseases. Farmed animals 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. Farm facilities should be organized in a way that all these requirements are met adequately as this will greatly help to prevent stress and therefore immuno-suppressed status and disease opportunities. For species, which have been cultured for long time, conditions are better known and the design and organization of the farm is part of the strategy for efficiency. Other species depend on the increase of their culture knowledge. In a similar way, culture procedures such as grading, feeding systems and feed quality, water control, handling etc., should be performed with minimum disturbances. In addition health safety practices, quarantine, cleaning procedures and slaughtering should be integrated in the current farm management and therefore they should be known and assumed by the operators.
Therefore, changes in the flesh, or alteration of the tissue biochemical properties like acidification will take place, which will significantly influence the quality of the final product and the organoleptic properties.
How farm conditions and practices may induce stress:
Almost all fish live their entire 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 stress hormones in blood and vigorous muscle contractions, which could result in escape and return to water and in production of acids and impair ionic balance. Some parts of the short-term emergency responses are shown in other disturbing circumstances. Low water quality can also lead to other emergency episodes, as do predator attacks. Vigorous avoidance and the associated physiological changes, whether shown in response to a sudden disturbance or to stimuli, can initiate maximal emergency response.
In some respects, fish are very different from birds and mammals because of their adaptations for aquatic life. However, 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 that are not specific for use in the aquatic environment, function in very similar ways in fish and in mammals. Farmed aquatic organisms are at equilibrium with their environment, they live in equilibrium with their disease organisms. Cold-blooded animals are very susceptible to external changes. When the environment changes, the equilibrium shifts.
Stress is a reaction. When fish are exposed to environmental stressors (unfavourable conditions) a compensating response has to be made. Exposure of fish to these conditions causes changes in the fish's physiological mechanisms. It is important to know that fish, as other animals may be subjected to acute stress (short-term, intense effect, i.e. sampling a fish out of water), or chronic stress, (low intensity longer term, i.e. high densities), as the loss of balance will occur sooner (acute) or later (chronic). Physical and social problems that 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 immuno-suppression, with increased susceptibility to disease, and inhibition of reproduction. There may also be effects, via changes in growth hormone levels, on fish growth. As for all other vertebrates, maintenance of stressors may generate imbalance in at least two important physiological compartments, the energy budget, which will impair growth, and the immune defence, which will reduce health. All of the scientific evidence concerning such effects makes it clear that the term stress is certainly very relevant to fish and that the means by which stress effects are mediated are very similar to those in mammals.
Water quality has a direct effect on fish performance. Poor water quality resulting either from deficiencies or excesses will cause stress, which in turn will affect energy allocation processes (resulting in poor growth, poor reproductive performance, disease, mortality and thus reduced production). Factors that affect water quality are on the one hand, extrinsic factors (influenced by conditions outside the farm and that can be modified): pH, temperature, alkalinity, hardness, salinity, and metals. On the other, intrinsic factors, governed by metabolic processes from the stock, influenced by what goes on inside the farm (dissolved oxygen, CO2, ammonia, nitrite)
The overall effect of a stressful environment is to reduce fish performance. As has been said, fish in aquaculture systems live in equilibrium with their disease organisms. Stress induced disturbances in this equilibrium can promote disease. Stress in nature is adaptive, but in aquaculture fish are restricted by the system. A minimum chronic stress is unavoidable in any aquaculture system. As has been stated, stress in fish involves predictable physiological changes. Stress causes an elevation of cortisol in blood; it has been shown that high cortisol result in depressions in number of circulating lymphocytes (immuno-supression). This means that cortisol plays a role in the immune system.
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 dangers. The means of meeting the various requirements are interlinked 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.
Report on the welfare of farmed fish. Surrey, UK: Farm Animal Welfare Council, 1996.
http://www.fawc.org.uk/fish/fishrtoc.htm
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