Agriculture Business Week

How the correct application of natural feed supplement can help promote stress resistance in aquaculture.

Stress is a major problem in intensive fish production systems, leading to growth reduction and increased susceptibility to infectious diseases. The goal of a successful aquaculture enterprise is to avoid a disease outbreak and significant economic losses. Although preventive measures through good management practices are still most effective. The correct application of natural feed supplement can help to promote stress resistance.

Stress and aquaculture
Many potential fish pathogens are ubiquirous in nature fish are able to resist these pathogens well as they are able to seek out the best environmental conditions through freedom of movement. Fish in an aquaculture environment are confined to the production unit and are often exposed to a regime of stressors including high fish density, reduced water quality, handling and sub-optimal nutrition. A stressor can be defined as an adverse stimulus acting on a biological system reacting to that stimulus. Stress often changes a biological system from one that requires energy to one that uses it, affecting the fish behavior and feeding. Stressors may have extensive adverse effects on growth, reproduction and flesh quality, changing the body to one that starts using energy, rather than storing or investing it. Furthermore, stressors may also suppress the immune system leaving the animal at greater risk of diseases.

In intensive aquaculture, stressors are common and as fish cannot escape these, stress is therefore an intrinsic problem which results in reduced fish performance and threatens fish health. In addition, stress is cumulative. A fish that is suffering from low dissolved oxygen will show a further increased stress response if it is then handled for example. Stressors have considerable adverse consequences for the profitability of the fish farmer, therefore, stress reduction is of paramount importance for a successful aquaculture system.
Stress response in fish
Two types of stressors can be distinguished : acute (e.g handling) and chronic (e.g. continuous poor water or feed quality). The stress response is essentially similar in all vertebrates and physiological and behavioral responses directed at restoring the physiological equilibrium of an organism.

Fish are able to adapt to stress for a short period of time. The may look and act normal but, under chronic unavoidable stressor exposure, the stress response can be maladaptive. The energy reserves are eventually depleted and hormone imbalances occur. This is then responsible for damage to the cardiovascular system, loss of appetite and reduced growth or weight loss, as well as reduced reproduction. Furthermore, the immune system of the fish is suppressed and the susceptibility to infectious diseases is increased.

Fish are relatively sensitive to stressors when compared to terrestrial vertebrates and the main reasons for this are the presence and importance of the gills and the structure of the skin.

Stress signalling by gills and skin
The gills consist of very delicate and extensive structures with epithelial layers forming a barrier of only a few micrometers between the blood and the ambient water. Furthermore, the structure of the skin of fish is very thin compared to that of terrestrial vertebrates, and only covered by a mucus layer instead of well protected by keratinised layers such as feathers and hairs. The advantage of the skin, with its complicated structure and many different cell types, is its role as an important interface between the fish and its environment. The skin contains many sensory cells and organs, with fine sensitivity for the detection of water movements, chemicals and signals from other fish, prey animals or predators. However, the skin is also easily damaged mechanically and through infection, leading to epithelial ulcers which arise rapidly.

Acute as well as chronic stress, is usually associated with marked structural changes of the skin and gills and structural damage to the gills. The latter effect is mainly due to the high blood pressure and increased blood circulation as a result of the stimulated heart rate and ventilatory movements. This may lead to the swelling of the lamellae and even to the disruption of the vascular tissue of the gills. It may also lead to increased permeability of the branchial and, most likely, the skin epithelia to water and ions which causes severe disturbance of the hydro-mineral balance, a typical phenomenon of the stressed fish. Skin and gills are also primary targets for infectious agents from viruses to crustacean ectoparasitcs.

Improve performance by stress reduction

Selection of fish
When compared to the farming of most farm animals, intensive aquaculture is a relatively new innovation. The process of domestication has only just begun and considerable effort is devoted to only selecting for and defining the heritability of economically important traits in these fish. These are for growth rate, age of maturity, fillet composition and disease resistance. Selection could be performed by selective breeding from desirable phenotypes or by more recent techniques such as application of biochemical/molecular techniques in genetic engineering. However, the selection of strains of fish with a low stress response and the propagation of stress resistant strains is still in its infancy and only few successes have been reported.

Management
Preventive measures through good management practices are still most effective in stress reduction. In this way, disease problems and mortalities are minimized while economic benefits are maximized. Good management involves maintaining good water quality, preventing injury and limiting stress during handling, providing well-balanced nutrition at a proper rate and using appropriate sanitation procedures. The use of the common sense of the farmer plays a vital role here.
Feed supplements, additives and nutraceuticals
The potential for reducing stress and enhancing immunity and disease resistance by a wide range of nutritional factors/food additives has been clearly demonstrated in warm-blooded animals, while relatively little work in this area has been conducted in fish. However, the positive effects of dietary nutrients and additives in terrestrial animal production together with the encouraging results of those ingredients and additives that have been tested in fish show great promise for some stress resistance in aquaculture.

To reduce the impact of stressors and to facilitate the compensatory adjustments of fish in aquaculture, several feed supplements have been advocated. Substances derived from yeasts, bacteria, algae and fungi, or lyophilized preparations of these organisms have been introduced to activate the innate defense mechanisms. Also nucleotides as a diet supplement have been shown to stimulate the immune system and to counteract the suppressive effects of stressors.

Conclusion –> Combating Fish Stress (Conclusion)