The environment in which an animal is cultured plays a critical role in the degree to which that animal is susceptible to pathogens and the occurrence of clinical disease. An understanding of the relationship between host, pathogen and environment is important for understanding the cause, prevention and treatment of most aquatic animal diseases. In general, there occur is a profound and inverse relationship between environmental quality and health status of fish. The deteriorated environmental condition will exert stress on fish. Some commonly known stressors present in the aquatic environment are unionized ammonia, inadequate oxygen, rapidly changing or extremes of pH or rapidly changing the water temperature. Such environmental stresses on fish increase geometrically when environmental conditions approach the tolerance limit of the host.
The relationship between pathogen, host and the environment can be diagrammed as follows:
Where (1) is fish population, (2) is pathogen and (3) denotes the culture environment.
Serious losses only occur when factors (1) and (2) are present in an environment (3) which favours the disease. When the pathogen and host are present (1–2) but the environment isn't favourable for the disease, no outbreak occurs. Also, when the environment is favourable for disease and the host is present (1–3), no outbreak occurs unless the pathogen is also present. As long as the environment remains unsuitable for the pathogen, no disease occurs. With this disease, all that remains is for the environment to deteriorate and disease will occur. Under conditions of (1– 2 –3), the outbreak would be impossible to prevent.
In general, diseases affecting aquatic animals may be grouped into:
- disease resulting from poor environmental conditions leading to direct effects;
- disease resulting from stress leading to infection by opportunistic pathogens (g., ulcer disease in fish);
- pathogens causing disease only when animals are stressed (g., MBV in shrimp); and
- primary pathogens causing disease without environmental stress. These are comparatively rare, although some recently reported shrimp viral infections such as yellow head baculovirus may fall into this category (even then, environmental management may be required to control the entry of such pathogens to the culture system).
Apart from creating stress, which deteriorates the health status and causes disease, the poor environmental conditions can also have a direct effect leading to disorders in the cultured fish. The range of water and soil quality parameters, suitable for most of the cultivable species are mention below for ready reference:
SOME DISORDERS ASSOCIATED WITH ENVIRONMENTAL CONDITIONS ARE DISCUSSED BELOW:
Temperature is probably the single most important factor in the farming situation. Fish are poikilotherms and mirror the temperature of the surrounding water. Nevertheless, each species has its own preferred range. Sustained periods at the extremes of this range, or probably more important, rapid changes (even very small ones sometimes) within this range, represent stressful conditions. In general terms, fish will tolerate a temperature drop better than a rise. Some species are more susceptible to temperature stress than others and this is thought to be due to a poorer ability to osmoregulate at the new temperature. Higher temperatures cause an increased metabolic rate and hence an increased O2 demand necessitating increased “irrigation” rates.
Fish also appear much more susceptible to bacterial diseases in conditions of rising water temperatures. The reason for this susceptibility is unknown in precise terms but it is thought to be the result of an enhanced rate of replication or enzymic production by the pathogen which is not matched by that of the fish's immune mechanisms.
High dietary protein combats the stress of Labeo rohita fingerlings exposed to heat shock (Kumar et al., 2011). Dietary microbial levan enhances tolerance of L. rohita (Hamilton) juveniles to thermal stress (Gupta et al., 2010). Even dietary pyridoxine supplementation at 100 mg/kg diet may reverse the negative effects caused by elevated temperature and may protect the haemato-immunological status of L. rohita fingerlings reared at higher water temperature (Akhtar et al., 2011).
In warm or polluted waters, where O2 levels are low, the fish suffer from ‘respiratory distress syndrome’. Increasing water temperature cause an increased demand for O2 for the increased metabolic rate so produced, but a decrease in the holding capacity of water for O2. Appetite also increases, but this extra intake of food may place a burden on the ability of the fish to extract enough O2 from the reduced levels available to metabolize the food properly.
Fish can acclimate to lower O2 levels to a certain extent. Diel variations, however, depress appetite and growth. If the low O2 levels persist for a long period of time the fish may die.
PUFA and/or vitamin E contribute significantly to the regulation of metabolism in hypoxia (Randall et al., 1992)
Rapid increases in water temperature or reduced pressure may lead to a situation of supersaturation. Supersaturation may also be seen in the wild associated with very rapid photosynthesis by plants and algae, or more commonly in an intensive culture operation, associated with leaky valves or pumps, in those farms where pumping is a feature. Water which is supersaturated with gas, either O2 or N2, may cause the condition gas-bubble disease. Fish normally equilibrate quickly with supersaturated water and it should, therefore, cause little problem. The reason for the gas in the blood coming out of solution is not therefore completely understood but is thought to be associated with the great drop in pressure experienced by the blood when crossing the gills. Whatever the reason, bubbles of gas cause emboli in the vessels of the gills pseudobranch, choroid gland and elsewhere.
An apparent contraindication, vigorous aeration or agitation of the incoming water, or replacement of the leaky valves are considerations in curing the condition.
4. Suspended solids
The high amount of suspended solids can kill fish, by clogging the gills. Some other effects of suspended solids include (1) settling on eggs and young larvae and possibly suffocating them, (2) reducing light penetration and therefore an abundance of food, (3) modifying behaviour patterns and natural movements. However, evidence for a direct damaging effect on fish is less convincing.
Fish excrete this via the gills, and where there is plenty of water to remove it there is no problem. Toxicity arises however, in situations of overcrowding, or where for example, the chicken slurry is added to the water. Young fish are relatively quite susceptible.
Free ammonia or NH3 is highly toxic, whereas bound ammonia NH4+ is much less so. Free ammonia combines with water as shown in the above equation and dissociates or recombines as shown. In acid water, most ammonia is in the bound or non-toxic form whereas in alkaline water, free ammonia may be more of a problem. Similarly, as water temperature increases, the amount of free NH3 also increases, whereas elevated calcium levels (seawater, hard water areas) increase the tolerance of fish to ammonia, possibly by decreasing the permeability of the gills to the toxin. High ammonia levels cause epithelial hyperplasia of the gills, thus effectively increasing the diffusion distance to O2. At the same time, it is thought that NH3 affects the ability of haemoglobin to bind O2, but the precise toxic action of NH3 is unknown. Chronic levels have been shown to cause meningeal proliferation in young minnows. Fish suffering from ammonia toxicity may exhibit behavioural abnormalities, such as hyperexcitability, anorexia, reduced growth, and increased susceptibility to pathogens.
Astaxanthin can become essential for P. monodon when the animal is under ammonia stress (Pana et al., 2003)
This is an intermediate in the oxidation of ammonium to nitrate, a process which is carried out naturally and by the bacteria in biological filters Nitrosomonas spp. converts ammonium to nitrite. Nitrobacter spp. converts nitrite to nitrate.
Alkaline conditions lead to a higher amount of un-ionized ammonia, and this, in turn, has a greater inhibiting action on Nitrobacter than on Nitrosomonas spp. Thus alkaline water see an increase in nitrite levels. Other parameters which affect the bacteria differently also have an impact. Nitrite is actively taken up by the gills (chloride cells probably) and blood levels maybe 10 times those of the surrounding water. Nitrite oxidizes the iron in haemoglobin to form methemoglobin and this pigment lacks the ability to bind reversibly to oxygen. Grossly, high levels of methemoglobin result in a brown colour to the blood and results in so-called “brown blood disease”. Erythrocytes of fish have a reductase which reconverts methemoglobin to haemoglobin but this takes 24–48 hours if fish moved to normal water. Nitrite also causes a leukopenia, and although in the short term this is probably inconsequential, in the long term with chronic low-level exposure, it may increase susceptibility to disease.
Tolerance to nitrites increased when the concentration of ascorbic acid was high in the diet (Blanco & Meade, 1980)
7. Carbon dioxide
CO2 is of course excreted by the gills, and with water, it forms carbonic acid, which reduces the pH in the microenvironment of the gills. Increasing levels of CO2 in the blood (or a decreasing pH) decreases the affinity of haemoglobin for O2 (Bohr effect). Fish haemoglobin is very sensitive to CO2 (large Bohr effect) by comparison with mammals, which are relatively tolerant of CO2. High environmental levels correlate with nephrocalcinosis in intensively cultured fish.
8. Hardness alkalinity and pH
Improper hardness and alkalinity may lead to acute to chronic stress responses. Disturbance in the osmoregulation may also occur.
A pH range of 5–9 is generally considered the “safe” range for fish, although maximum productivity is seen from 6.5–8.5. Fish populations are found naturally however from 4–10.
Acutely low pH can result in acute mortality with tremors and hyperactivity, dyspnea or acute stress response whereas chronically low pH can result in increased mucus production and chronic stress response. High pH can lead to the cloudiness of skin and gills and stress response. Recently a team of Department of Fish Health and Environment, College of Fisheries, CAU, Lembucherra, Tripura have formulated a nutraceutical mixture (Nutra-X) which makes fish tolerant to acid stress after feeding. Thus, Best Management Practices (BMPs) or Nutraceuticals through feed are the major strategies for ameliorating the stress responses in aquatic life for maintaining growth and sustainable production from aquaculture systems.
Akhtar, M. S., Pal, A. K., Sahu, N. P., Ciji, A. and Kumar, N. (2011) Effects of dietary pyridoxine on haemato-immunological responses of Labeo rohita fingerlings reared at higher water temperature. J.l of Animal Physiology and Animal Nutrition (DOI: 10.1111/j.1439-0396.2011.01181.x)
Blanco, O. and Meade, T. (1980) Effect of dietary ascorbic acid on the susceptibility of steelhead trout (Salmo gairdneri) to nitrite toxicity. Rev. Biol. Trop., 28(1): 91-107.
Gupta, S. K. Pal, A. K., Sahu, N. P., Dalvi, R. S., Akhtar, M.S., Jha, A. K. and Baruah, K. 2010. Dietary microbial levan enhances tolerance of Labeo rohita (Hamilton) juveniles to thermal stress. Aquaculture, 306: 398-402.
Kumar, S., Sahu, N. P., Pal, A. K., Subramanian, S., Priyadarshi, H., Kumar, V. (2011) High dietary protein combats the stress of Labeo rohita fingerlings exposed to heat shock, Fish Physiol. Biochem. 37:1005–1019 (DOI 10.1007/s10695-011-9504-1).
Pana, C. H., Chien, Y. H., Hunter, B. (2003)The resistance to ammonia stress of Penaeus monodon Fabricius juvenile fed diets supplemented with astaxanthin. Journal of Experimental Marine Biology and Ecology 297(1): 107–118
Randall, D. J., Mckenzie, D. J., Abrami, G., Bondiolotti, G. P., Natiello, F., Bronzi, P., Bolis, L., and Agradi, E (1992). Effects of Diet on Responses to Hypoxia in Sturgeon (Acipenser naccarii), J. Exp. Biol. 170: 113-125.
Saha, R. K. 2010. Soil and water quality management for sustainable aquaculture, 1st edn. Narendra Publishing House, Delhi, pp 33–186