While fish will grow in water of sub-optimal quality, their growth rate will not be maximized. With high investments costs involved in the running of rearing units it is of course vital that the production per unit of farming volume is as high as possible. Fish can live well in the wild even if water quality is sub-optimal. However, the food supply is usually limited and the growth rate will be much lower than that possible under optimal conditions.
There is often some kind of stress response involved when there is an outbreak of disease. Disease can be latent in the stock but can become a problem if the fish are exposed to some kind of stressor, for instance sub-optimal water quality. In fish farms, where fish are grown as quickly as pos-sible, they are already under stress and so disease outbreaks are more likely due to sub-optimal environmental conditions. It has been shown that by catching wild fish and holding them under farming conditions at a high stocking density, it is very easy to cause a disease outbreak, even if the water quality is equal to that found at the wild site where the fish were caught.
Many experiments have been carried out for the species farmed today, and there are quite good data for recommended water quality.8,9 However, as the amount of new water added per kilogram of fish is continuously decreasing, research is focused on accurately documenting lowest acceptable levels of nutrients, etc. to maintain optimal growth. Norwegian salmon smolt production can be taken as an example. In 1985 the average size of the fish was 40 g while today is it over 100 g, as the result of improved feed and increased individual growth rate. Most of the sites used have limited fresh water resources and therefore the amount of new water supplied per kilogram of fish has been reduced,mainly by adding pure oxygen: in 1985 the licence requirement for new water was 0.38 l/kg fish/min, while today in dry periods it is down to 0.1–0.2 l/kg fish/min.
Water quality requirements depend upon the species. The same is the case regarding requirements for the various life stages; the early stages normally have the highest requirements for optimal water quality. Even if the quality requirements vary, it is better to have good quality water than bad, if this is possible.
Optimal water temperature is species specific and so general advice is impossible. Species can be defined generally as warm water species (>20°C) and cold water species (<20°C). Some species tem-peratures prefer below 10°C. If the water tempera-ture falls below 0°C freezing will be a problem. The oxygen content of the water enclosure (see full saturation, p. 119) is reduced with increasing tem-perature. At 5°C the available oxygen content is 12.8 mg/l while at 25°C is it reduced to 8.2 mg/l.
The water ought to be fully saturated or super-saturated with oxygen gas. It is very important that the oxygen content of the rearing water is high enough: for instance, 7 mg/l (70% saturation) is the typical value for the outlet water in salmonid farming, 30% having been consumed by the fish.
Of the other gases dissolved in the water, the con-centrations of nitrogen (N2) and carbon dioxide (CO2) must not be too high. The nitrogen gas con-centration should be below 100.5% saturation. For carbon dioxide, levels are not only dependent upon the inlet concentration, they also increase in the tank as a result of fish metabolism which releases CO2 into the water; the outlet concentration must not therefore be too high.
Water pH must neither be too low nor too high (the latter is seldom the case). This applies to fresh-water, since seawater has stable pH values of 7.5–8.2. Sufficient alkalinity in the water helps to control fluctuations in pH.
Too many particles in the inlet water may have negative effects on the fish, for instance by clogging their gills. Fish faeces will increase the particulate content of the water, the outlet concentration of which must not be too high.
Ammonia may be a problem in production unit outlet water because of waste products from fish metabolism, but only with very limited supply and exchange of water. With normal water sources thereare no problems with ammonia concentration in the inlet water.
The concentration of metal ions in the inlet water may be of levels that are toxic to fish; low pH may increase this toxicity. Problem metals include aluminium, copper, iron, zinc and cadmium.
Micro-organsims, including parasites, bacteria, viruses and fungi, may be present in the inlet water at concentrations unfavourable for aquaculture.
Interactions between several water quality para-meters, for instance between pH and metals, may also pose quite a challenge. To fully understand water treatments effects, a good knowledge of basic water chemistry is an advantage; this topic is not covered in this book but extensive literature is available, for example refs 11-13.