The key water quality parameters for fish farming in Pakistan are temperature, oxygen, pH, alkalinity, hardness and nitrogenous wastes.
Oxygen levels higher than 5 mg/L are good for eggs, larvae, fry and fingerlings. When dissolved oxygen levels are low, fish start gulping air at water surface. Even adult fish will perform better when the dissolved oxygen levels are adequate. Oxygen levels of 0 – 1.5 mg/L can be lethal especially if exposed for long periods. At 1.4 – 5 mg/L fish survive, but reduced feed intake, higher FCRs, slow growth, stress, and increased susceptibility to disease has been observed. Gas bubble trauma occurs when the water is supersaturated to levels of 300% and above. During day, oxygen deficiency may not be problem in well fertilized ponds due to oxygen production from phytoplanktons and water perturbations but problem may occur when this process reverses at night so some sort of artificial aeration will be required during night or even during day if stocking densities are high. Daily monitoring of oxygen especially during hot and humid days is very important.
Fish are cold blooded animals. Their rate of metabolism is directly influenced by water temperature. Rate of breaking down of wastes in pond and dissolution of chemicals is faster in warmer waters. Temperatures at 26 oC or higher affects solubility of oxygen in water. If the temperature is below 20 C there is reduced feed intake which stops growth. At lower or higher temperatures than optimum, feed intake is lower and FCRs are higher. At extreme temperatures fish is more susceptible to disease and sometimes death ensues.
Water pH Level
It affects the solubility and chemical forms of various compounds. The pH range from 6.5 to 9 is acceptable, below 4 is called ‘Acid Death Point’. Fish can survive from 4 – 6.0 but remain stressed, growth is slow, there is reduced feed intake and FCRs are higher. Low pH indicates high levels of dissolved carbon-dioxide, hence, pH values from 9 – 11 are stressful for fish and considerably reduces growth rate. The pH value above 11 is ‘Alkaline Death Point’.
Alkalinity and Hardness
In combination, alkalinity and water hardness influence the buffering capacity of the pond water. Hardness is composed mostly of calcium and magnesium, which affect the physiological condition of the fish. Alkalinity also controls the amount and form of carbon dioxide in water. Alkalinity > 20 ppm, hardness > 20 ppm, total alkalinity and total hardness above 60 ppm is desirable. Well buffered water will minimize diurnal fluctuations in pH.
Ammonia Nitrogen (TAN)
Ammonia occurs in both toxic (ammonia) and nontoxic form (ammonium) depending on the pH of the water. Toxic ammonia more than 0.3-0.5 is not acceptable. The proportion of TAN in the form of ammonia tends to be higher as the pH of the water increases above 7. The fish become susceptible to pathogenic attacks and fail to eliminate ammonia from their blood because there is too much ammonia already in the water. Ammonia is excreted by fish as a byproduct of protein metabolism primarily through their gills. High concentrations of ammonia in water reduce the ability of the gills to do so.
When organic matter is added into ponds, it results into an increased demand for the available dissolved oxygen and an increased likelihood of pollution. The likelihood and degree to which pollution might occur becomes reduced if the organic matter added can be broken down into smaller, less complex particles that are less toxic. Oxygen and bacteria are required for the effective break-down and assimilation of organic matter. Controlling input levels of organic matter is a pre-requisite for managing dissolved oxygen levels as well as other water quality parameters within the pond. Once feed has been fed to the fish in a pond, the excess feed cannot be removed. It drops to the bottom of the pond as organic matter. Accumulated wasted feed, subsequently results into pollution instead. Feeding fish by response is one way of ensuring that the fish are only fed what they can consume.
There are reasons to exchange water in specific instances, notably, to flush out excessive nutrients and plankton and to reduce ammonia concentrations. Daily water exchange usually does not improve water quality in ponds because water temperatures and dissolved oxygen levels in in-flowing water are lower than those in ‘static water’ ponds that have not yet attained their carrying capacity. Pumping costs are also a liability. Ponds are highly efficient in assimilating carbon, nitrogen, and phosphorus inputs not converted to fish flesh, but if water exchange is great, phytoplankton is reduced and these nutrients are discharged from ponds before they can be assimilated. In well fertilized ponds and with high stocking density water exchange is highly important and useful too.
Turbidity is a measure of transparency of water in the pond. The color of the water gives an indication of what sort of turbidity it is. If it is brown it is often due to clay and if it is greenish, it is due to plankton. Make sure that water is light green. Dark green or brown color water is not good for successful fish culture.