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Once the species for culture has been selected and the culture system has been constructed and stocked, aquaculturists must address various management concerns. In term of things that should be monitored by the aquaculturists, perhaps the most important water quality variables are temperature, dissolved oxygen and ammonia. Other variables can be important under certain circumstances, but the three mentioned generally provide a good indication of the performance of the animals in the culture system.

Temperature. Aquaculture species are all “cold-blooded” or poikilothermic. That means that their body temperatures are virtually the same as the temperature of the water that surrounds them. Basically, there are two primary types of culture species with respect to temperature: warm water species and cold water species. Carps, tilapia, channel catfish and freshwater shrimp are examples of warm water species. Trout, salmon and American lobsters are examples of cold water species. The optimum temperature for warm water species tends to be about 86°F (30°C), while that for cold water species is often 59°F (15°C). Some species of aquaculture interest, such as the yellow perch, have temperature optima between the warm and cold water species and are known as mid range species. Few mid-range species are currently being cultured.

When temperature changes dramatically and, in particular, when it moves out of the optimum range, aquatic animals are placed under stress. It is at such times that disease resistance is lowered and problems often arise. Knowledge of temperature requirements of the species under culture and of the temperature at any given time will not only provide the culturist with valuable information about how well the animals are growing and how much of feed them, it will help to establish the disease resistance status of the animal.

Dissolved Oxygen. Oxygen enters water by dissolution from the atmosphere and through the release of that element by plants during photosynthesis. Animals with gills respire by absorbing oxygen that has been dissolved in water directly into the blood stream through diffusion as a general rule, if the water contains 5 parts per million (ppm) of oxygen it will support aquatic organisms. Some fish, such as tilapia, can survive at very low concentration of oxygen, while others, such as trout, are stressed if the concentration falls below 5 ppm.

Daily changes in temperature are very small relative to the changes that occur seasonally, particularly in temperature climates. Daily changes in dissolved oxygen, on the other hand, can be substantial. Dissolved oxygen begins to increase at about dawn, when photosynthetic production of oxygen by the plant community begins. As the sun rises photosynthetic oxygen production increases with the increasing amount of light energy available. While both plants and animals respire continuously, the rate of oxygen production exceeds respiration and there is a net increase in the dissolved oxygen level.

At dusk, when there is insufficient light for photosynthesis, the oxygen level begins to drop because of respiration demands, and the drop continues through the night. As long as the lowest level is not below about 5 ppm, there should be no problem. However, the lowest level of dissolved oxygen can change dramatically from one day to the next day. Daily production of oxygen can be influenced by the weather (cloudy days don’t support as much photosynthetic activity as clear days) and by the biomass of culture organisms present. As the fish or shellfish being raised grow, they extract more oxygen from the pond each day.

Ammonia. Ammonia occurs in two forms, unionized (NH3) and ionized (NH4). The ratio between the two depends on temperature, pH and a few other factors. Ammonia is rapidly converted to nitrate by plants and bacteria in aquatic systems. Thus, in ponds where there are plenty of plants and bacteria present ammonia toxicity is not usually a problem. In raceways and other water systems where animals are reared at high densities, ammonia removal is often not as efficient as in a pond, and toxicity can occur. Different species of aquaculture interest have different tolerances for ammonia. Tilapia can tolerate high concentrations of total ammonia (several ppm), whereas trout are highly susceptible to levels well below 1 ppm.

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