Drain design important for tilapia in tank
Aeration requirements depend on the rate of water exchange. If water is exchanged rapidly, one to four times per hour, in a tank with moderate fish densities, aeration devices may not be required. The oxygen supply will be renewed by the DO in the incoming water. A flow rate of 6 to 12 gallons/minute is needed to support the oxygen requirement of 100 pounds of tilapia. DO, which should be maintained at 5 mg/liter for good tilapia growth, is the primary limiting factor for intensive tank culture. Flow- through systems should ideally be located next to rivers or streams to take advantage of gravity-fed water supplies, but pumping is practical in many situations. Limited water supplies frequently restrict exchange rates to a few times a day or as little as 10 to 15 percent per day.
In this case, aeration is needed to sustain tilapia at commercial levels. Paddlewheel aerators, agitators and blowers (diffused aeration) are some of the devices used to aerate tanks. Aerators are rated according to their effectiveness (pounds of oxygen transferred into the water per hour) and efficiency (pounds of oxygen transferred/horsepower- hour). Aeration requirements can be estimated by using aerator ratings and oxygen (O2) consumption rates of tilapia, which consume 4.5 grams O2/100 pounds of fish/hour while resting and several times more oxygen while they are feeding and active. For example, a tank with 1,000 pounds of tilapia would consume 45 grams of O2/hour at resting, but maximum oxygen consumption may be at least three times higher (135 grams O2/hour) depending on water temperature, body weight and feeding rate. Aeration efficiency (AE) of diffused-air systems (medium bubble size) ranges from 1,000 to 1,600 grams O2/kilowatthour under standard conditions (68°F and 0 mg/liter DO). However, AE declines to 22 percent of the standard at 5 mg/liter DO and 86°F. Therefore, AE would range from 220 to 352 grams O2/kilowatt-hour under culture conditions. Dividing the maximum oxygen consumption rate (135 grams O2/hour) by the median AE (286 grams O2/hour) gives 0.47-kilowatt (0.63-horsepower) as the size of aerator needed to provide adequate DO levels.
A current trend for intensive tank systems has been the use of pure oxygen for aeration. Oxygen from oxygen generators, compressed oxygen tanks, or liquid oxygen tanks is dissolved completely into the culture water by special techniques to help sustain very high fish densities.
An Open Letter regarding recent reports that low-fat fish like tilapia are unhealthy. (July 16, 2008)
Eating fish, especially oily fish, at least twice per week is recommended for heart disease prevention. Fish is low in total and saturated fats, high in protein and essential trace minerals, and contains long-chain omega-3 fatty acids (EPA and DHA). Oily fish rich in these healthy omega-3s include salmon, trout, albacore tuna, sardines, anchovies, mackerel and herring. Our omega-3 needs can also be met by eating less-oily (lower-fat) fish more often.
Tilapia and catfish are examples of lower-fat fish that have fewer omega-3s than the oily fish listed above, but still provide more of these heart-healthy nutrients than hamburger, steak, chicken, pork or turkey. Actually, a 3 ounce serving of these fish provides over 100 mg of the long chain omega-3 fatty acids EPA and DHA. Considering that this is about the current daily intake of these fatty acids in the US, even these fish should be considered better choices than most other meat alternatives. Since they are also relatively low in total and saturated fats and high in protein, they clearly can be part of a healthy diet.
US Department of Agriculture statistics indicate that farmed tilapia and catfish contain somewhat more omega-6 fatty acids than omega-3. Most health experts (including organizations such as the American Heart Association and the American Dietetic Association) agree that omega-6 fatty acids are, like omega-3s, heart-healthy nutrients which should be a part of everyone's diet. Omega-6 fatty acids are found primarily in vegetable oils (corn, soybean, safflower, etc) but also in salad dressings, nuts, whole-wheat bread, and chicken.
Replacing tilapia or catfish with "bacon, hamburgers or doughnuts" is absolutely not recommended.
William S. Harris, PhD, FAHA
Sr. Scientist and Director
Metabolism and Nutrition Research Center
Sioux Falls, SD