Recirculating systems for tilapia in tank
Nitrate is relatively non-toxic to fish, but an accumulation of ammonia and nitrite can cause mortality. Tilapia begin to die at ammonia concentrations around 2 mg/liter (expressed as NH3-N) and nitrite levels of 5 mg/liter (as NO2-N). Gravel biofilters, which once were common, are being replaced by plastic- media biofilters because they are lightweight and easy to clean. Biofilters now consist of self-supporting stacks of honeycombed modules, columns or tanks containing loosely packed rings, or a series of discs on an axle that floats at the water surface and rotates, alternately exposing the media to water and air.
Regardless of design, biofilters generally have the same requirements for efficient vitrification: 1) DO of not less than 2 mg/liter or 3 to 5 mg/liter for maximum efficiency; 2) pH 7 to 8; 3) a source of alkalinity for buffer since vitrification produces acid and destroys about 7 mg of alkalinity for every mg of NH3- N oxidized; 4) moderate levels of organic waste (less than 30 mg/liter measured as biochemical oxygen demand), thereby requiring good clarification; 5) water flow velocities that do not dislodge bacteria. Biofilters can be sized by balancing ammonia production rates with ammonia removal rates. Unfortunately, these rates are highly variable. In a growout study on tilapia in tanks, ammonia production averaged 10 grams/100 pounds of fish/day (range:4 to 21). Ammonia production depends on quality of feed, feeding rate, fish size and water temperature, among other factors.
Ammonia removal rates may range from 0.02 to 0.10 grams/ft2 of biofilter surface area/day depending on type of media, biofilter design, and the factors that affect vitrification. The required biofilter surface area can be obtained by dividing total ammonia production for the maximum standing crop by the ammonia removal rate. The filter volume can be determined by dividing the required biofilter surface area by the specific surface area (ft2/ft3) of the media. For example, assume that a biofilter containing l-inch pall rings is being designed to support 1,000 pounds of tilapia. The ammonia production rate is estimated to be 10 grams/100 pounds of fish/day. Therefore, total ammonia production would be 100 grams/day. The ammonia removal rate is estimated to be 0.05 grams/ft2/day. Dividing total ammonia production by the ammonia removal rate gives 2,000 ft2 as the required biofilter surface area. One-inch pall rings have a specific surface area of 66 ft2/ft3. Dividing the required biofilter surface area by the specific surface area gives 30 ft3 as the biofilter volume needed to remove ammonia.
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