Tank Culture of Tilapia (part 4)
Feeds and feeding
One of the characteristics that make tilapias suitable for simple hatchery production is that new fry do not need specialized live feeds such as Artemia, rotifers or microalgae. They can be given commercial dry feeds, size 00 or 0, after they have absorbed their yolk sacs. The fine powder form allows some of the feed to float, encouraging surface feeding. Fry this size may eat as much as 20 percent of their body weight (biomass) per day.
At this stage, fry can be given special feed to reverse their sex. This special feed, which must be prescribed by and produced under the direction of a veterinarian, contains a small concentration of male androgen. For up to 28 days post-hatch, this feed is offered to the quickly growing and sexually undifferentiated fry. Properly administered, this feed produces populations of more than 90 percent male fish.
In their natural environments, tilapias are detritus feeders. They feed by “grazing,” and for longer periods than predator fish that pursue and capture prey. Therefore, it is possible to use an extended feeding regime for tank-cultured tilapia, feeding a daily ration over a period of 10 to 12 hours or longer. Some researchers have reported that better feed conversion will result if tilapia are not fed continuously, but are given a rest period, during which their metabolism decreases. The method of feeding used, by hand or by automatic feeder, determines whether a feeding schedule can be extended. Each method has advantages and disadvantages. Feeding manually over longer periods of time increases labor cost but allows feeding to be monitored. Using mechanized feeders makes it easier to schedule feed delivery to individual tanks throughout a daily cycle.
In both flow-through and water reuse systems, extending the feeding schedule over a longer period also spreads out the impact of feed on water quality parameters. Feed can be delivered to the system at more even intervals to moderate the high and low variations of these parameters.
When oxygenation is used, there is less need to adjust oxygen input to maintain optimum dissolved oxygen levels. When dissolved oxygen concentrations are low, feed conversion ratios can be affected; when dissolved oxygen concentrations are high (supersaturation), oxygen is being wasted.
With hand feeding, an operator can observe feeding behavior and reduce or increase feed depending upon the reaction of the fish. Feed can also be spread over the tank surface to allow more fish to feed. Should fish concentrate and feed in one area, feed can be applied in a different area to attract those fish that were crowded out. Trained feed applicators can also become skilled at identifying changes in water quality from the feeding behavior of the fish. As feeding slows, feed application can be decreased or stopped to reduce the amount of uneaten feed.
Tank-cultured tilapia can have very efficient feed conversion ratios (FCR). The time period for FCR can be days, weeks, the length of the tank production cycle, or a year. FCRs in the range of 1.4:1 to 1.8:1 are common with tilapia and are some of the best in animal agriculture. While FCR is one of the most important benchmarks for measuring the efficiency of an operation, FCR alone does not give a true measure of production. An artificially low FCR can be created by underfeeding, so it is important to consider the growth rate also.
The cost of feed is a large part of an operating budget so it must be used wisely. Table 1 contains guidelines for feed sizes and feeding rates for tank culture of tilapia.
To achieve projected weekly weight gains, the corresponding total amount of feed must be fed and consumed by the fish population during that week. In systems that are not properly designed for good solids removal, biofiltration, dissolved gas stripping, and aeration or oxygenation, poor water quality often occurs before the required daily ration is fed. This is especially true near the end of the growing period when biomass is nearing maximum. When this occurs, the manager can 1) reduce or suspend feeding or 2) begin water exchanges.
To ensure that targeted growth rates are achieved for a given week, the manager will estimate the beginning and ending biomass of the tank based upon estimated growth rates, assume a realistic FCR, and calculate the total amount of feed required for that week. For example, if a growth rate of 5.0 grams per day per fish is expected and the tank contains 10,000 fish, a total of 350,000 grams (350 kg) of weight should be gained for the week. Assuming an FCR of 1.7:1, the manager would need to feed 595 kg of feed for the week (1.7 x 350 kg = 595 kg). The daily feed rate would have to average at least 85 kg (595 kg ÷ 7 days/week = 85 kg). If that feed rate cannot be reached, the manager should look for an explanation so the problem can be remedied. Often poor water quality is the cause of the problem. The fish may not be eating aggressively due to the stresses of high ammonia levels, nitrite toxicity, low dissolved oxygen, high levels of carbon dioxide, or other water quality problems.
Water quality requirements
Tilapia are some of the hardiest fish being cultured; they can withstand water quality conditions and physical handling that would create serious challenges for other species. However, tank culturists need equipment that analyzes the minimum basic water quality parameters of dissolved oxygen, temperature, pH, ammonia, nitrite, alkalinity, chloride concentration, and calcium hardness. The equipment should be of good
enough quality to allow daily measurements, so that daily readings can be compared to determine the effect of increased feed and fish biomass on water quality from one day to the next.
Strict water quality parameters for tilapia culture are difficult to define. Experience at one site may not reflect the same results as those reported in a scientific publication or from another system at another location. There are variables that influence the effect a particular parameter, such as ammonia concentration, has on various fish. Water quality variables interact in complex and often poorly understood ways. Variables such as water temperature, pH, hardness, general fish health, feeding history, and sound and light stressors all have a role in determining whether the lethal level of a particular parameter has been reached.
The following water quality guidelines are based on published information as well as the authors’ experience in the tank culture of tilapia. For further information, the following texts are recommended:
C. Lim and C.D Webster (eds). 2006. Tilapia–Biology, Culture, and Nutrition. New York: The Haworth Press.
M.B. Timmons and J.M. Ebeling (eds). 2007. Recirculating Aquaculture. NRAC Publication No. 01-007. Ithaca, New York: Cayuga Aqua Ventures.
Temperature — Optimum growth for tilapia is achieved at 81 to 84 °F (27 to 29 °C), but acceptable growth rates are reported at 77 to 90 °F (25 to 32 °C). Temperatures in the extreme upper range make it more difficult to maintain dissolved oxygen concentration.
Dissolved oxygen — Operating levels of between 5.0 and 7.5 milligrams per liter (mg/L) are recommended. Growth and feed conversion will be affected by chronically low DO concentrations below 3.5 mg/L. Survival and recovery are possible with short-term exposure (less than 10 minutes) to DO concentrations as low as 0.8 mg/L6.
pH — Tilapia can survive a wide range of pH, from 5 to 10, but are said to grow best at pH 6 to 9. In tank systems, dissolved carbon dioxide causes pH to decline because of the formation of carbonic acid (H2CO3) in solution. Low pH is not as serious a problem in flow-through systems as in water reuse systems, in which a minimum pH of 6.8 is suggested as the lower limit of tolerance for the nitrifying bacteria of the biofilter. Due to the presence of dissolved carbon dioxide, high pH is generally not a problem in tank systems. (See also Carbon dioxide.)
Ammonia (NH3) — Ammonia exists in two forms in the tank environment, un-ionized NH3 (highly toxic) and ionized NH4+(less toxic). Avoid concentrations of un-ionized ammonia greater than 1.0 mg/L. Consult other sources to understand the relationship between pH and the toxicity of Total Ammonia Nitrogen (TAN), un-ionized ammonia and ionized ammonia.
Nitrite (NO2-) — Avoid concentrations greater than 5 mg/L nitrite-nitrogen if chloride (Cl–) is low (less than 10 mg/L). Add rock salt to maintain chloride concentration of 150 to 200 mg/L under normal operating conditions, and increase chloride concentration when nitrite is elevated. The chloride ion alleviates nitrite toxicity and can be added as sodium chloride (NaCl) or calcium chloride (CaCl2).
Nitrate (NO3-) — Nitrate toxicity can occur if levels in water reuse systems exceed the 300 to 400 mg/L nitrate-nitrogen range. Normal water exchanges during filter backwashing or solids removal generally control nitrate concentrations. Water exchange or a denitrification process may be required.
Carbon dioxide (CO2) — Maintain at less than 40 mg/L. Elevated carbon dioxide levels cause lethargic behavior or slow feeding response in fish. While tilapia can tolerate a wide range of pH, dissolved carbon dioxide gas stripping is required in water reuse systems to keep pH above 6.8 and promote conditions favorable to nitrifying bacteria in the biofilter.
Alkalinity — This is the measure of the pH buffering capacity of water, and should be maintained at 100 to 250 mg/L by adding a soluble carbonate or bicarbonate source. Sodium bicarbonate is commonly used because it is readily available, highly soluble, and safe to handle. Dissolved carbon dioxide reduces pH, so higher alkalinities must be maintained if CO2 stripping is poor. Choosing a water source with higher alkalinity reduces operating expenses because less supplemental alkalinity will be needed.
Further water quality guidelines for recirculating aquaculture systems can be found in SRAC publication 452, Recirculating Aquaculture Tank Production Systems: Management of Recirculating Systems.
Continue to Tank Culture of Tilapia (part 5)