After proper stocking, the most important aspect of cage culture is providing good quality feed in the correct amounts to the caged fish. The diet should be nutritionally complete, containing vitamins and minerals. Commercial pellet diets for tilapia, catfish, or trout are best. Protein content should be 32 to 36 percent for 1-to 25-gram tilapia and 28 to 32 percent for larger fish. Feeds and feeding are the major costs of production. Floating feeds allow observation of the feeding response and are effectively retained by a feeding ring. Since it takes about 24 hours for high quality floating pellets to disintegrate, fish may be fed once daily in the proper amount, but twice-daily feedings are better.
Good results can be obtained from sinking pellets, but extra care must be taken to ensure they are not wasted. Sinking pellets disintegrate quickly in water and have a greater tendency to be swept through the cage sides. More than one feeding is needed each day; tilapia cannot consume their daily requirement of feed for maximum growth in a single meal of short duration. Fish less than 25 grams should be fed at least three times daily.
Sinking pellets may be:
- slowly fed by hand, allowing time for the fish to eat the feed before it sinks through or is swept out of the cage,
- placed in shallow, submerged trays, or
- placed in demand feeders.
Feeding slowly by hand is inefficient. Use of a tray allows quick placement of feed onto the tray, but multiple daily feedings are still required.
The correct amount of feed must be weighed daily. Feeding rate tables or programs are required to make periodic increments in the daily ration. Feeding adjustments can be made daily, weekly or every 2 weeks. The fish should be sampled every 4 to 6 weeks to determine their average weight and the correct feeding rate for calculating adjustments in the daily ration. Adjustments can be made between sampling periods by estimating fish growth based on an assumed feed conversion ratio (feed weight divided by weight gain).
Example: with a feed conversion ratio of 1.5, the fish would gain 10 grams for every 15 grams of feed. The correct feeding rate, expressed as percent of body weight, is multiplied by the estimated weight to determine the daily ration. Recommended feeding rates are listed in Table 2. Feeding rate tables serve as guides for estimating the optimum daily ration, but are not always accurate under a wide range of conditions, such as fluctuating temperatures or DO. Demand feeders can be used to eliminate the work (feed weighing, fish sampling, calculations) and uncertainty of feeding rate schedules by letting the fish feed themselves.
The demand feeder in Figure 1 consists of an 11-inch polyethylene® funnel with a toggle inserted into a 5-gallon plastic bucket which is mounted on the cage top.
The bucket holds 12 pounds of feed, about 3 days’ supply for a l-cubic meter cage. Fish quickly learn that feed is released when they hit a brass rod that extends from the funnel into the water. Demand feeders and feeding rate schedules produce comparable growth and feed conversion, but demand feeders reduce labor by nearly 90 percent. Feeding rate schedules may still be used with demand feeders by adding a computed amount of feed daily instead of refilling the feeder whenever it is nearly empty.
Because floating pellets are round and uniform in size, they are best for demand feeders, but sinking pellets will also work. Sinking pellets disintegrate rapidly and clog the feeder if they are splashed; and the less uniform size of sinking pellets makes adjustment of the trigger mechanism sensitivity more difficult.
With high quality feeds, good growing conditions and effective feeding practices, feed conversion ratios as low as 1.3 have been obtained. Generally, feed conversion ratios will range from 1.5 to 1.8.
Sampling and harvesting
To remove fish during sampling or harvesting, the cage is partially lifted out of the water and fish are captured with a dip net. A sample of fish may then be counted, weighed and returned to the cage for further growth, or all of the fish maybe harvested. If size uniformity is important, 4 weeks or more maybe required for complete harvest, because not all fish reach the desired harvest size at the same time.
Southern Regional Aquaculture Center, Publication No. 281
Download: Cage Culture of Tilapia