Harvesting and marketing
In the southern U.S., tank culture of tilapia is carried out primarily for the live fish market. The harvesting of tilapia requires a certain amount of preparation and special handling. The generally accepted size for the live tilapia market is approximately 1.5 pounds (680 g), although fish weighing as little 0.75 pound (340 g) and as much as 2.0 pounds (908 g) will find acceptance in some areas. As the average weight of the stock in a tank approaches market size, buyers, harvest crews and live haul transporters should be coordinated.
Tilapia are usually prepared for sale by purging them, or withholding feed for a period before harvest and transporting. This allows the fish to rid their digestive systems of wastes and improves conditions in the hauling hauling tanks during transport to market. Purging time varies and is influenced by water quality conditions, the type of feed and its ingredients, and the preferences of the marketplace. Withholding feed for 3 to 5 days before harvesting and marketing is common. During that time, water should be exchanged to improve water quality and reduce temperature. Lowering the temperature to about 72 °F (22 °C) slows the activity and the metabolism of the fish and increases the dissolved oxygen in the hauling tanks. Fish grown in recirculating systems often develop off-flavor, which is thought to be caused by the fish feed or bacterial content of the culture system. Purging helps reduce off-flavor problems and may cause the fish to begin utilizing fat reserves where the compounds that cause off-flavor may be concentrated.
Rock salt or non-iodized salt should be added to the hauling tank to help alleviate stressful conditions. Food grade salt that contains the anti-caking agent yellow prussiate of soda (sodium ferrocyanide) should not be used, because this cyanide-based compound is toxic to fish. Although it is not currently approved for food fish, rock salt is classified as Low Regulatory Priority (LRP) by the U.S. Food and Drug Administration. Dissolving salt in the transport water brings the salt concentration of the hauling water up to the salt concentration of the fish’s blood. Rates of about 6.5 pounds of salt per 100 gallons of water are recommended. This equates to a salinity of about 7.8 parts per thousand (ppt). With appropriate preparation, live tilapia can be transported by truck for 18 hours or more with little mortality, and can be held for live sale for up to 1 week after harvest.
Proper handling of fish during the harvesting stage enhances survival and ensures that strong fish will be available to the buyer in the event that fish must be held in a live facility before sale. Tanks should be drained to a water depth that is comfortable and safe for harvesters. A depth of about 50 inches (127 cm) is workable if fish are not so crowded that they jump excessively and risk injury to themselves or workers. Close attention should be paid during the draining and crowding process so that dissolved oxygen levels of 4.0 mg/L or greater are maintained. Stressing fish and causing mortality at this stage risks the resources that have been invested in the operation. Minimizing the number of times the fish are handled between the time they are captured in the culture tank and released into the live haul tank will significantly improve their survival when they reach the fish dealer’s facility. Scooping the fish in the same basket in which they are weighed and carried to the live haul tank reduces handling and the abrasion and removal of the fish’s slime coat.
Harvesters should wear protective eyewear or face shields, gloves, and chest waders or long pants to protect themselves from injury.
All parties involved—the fish producer, the live hauler and the fish dealer—should regard the live fish as a sensitive item, worthy of great care and proper handling, so that all are rewarded with the reputation of producing and delivering a high-quality product.
Much has been learned about the tank culture of tilapia since the early days of production. Successes and failures have added to our knowledge of system design, species selection, stock management and nutrition. There are a number of recurring short courses, workshops and conferences where prospective tank aquaculturists can learn to plan, design, build and operate facilities with confidence. Before putting significant capital at risk, it is to your advantage to use these resources, along with resources available on the Web and from your state Cooperative Extension Service.
Southern Regional Aquaculture Center, Publication No. 282, Revision June, 2009
Download: Tank Culture of Tilapia