In Alabama, channel catfish are produced in static water ponds that are drained once every eight to ten years. The water becomes eutrophic with natural pond production (plankton) that is fertilized with the nutrient byproducts of feed and fish wastes. Catfish make little use of plankton because most of it is microscopic and catfish do not possess the feeding apparatus to filter it from the water. The plankton may reach such high concentrations that they suddenly die, usually because of poorly understood dynamics. When the oxygen-producing plankton die, producers have a crisis with low dissolved oxygen that could result in major fish mortality. Many producers routinely apply algaecides to reduce plankton populations and the chance of die-offs. Algaecides are costly and may have long-term detrimental effects on the pond ecosystem.
Plankton are excellent nutrition for fish equipped to filter them from the water. However, attempts to co-stock filter feeding fish, like tilapia, in catfish ponds to consume plankton have been frustrated in the past. Producers have been assessed fees by processors for separating the filter fish from the catfish, and some seining crews refuse to harvest ponds containing large tilapia. Lastly, some filter feeding fish have no obvious market value.
The goal of this project was to manage excessive plankton by converting it into another fish product that could be marketed. The producer put tilapia in cages near the aerators where plankton-rich water flowed past them and they remained separated from the catfish. They also didn’t compete for feed, and–because they were in cages– were easily removed at harvest.
The producer evaluated the growth and production of tilapia in cages placed near aerators and at increasing distances from them and found distance from the aerators made no difference on total biomass, size or survival. The highest density of tilapia was 500 per cage. Density of tilapia in cages made no difference on size of tilapia individuals although total tilapia biomass was greater and mortality was higher with increased density. Tilapia are well known filter feeding fish, tolerant of water quality extremes encountered in catfish ponds, and highly marketable. This project found that, when placed in cages, tilapia can be used to manage plankton growth in lieu of algaecides. To be economically profitable, at least fifty percent greater densities of caged tilapia than were used in this study would have to be considered.