MISSISSIPPI STATE -- Mississippi producers had been using 60-year-old recommendations from Alabama to fertilize ponds in preparation for young catfish, but now are seeing good results with updated recommendations made for local soils.

Fish producers fertilize nursery ponds to stimulate the growth of oxygen-producing algae. These phytoplankton become food for zooplankton, tiny animals that are eaten by catfish fry, or those less than 1 inch long. Fertilizer is used to alter the nutrient content of the water, making it more desirable for this tiny plant and animal life.

Chuck Mischke, water quality scientist with the Mississippi Agricultural and Forestry Experiment Station, evaluated phytoplankton and zooplankton response to fertilization in channel catfish nursery ponds.

"We found that ponds responded more to nitrogen than to phosphorus additions, and increasing the nitrogen application had better results," Mischke said. "Based on these studies, the recommended fertilization practices for Mississippi Delta catfish ponds were changed."

Ponds are fertilized to stimulate a good algae bloom that produces oxygen the pond needs and shades the pond, preventing rooted plants from becoming established and interfering with harvest. Fertilization also encourages a good zooplankton population.

Mischke said dramatically changing pond fertilization practices should improve fry growth, survival and health. He said the research was prompted by the number of Mississippi producers having difficulty getting consistent algae blooms, so he began looking at what was being recommended to add to ponds.

"We're still fine-tuning those recommendations, and we'll study catfish ponds in east Mississippi next," Mischke said. "Following the new recommendations that take into account Delta soils, we are seeing much quicker phytoplankton bloom, more consistent blooms, and more preferable forms of algae and zooplankton."

David Wise, MAFES fisheries biologist, said there are no comparative studies in actual production ponds comparing fish production results of the former fertilization recommendations with current ones. However, the new recommendations are very efficient at producing phytoplankton blooms.

"We used to have problems getting good blooms," Wise said. "You've got to have two good blooms on the pond, one of phytoplankton and one of zooplankton. You need the color from the phytoplankton to shade out the pond and prevent too much growth of algae and pond plants.

"The zooplankton bloom is a food source for the fry. Fertilizing the pond properly optimizes the conditions for the zooplankton to grow," Wise said.

Building good zooplankton populations in nursery ponds has tremendous benefits to fry. Mischke said the U.S. catfish industry has largely ignored the importance of zooplankton to catfish fry, but the young fish thrive on the nutrient-rich organisms.

In a series of MAFES studies, researchers looked at the role of natural foods such as zooplankton in catfish fry production. An analysis of the organisms showed them to be excellent sources of protein, niacin and vitamin E, and zooplankton met or exceeded all other known requirements of catfish fry.

"In laboratory feeding studies, catfish fry grew 50 percent more over a 20-day feeding study when fed commercial feeds supplemented with zooplankton compared to commercial feeds alone," Mischke said.

Because of the excellent nutritional value of zooplankton and the positive impacts of zooplankton on fry growth and health, more emphasis will be placed on pond zooplankton populations for catfish production.

Contact: Dr. Chuck Mischke, (662) 686-3560