“As you feed fish, phytoplankton also grow—producing oxygen in the daytime and consuming it at night,” said fish biologist Eugene Les Torrans, who is in the Agricultural Research Service’s Warmwater Aquaculture Research Unit (WARU) at the Thad Cochran National Warmwater Aquaculture Center in Stoneville, Mississippi. “If you feed too much, oxygen gets too low, and fish can die.”
Traditionally, oxygen management was based on farmers’ observations. If fish were seen at the water surface sucking oxygen in the morning, aeration equipment was turned on. If no fish were seen, it was presumed there was enough oxygen.
Mr Torrans’s experience and research has shown otherwise. “There’s a DO concentration at which fish die and a little higher DO at which they survive, but are responding to stress,” he says.
“In the daytime, DO can get as high as 14 parts per million (ppm), and if it gets down to 3 ppm at night, fish are still fine. But when DO drops lower, the fish become partially asphyxiated.”
Fish first respond to oxygen stress by losing their appetite. When DO drops too low in the morning, fish eat less feed. “As a result, the production cycle increases,” Mr Torrans says. “Instead of fish growing out in two years, it takes four years and sometimes five years.”
Mr Torrans examined the impact of DO concentration on channel, blue, and hybrid catfish growth, yield, feed consumption, and feed conversion. A computer-controlled pond-oxygen monitoring system was used to maintain precise DO levels in the morning—3.0, 2.0, and 1.5 ppm.
Scientists found that a DO level of 3.0 ppm is required for optimum production. This minimum DO concentration improves growth, significantly shortens the production cycle, reduces fish losses, and greatly improves feed conversion.
“This revelation has changed the industry’s oxygen-management practices,” says WARU research leader Craig Tucker.
“We have gone from keeping fish alive to keeping them growing. You can double the growth rate of the fish in your pond by managing oxygen. It’s pretty incredible.”
“We’re trying to reduce production costs as much as possible,” Mr Torrans says. “We now have exact numbers in terms of how much aeration is needed to maximize fish feed intake, growth, and production.