THORNTON, Miss. -- Mother Nature works in mysterious ways, as in the case of the tilapia fish. They spend their entire lives at one Mississippi hatchery cleaning up after catfish, only to be eaten by them in the end. Whatever the reason, it's a system that seems to work for Louie Thompson, who uses tilapia to perform a myriad of duties at his catfish hatchery.

Thompson Fisheries in Holmes County, Miss., has 57 water acres of brood stock, 280 acres of fingerlings and 10 acres of grass carp. Because tilapia is a tropical fish, Thompson holds them in 1,000-gallon water tanks held at about a 90-degree temperature.

The tilapia fish clean egg baskets at Thompson's catfish hatchery, and he knows from experience that it's no easy task. "I grew up cleaning these baskets," he says. "We used to have one person working the better part of a day spraying down and cleaning egg baskets. Now, with the tilapia, it takes only seconds."

That's readily apparent as the tilapia rush immediately to a dirty egg basket tossed by Thompson into one of the tanks, causing the water to erupt with bubbles of activity. Within seconds, the baskets are sparkling clean, with no remaining residue.

Thompson also uses tilapia as catfish feed when protein needs are high. "Tilapia are very prolific, and will multiply quickly during the summer months. Then, when the weather cools, and the tilapia become sluggish, they are slow enough that the brood catfish can feed on them. That's the same time the brood catfish are building up their egg supply for spring," he says.

"Tilapia are a supplemental, high-protein food source for our breeder stock at exactly the right time."

When springtime rolls around again, Thompson says, the tilapia have been completely eliminated from the ponds. "There are no trash fish left come spring. If the brood fish don't get the tilapia, the cold weather will," he says. "Their whole system shuts down when temperatures become cooler. If we get a hard cold front, they'll die quickly. Ideally, we like to see temperatures cool down slowly, giving the brood fish time to consume the slower tilapia."

Les Torrans with the Thad Cochran Warmwater Aquaculture Center in Stoneville, Miss., researched the benefits of adding blue tilapia to supplement commercial catfish feed, and recommends the practice.

"The group fed tilapia, in addition to the commercial fee, had a higher spawning success. They also produced larger eggs, with a higher hatch rate, and larger fry," Torrans says. "Overall, the group fed tilapia produced 15 percent more sac-fry per pond than the control group, and over three times as many as the industry average."

A warmwater fish, tilapia begin to die when the water temperature drops below 50 F, according to fish biologists. In most parts of the Mid-South, tilapia will not survive in ponds unless they are supplied heated water, although they are sometimes able to successfully overwinter in ponds and tanks located along the Gulf Coast.

Thompson also keeps tilapia under the nets holding newly hatched catfish in their concrete tanks to clean up any excess feed that gets through the nets. This improves the water, he says, and helps to keep the tanks clean. This system also helps to eliminate problems caused by excess feed in a tank, including the development of high ammonia levels, fungus and bacteria that can harm catfish.

"It improves overall fish health," he says. "A tilapia is like a fresh-water hog. It will eat just about anything."

Thompson uses another fish — the grass carp — in his fingerling ponds and brood ponds as a biological grass control. He uses a stocking rate of 40 sterile grass carp per surface-water acre to eat pond vegetation.

The grass carp, also known as the white amur, was imported into the United States from Southeast Asia in the early 1960s as an experimental aquatic weed control method, according to Martin Brunson, fisheries specialist with the Mississippi Department of Wildlife and Fisheries.

"Grass carp are a viable and economical means of controlling the growth and spread of certain aquatic weeds," Brunson says. "For most farm pond situations where weeds have already become a problem, five to 10 grass carp per surface acre will achieve desired weed control, without resulting in crowded conditions of the fish. In severely weed-choked cases, higher rates of 15 to 20 grass carp per water acre may be necessary to attain control."

Thompson adds, "You're going to end up loosing up to 90 percent of your fish if your pond gets choked up with aquatic weeds, but we don't use any herbicides. There are a lot of potential problems with using herbicides in fingerling and brood ponds. You can experience severe oxygen problems if you kill off too many aquatics at one time. Chemicals are often not very consistent in their performance, and you're putting chemicals where they probably shouldn't be chemicals."

e-mail: dmuzzi@primediabusiness.com