U.S. doctors have used ultrasound for years to keep track of an unborn baby's size and development. More recently they use it to identify organs and diagnose diseases. Now farmers use it on fish.

"We use ultrasound technology to determine the sex of channel and blue catfish," said Dr. Gary Burtle, an aquaculturist at the University of Georgia Coastal Plain Experiment Station in Tifton, Ga. "That allows more efficient reproduction and more fish getting to the markets."

In 1996, Georgia farmers sent about $20 million worth of catfish to processors. Nationwide, the farm catfish crop was worth more than $364 million.

Until now, the only effective way to know if a fish was male or female was by dissection -- cutting open the fish to see the reproductive organs. But that's not very efficient.

So why do catfish farmers need to know if they have males or females in their ponds?

Burtle said new hybrids result in larger fish that can be more disease-resistant. When a male blue catfish is bred to a female channel catfish, a new hybrid results: a blue channel. This hybrid can grow 15 percent to 30 percent faster than the parent species.

In the past, catfish farmers had to wait five years for the blue catfish to mature sexually to know if they had males or females. And by that time, the farmer had paid to raise fish he might not need.

When the two fish species are about a year old, Burtle said, farmers can use ultrasound equipment to separate the fish by gender into pens. Then they can sell the female blue catfish and the male channel catfish.

It costs about $1.25 per year to raise a catfish to maturity.

"This technique allows the farmers to market their fish earlier, saving about $5 per fish in carrying costs," Burtle said. They can keep the fish they need to breed faster-growing hybrids and sell the others while they're young.

Those young fish hook diners, too. Most catfish lovers prefer their catfish small, especially their fried catfish. Some claim the milder meat tastes sweeter. So the young fish may reel in premium prices for the producer.

Extension experts say catfish is a fairly low-fat meat. It has about the same fat content as skinless white chicken meat.

"Fat content varies greatly with the cooking method, though," said extension foods specialist Judy Harrison. "If you just can't resist the crunchy skin of fried chicken, broiled or baked catfish removes the temptation."