For John Davis, raising and harvesting shellfish is a way to give back to the sea, rather than taking away.

The 72-year-old diver and fisherman figures every mussel or oyster grown on his 6-acre "aqua farm" in Carlsbad is one less that will be taken from the ocean.

"I've always been involved with the ocean, and it has always treated me well," Davis said last week. "This was an opportunity to give back to the ocean."

Davis started Carlsbad Aquafarm Inc. about 18 years ago in a section of the Agua Hedionda Lagoon that he leases from San Diego Gas & Electric, the owner of the Encina Power Plant just south of downtown Carlsbad.

Each year, the farm sells millions of shellfish to restaurants across the country.

Davis runs the business with help from his son and nearly 20 other employees. They primarily raise mussels and oysters, though they also grow sea horses, abalone, algae and brine shrimp.

The sea life is sold to nearly 50 distributors that transport it to restaurants and stores nationwide. Some of the shellfish also has been sold at local grocery stores, Davis said.

His biggest competition comes from Canada and Mexico, where the federal governments subsidize shellfish farms, Davis said. Still, he said he's able to turn a decent profit.

The farm isn't easy to spot. Those driving or walking along Carlsbad Boulevard might miss the rows of white buoys that float in the lagoon. Mussels hang from them in 8-foot-long mesh sleeves submerged in the salty water.

Still, the operation is a valued part of the lagoon, which supports a variety of uses, said Eric Munoz, chairman of the Agua Hedionda Lagoon Foundation, a nonprofit group that works to preserve and enhance the lagoon.

"We are very happy that they're there," he said. "We support them up and down, left and right."

Davis started the farm after retiring as a commercial airline pilot. He and his sons built the farm from the ground up, including heavy-duty filtration systems.

"We put water back into the ocean cleaner than it comes in," Davis said. "We're a pretty doggone green operation."

Also, Davis said, the "enhanced" habitat benefits all of the surrounding wildlife.

In the beginning, the farm raised only mussels. In the years since then, that part of the business has greatly expanded and the farm has begun producing a variety of other sea life.

Davis said he doesn't expect to add any other shellfish, largely because he only wants to grow things that are indigenous to the area.

"At one point there was an effort to grow East Coast lobsters here," Davis said, shaking his head in disapproval. "If God wanted East Coast lobster here, he would have put it here."

Call staff writer Stacy Brandt at 760-901-4009.