KAPOLEI — The little heads of lettuce were green with the promise of salads, and Sam Moku smiled at the thought. They were growing nicely, almost without the need for care, thanks to the most unlikely gardeners he had ever had seen: A small school of baby tilapia.

The fish live in a large black tub beside the lettuce, which had been planted in a homemade tray of water about the size of a pool table. A pump quieter than a carpenter bee sent water from the tub to the wood tray, where a small drain allowed the water, now rich with nutrients from fish waste, to trickle back to the tilapia.

Here in a courtyard at the Department of Hawaiian Home Lands offices in Kapolei, where Moku is a program manager, fish and plants offer a telling demonstration of sustainability that some view as the future of farming. It's called aquaponics, and DHHL likes the practice so much that it wants to include it in a new 18-home subdivision it plans to build in Wai'anae early next year. If the DHHL model succeeds, aquaponics could become a fixture in backyards across the Islands.

"What we really want is to get our homesteaders involved so they can have them in their backyards," Moku said. "It's beneficial. You have your own source of food. Protein from the fish, and you have your vegetables. You are really promoting sustainable food in your own home."

Through a marriage of raising fish and growing plants, water saturated with fish waste fertilizes floating plants, which clean the water before it's returned to the 110-gallon tub. Vegetables can be harvested in six weeks and fish in a few months. Proponents love it for its simplicity.

"Once you get it set up, it runs by itself," Moku said. "It's not complicated at all."

Aquaponics is so efficient — and potentially compact — that it's catching on all over Hawai'i. Regular classes for homeowners are taught in Waimanalo, and the practice is spreading commercially on the Big Island.

'IT'S EASY'

The broad appeal of aquaponics is easy to grasp. A system built on a 4-by-8 sheet of plywood and drawing fertilizer from five pounds of fish living in a 100-gallon tub can grow 50 heads of lettuce. Every seven months, you can harvest 30 pounds of fish — tilapia, catfish, gourami, carp and just about any other freshwater variety.

"It's green and popular and it's easy if you do it right," said Harry Ako, a science professor at the University of Hawai'i-Manoa who is studying how Pacific island cultures can use aquaponic systems.

He's also advising DHHL and the Institute for Human Services, which plans to put an aquaponics system on the roof of its women's shelter to help feed 300 to 400 residents.

"It's compact agriculture," Ako said. "It is island agriculture."

The largest aquaponics operation in Hawai'i is also generally recognized as the oldest — Friendly Aquaponics in Honoka'a, owned by Tim Mann and his wife, Susanne Friend.

The couple, who also own a crane company and a drafting service, got into aquaponics in August 2007 when the Big Island construction industry dried up. They knew a little about the process and flew to the University of the Virgin Islands to take a course from James Rakocy, who runs the world's most successful large-scale project.

Mann and Friend planted their first crop in January 2008 and spent much of the year expanding their operation and experimenting with 100 different kinds of vegetables to see what worked best. They grew about 4,000 pounds of vegetables.

The couple settled on lettuce as their cash crop of choice and landed a contract with Costco on the Big Island, which wants more lettuce — 800 pounds a week — than Friendly Aquaponics can grow at the moment, Friend said.

They continue to "expand like crazy," confident they can meet Costco's need by the end of this year, but they also are discovering the universal challenge of farming — the weather.

Last summer, Mann and Friend harvested up to 600 pounds of lettuce a week, but discovered this summer that cooler temperatures limited their yield to about 320 pounds a week and slowed fish growth as well.

"We thought all we had to do was warm up our water, but we found we needed more sunshine," Friend said.

Still, the business gets a lot of bang for the buck. It needs only 2,816 square feet to grow crops that would require nearly 3 acres of land if grown traditionally, Mann said.

"You can plant about four times as densely as you do on the ground," he said. "The result of that is that in a very small area you can produce about eight times the vegetables as you do in the same amount of ground."

GREENBACK HARVEST

Because they want to expand production, Friendly Aquaponics has yet to market its two types of tilapia, which it hatches on site, Friend said. But most of their income — 80 percent to 90 percent — comes from plants, not fish, she said.

"At this point we are not making that much money from ... (tilapia), but we make a whole bunch of money from the lettuce," she said. "But the fish make it possible, so we love our tilapia."

As Friend sees it, one of the problems with aquaponics is that she has to buy imported fish food, which is neither organic nor of good quality.

To solve that, the couple will install a methane biodigester, which can generate renewable energy while converting waste from a nearby slaughterhouse into fish food, Friend said. As a bonus, the large amount of heat byproduct from the biodigester can be used to warm the operation's 12,000 fish.

Last October, Friendly Aquaponics, which Oregon Tilth certified as the nation's first organic aquaponics operation, was the only business of its kind on the Big Island. But Mann and Friend wanted to share what they learned and offered classes on aquaponics.

Now there are seven new commercial operations on the Big Island, Friend said.

"We teach people everything we know," Mann said. "We don't hold anything back."

One of their students was Glenn Martinez, who runs a five-acre farm in Waimanalo called Olomana Gardens. At the time, he was using diverted streamwater, which he ran through fish and duck ponds and then into his crops, but discovered the water contained unsafe levels of E. coli. The bacteria can cause severe food-borne illness.

He turned to aquaponics. Martinez filled a 1,500-gallon tank with clean water and stocked it with 800 tilapia. The fertilized water flows through his grow beds, which use volcanic cinder to hold the plants and help filter contaminants.

"My vegetable garden essentially is a wetlands cleaning up the water," Martinez said. "It's my biological filter."

He grows organic taro, lettuce, cucumbers and tomatoes. And his tilapia, which he sells to restaurants when they reach three pounds, fetch up to $25 a fish.

"Why do restaurants want big fish?" he said. "To fillet them. There is no way they are going to sell a tourist bones. It's nice white, flaky meat."

Now Martinez offers his own classes. He's a big proponent of what he calls "barrelponics" — a 55-gallon drum with 30 fish placed under a typical, 6-foot-long "lu'au table" and three trays on top for plants. Any homeowner can do it and no permits are needed, he said.

Starting backyard aquaponics isn't cheap, though. The average cost of set-up is $700, Martinez said.

"But the most green thing about it is it doesn't matter what land you are on," he said. "It can be on any land, anywhere. It can be in an asphalt parking lot, on rooftops."

And one more thing, he said, as if to complete a pitch for perfect gardening.

"In aquaponics, I'd say 90 percent of the tables are at waist level, so there is no bending over," he said. "And there is no weeding."

'GROWING LIKE CRAZY'

In Kapolei, the lettuce growing in the DHHL courtyard is about the size of a prizefighter's fist. The plants in the water-filled grow bed are suspended by a large, thick sheet of Styrofoam.

When an admiring Sam Moku lifts the sheet, his eyes widen at an array of roots, thin and wispy as an old man's beard.

"Look at that root system," he said. "Oh my gosh, it's growing like crazy."

When Moku first heard about aquaponics in June, a few months after the department started researching the practice, he immediately set one up at his Kane'ohe home.

His family is waiting for the first crop of lettuce, tomatoes, green onions and basil, but has already eaten one of the larger tilapia.

"I was looking at a way to be as self-sufficient as I could be in this economy, by producing my own food," he said. "I really found that it was very rewarding that we were able to produce something out of our own backyard."

The hardest part was not the gardening, it was getting used to the idea of eating the tilapia, a fish with a bad reputation because it thrives in foul waterways, including the Ala Wai Canal.

"There was a little tentativeness in taking that first bite," Moku said. "The first time we ate the fish, we filleted it with a little mayonnaise sauce. It tasted excellent."