It's a tough time for seafood lovers.

Prefer your fish from the ocean? That habitat is becoming a less hospitable place every day, according to a recent international State of the Oceans report. Water is getting warmer, more acidic. Dead zones are growing. A mass extinction of certain fish and coral species could happen sooner than scientists previously thought.

Don't mind a farmed fish? Maybe you should. It often grows in conditions so dirty it requires constant antibiotics and other chemicals just to grow big enough to eat.

Plus, 84 percent of U.S. seafood is imported and poorly regulated, which is why so much of it is mislabeled, disguised as pricier fish. You could be paying $23 a pound for red snapper that's really $3 a pound tilapia.

But there's a scientist in Brooklyn, N.Y., who says he has a solution to all of these problems.

For Martin Schreibman, the key is fish poop.

'Urban Aquaculture'

Schreibman's lab on the campus of Brooklyn College in New York is full of pipes and pumps whirring and clanking.

You're hit by a fishy smell when you walk in, and you quickly see why: Jacuzzi-sized tanks, filled with tilapia.

"These guys haven't been fed today, so you can see how voracious they are," Schreibman says, tossing food pellets in the water.

His tanks are part of a system very different from a fish farm or natural ecosystem. Schreibman's worked for years to develop an advanced water-recirculation system that eliminates the need for chemicals during the growth process. It filters plain old tap water in and out of a tank, constantly circulating and removing fish waste.

Over the course of years of work, Schreibman says, "it just occurred to me and my colleagues that we can grow a lot of fish in a very small area, on land, under controlled conditions. And there are no antibiotics, pesticides or hormones."

Schreibman works primarily with tilapia, which are resilient and good for research. But he says he can tweak temperature, salinity and other factors to grow most any kind of widely consumed fish — and grow them using only about a gallon of water per fish.

Schreibman calls this method urban aquaculture. And he thinks it could catch on as people grow increasingly concerned about where their food comes from and whether it's sustainably produced.

"This is the future," he says.

Just Add Water

Schreibman didn't exactly invent aquaculture — the term is basically a catchall that refers to any alternative method of fish farming — but he's thought as much as anyone else about how to make it urban, by making his recirculation system small enough to run anywhere on a municipal water source.

His utopian city is one with Jacuzzi-sized fish tanks on every roof, giving locavore owners more than 100 pounds of fish a year.

Schreibman further sweetens the deal with something called hydroponics. By tweaking his filtration system to leave a certain amount of fish waste in the water, plants can be grown in the same tank.

"We're talking plants floating on the surface of the water, using the fish waste as nutrition," he explains.

Lettuce, herbs, bok choi and kale can all be grown this way. The plants float on a foam sheet, their roots dangling into the water below.

"Fish poop a lot," Schreibman says. "People would be amazed at how much product you can produce in a certain area."

He says lettuce heads, for instance, can be grown six inches apart and cut in about six weeks. Herbs can be snipped for cooking and continue to grow.

There are drawbacks to such a system. A hydroponic garden is more expensive than dirt. A private company in Milwaukee will sell you one for about $3,000. And you get the pleasant task of gutting your own fish.

But Schreiman thinks it will catch on.

"The people I spoke to seven or eight years ago — their eyes used to glaze over — are now hearing me speak again and they're saying, 'Oh, I get it now,'" he says.

"It's clean. It's productive. You can get a head of lettuce from seed to your plate in six weeks — from a tank in your backyard."