By midcentury, mankind must double food production to fill the bellies of an exploding population of humans, according to the United Nations. A big part of that growth is likely to come from farmed seafood.

“With Earth’s burgeoning human population to feed, we must turn to the sea with new understanding and new technology,” famed oceanographer Jacques Cousteau said in 1973. “We need to farm it as we farm the land.”

Balancing accelerated food production with sustainability is a tricky act, but on Sunday scientists at the American Association for the Advancement of Science annual meeting in Boston described how aquaculture could possibly pull it off -- and what challenges lie on the road ahead.

U.S. Department of Agriculture, or USDA, aquaculture program leader Jeffrey Silverstein pointed out that 70 percent of the Earth is covered in water, yet only 1.5 percent of human food is drawn from it.

Finned fish are much more efficient sources of protein than other kinds of livestock -- for every pound of food you put into a fish, you get about a pound of body weight. In contrast, you have to feed chickens two pounds of food to get a pound of body weight, and you have to give pigs three or four pounds of food for each pound they put on, according to Silverstein.

However, that pound of fish food can require several pounds of flesh to produce, much of it from other fish, usually from processed pelagic fish such as anchovies. To increase sustainability, fish farmers have been turning to diets that consist of more and more plant products and less and less fish meal.

But using plant products brings its own host of problems, one of the more significant being that a farmed fish that dines more on plants than on animals tends to be less oily and nutritious. Frequently, farmed fish have to be fed fish oil near the end of their lives to help make them healthier for humans to eat. One of the ways to mitigate that dip in nutrition could be to turn to an alternative source of fish food: microbes.

Researchers are currently trying to perfect the production of various microbes that could bulk up a farmed fish’s diet, thereby cutting the amount of food and agricultural land used indirectly by fish food production.

Margareth Overland, a nutritionist with the Aquaculture Protein Center at the Norwegian University of Life Sciences, said one of the more promising microbial sources for fish food is yeast grown on processed spruce wood. Algae and bacteria are also being studied.

Microbial and bacterial diets not only pump a fish full of vitamins, but also seem to help reduce the intestinal inflammation in the fish.

“Not all that different from the probiotic yogurts we buy in stores,” Overland said.

However, microbe-based food is years away from being ready to supply large industries, and researchers still need to conduct taste tests to make sure there's nothing especially fishy about a salmon that's been dining on yeast.

Changing fish food will likely also help cut the carbon footprint of many fish farms. One of the biggest components of a Norwegian salmon farm's greenhouse-gas impact is associated with the transportation of food sources, so if microbial food is a success, it could lessen that impact.

Other options for alternative fish food exist, too. A British business magnate has been creating protein meal made from housefly larvae, better known as maggots, and selling that to poultry and salmon farms in South Africa, as NPR reported.

There may also be ways to tweak fish to get more nutrition from them. Preliminary research has identified a genetic variant in rainbow trout that prompts them to make more omega-3 oils, a trait that could be exploited through traditional breeding techniques, USDA's Silverstein said.

At the moment, “genetically improved” fish stocks make up just 10 percent of farmed fish in the U.S., Silverstein said.

That figure could change soon. In December, the U.S. Food and Drug Administration released a preliminary finding that AquAdvantage salmon -- a genetically modified Atlantic salmon that grows to full size much faster than normal fish thanks to the addition of genes from Chinook salmon and the ocean pout -- would have “no significant impact” on the environment. That move signals that AquAdvantage salmon is nearing final approval.

But AquAdvantage salmon has become a major flashpoint in the debate over genetically modified organisms. Last week, the FDA said it would extend the comment period on its environmental assessment of AquAdvantage salmon until late April.

Improvements in fish-farm construction are also expected to boost aquaculture production.

Catfish farmers have found success with split-pond designs. Such a design entails separating a smaller area, where the fish are kept, from a much larger area, where the water is treated. Split-pond farming has helped catfish farmers see their yields triple to 15,000 pounds per acre from 5,000 pounds per acre in the last several years, according to Silverstein.

Land-based enclosures are also catching on in aquaculture. Right now, you might be perturbed at the thought of a salmon grown in the Midwest, but such a scenario could be in the future of sustainable fish farming.

Steven Summerfelt, who oversees research into sustainable aquaculture techniques at the Arlington, Va.-based nonprofit environmental group The Conservation Fund, said the latest contained fish farms use less water and pollute far less than farms in open oceans.

“If done properly, there’s no discharge allowed,” Summerfelt said.

A single 3,300-metric ton next-generation salmon farm could produce enough fish each week to fill up an entire schoolbus, top to bottom, side to side, Summerfelt said -- a rate that could satisfy 1 percent of U.S. salmon consumption.

Human population explosion and declining wild-fish stocks around the globe practically dictate that fish farming will be a necessity in the near future -- and if the next generation of aquaculture realizes its promise, it could ensure that seafood stays on the plate for generations to come.

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