They calculated that marine aquaculture could produce 16.5 billion tons of fish per year, or about 4,000 pounds per person.
"And we were being very, very conservative in our calculations," says co-author Halley Froehlich, a postdoctoral researcher at UC Santa Barbara.
Froehlich says it's not likely that aquaculture will be practiced in every feasible location. "And we certainly would never need so much production," she says. "That number was really an overestimate to show what the potential is."
Still, even with a downsized calculation using a much more realistic fraction of the ocean's surface, the numbers are impressive: The scientists' math shows that an area of water about the size of Lake Michigan – roughly 1/67th of a percent of the ocean's surface – could produce about 110 million tons of fish and shellfish per year. That's about the amount of seafood caught annually by commercial fishermen, and about five times the globe's current aquaculture production, Froehlich says.
While the production potential of aquaculture is clearly massive, such volumes of fish and shellfish could not be grown without costs. Aquaculture can offer environmental benefits – but only under certain circumstances, and there are many ways in which aquaculture can go wrong.
Salmon farming in British Columbia has been associated with declines in certain streams' runs of wild salmon, since a parasite called the sea louse that sometimes thrives amid densely raised farmed fish can attack wild fish. (The issue is a contentious one, and scientists, activists and fish farming lobbyists still disagree over how directly salmon farms have impacted wild salmon.)
Many aquaculture operations also rely on wild-caught fish as feed. This has driven overfishing in some places, like Peru, whose anchovy population has been. Shrimp farming operations in Southeast Asia have become notorious for destroying mangrove thickets and pouring harmful effluent into estuaries.
"So, we know and we've seen how aquaculture can be done incorrectly, and we're looking at the potential for improvements," Froehlich says.
Max Troell, a scientist at the Stockholm Resilience Centre, co-authored an essay published in the same issue of Nature Ecology & Evolution that analyzes Gentry's and Froehlich's findings.
"The work of Gentry and colleagues shows that space is currently not a limiting factor for the expansion of oceanic aquaculture," Troell writes.
But there are other constraints. Growing fish means feeding them, and this, Troell tells The Salt in an email, requires either catching wild fish or growing high-protein vegetable crops on land. Since these are products already consumed by people, Troell notes in his commentary piece, "reducing competition with human food resources will be key for sustainability."
In a 2014 paper in the Proceedings of the National Academy of Sciences, Troell and several co-authors assessed aquaculture's potential to improve the resilience of the planet's food systems. In that paper they asked, "[D]oes continued growth in aquaculture enhance or undermine the potential of the global food system" to feed humanity?
The jury remains out on that question.
In an email interview, Troell tells The Salt that, if aquaculture production of fish is scaled up dramatically, "[t]he link to terrestrial feed sources will increase," and so will environmental impacts.
"For filter feeders like mussels, the story is different," he says.
Unlike fish, they don't need to be fed, since they filter naturally occurring nutrients and organic matter from the water.
This, says Troell, makes them "very beneficial species to scale up" in aquaculture.
Growing them could even be good for the environment. Froehlich tells The Salt that dense flotillas of shellfish pens could actually mitigate some types of pollution. For instance, such pens could be useful at river mouths, where nutrients from inland farmlands can cause algae blooms that, in turn, deplete the water's oxygen and create so-called "dead zones" – like the massive one that develops every summer in the Gulf of Mexico, thanks to polluted Mississippi River discharge.
Froehlich is continuing to study aquaculture's potential to sustainably feed the world, with some focus on different types of feed and how efficiently farmland can be used to help produce fish and shellfish. She notes that "fish are extremely efficient at converting feed material into body mass," and that some species can turn food into fat, bone, muscle and other tissue at a conversion ratio of nearly one to one. "That's a pound of feed in, and a pound of fish out," she notes.
Froehlich believes seafood consumption will eventually replace a considerable amount of land-based meat production, and she hopes to quantify the extent to which this could alleviate agricultural pressures on land and water resources.
"There's a discussion and a movement of people switching to pescatarian diets," she says. "So, we want to know, what will that translate into?"
Alastair Bland is a freelance writer based in San Francisco who covers food, agriculture and the environment.