Most carnivorous fish are sold to lucrative markets in the United States, Europe and Japan.
To maximize growth and enhance flavor, shrimp and salmon farmers use processed food made from less valuable fish species harvested from the sea - herring, mackerel, anchovy, sardine and other relatively small varieties.
The problem, explains Naylor, is that it takes a lot of mackerel and herring to feed farmed fish. As a result, the average farmer ends up using about three pounds of wild-caught fish to grow a single pound of salmon or shrimp.
About 29 million tons of finfish and shellfish were farmed worldwide in 1997 - a significant contribution to world fish supplies until you subtract the additional 10 million tons of wild fish harvested for feed that year.
In other words, says Naylor, 10 million tons of herring, mackerel and sardine that could have been directly consumed by people or wildlife ended up as processed fishmeal instead.
Another 22 million tons of wild fish also were used for pig and cow feed in 1997.
If that rate of harvest continues, then some populations of herring, mackerel and other fish low in the food chain could virtually disappear from the world's oceans, according to the Nature study. That, says Naylor and Mooney, would have a direct impact on humans as well as seabirds and marine mammals that depend on these wild fish populations.
"The growing aquaculture industry cannot continue to rely on finite stocks of wild-caught fish, a number of which are already classified as fully exploited, overexploited or depleted," warn the authors.
"What our study demonstrates is that you don't get something for nothing," adds Mooney, the Paul S. Achilles Professor of Environmental Sciences.
"It takes a lot of protein to produce protein," he explains. "We're calling upon the aquaculture industry to create a better feed, one that uses fewer fish."