Global demand for fish products has doubled since the 1950s and is still rising. Today more than 40 percent of the world's seafood comes not from wild catches but from land-based and offshore farms. With many wild fisheries already overharvested throughout the world, aquaculture is an important food source -- especially for poor countries -- and has made seafood more abundant and affordable. But some fish farms pollute surrounding waters, and escaped farm fish compete with wild stocks and spread diseases. Moreover, raising carnivorous fish can use up more fish protein for feed than it produces, further stressing wild fisheries. There are also growing concerns about whether imported seafood is safe to eat and whether the United States regulates fish imports strictly enough. Congress is considering legislation to expand ocean aquaculture, but many fish and marine experts urge caution, saying we know little about the potential impact on the oceans.

Jennifer Weeks is a freelance writer in Watertown, Mass., who specializes in energy and environmental issues. She has written for The Washington Post, The Boston Globe Magazine, Environment, On Earth and other publications, and spent 15 years as a congressional staffer, lobbyist and public policy analyst. She holds a B.A. from Williams College and master's degrees from the University of North Carolina and Harvard University.