Captive-raised specimens of some of the world’s largest and most critically endangered freshwater fish—including a five-foot-long Mekong giant catfish—were released this month into Cambodia’s Tonle Sap, the largest lake in Southeast Asia. Scientists hope the released fish can survive and begin to rebuild wild populations decimated by decades of overfishing, dam building, and other human actions.

Those threats still exist, and the released species—the giant catfish, giant barb, and striped catfish—are considered bellwethers of the danger to a larger fishery that sustains millions of Cambodians. 

“This release is significant, but it is just the first step of many actions that will be needed for the long-term recovery of these giant fish,” says Zeb Hogan, a fish biologist at the University of