GLOVER’S REEF, Belize — As Alex Tilley powers his 15-foot skiff over the turquoise surface, a dark form slips across the white sand floor below. “Sting ray,” Mr. Tilley says.

For the next half mile, en route to the Wildlife Conservation Society research station here at Glover’s Reef in Belize, at least half a dozen rays are spotted moving beneath the surface. To Mr. Tilley, the presence of so many rays says a lot about the state of the reef here.

Protected Reef Offers Model for Conservation -