A new discovery of thriving coral reefs off the coast of Puerto Rico may offer hope for other shallower reefs.

Scuba diving scientists discovered sprawling and diverse coral reefs at 100 to 500 feet (30 to 150 meters) below the ocean surface within a 12-mile (19-kilometer) span off the southwestern coast near La Parguera, Puerto Rico.

With the overall health of shallow coral reefs and the abundance of reef fish in Puerto Rico in decline, this finding brings hope that deeper fish stocks may help to replenish stocks on shallower reefs.

These mesophotic ecosystems — 'meso' for middle and 'photic' for light — are the deepest of the light-dependent coral reefs. Too deep for exploration with traditional scuba gear, these reefs have until recently remained largely unexplored because of the cost and technical difficulty of reaching them. Advances in diving techniques allowed scientists to safely dive and conduct the new survey.

"We had no idea how extensive, vibrant and diverse these mesophotic coral ecosystems are off La Parguera," said Richard Appeldoorn, the study team leader from the University of Puerto Rico, Mayaguez.

"At mesophotic depths in Puerto Rico, scientists are seeing fish species that were once common inhabitants of shallow reefs such as groupers, snappers and reef sharks," said Kimberly Puglise of the National Oceanic and Atmospheric Administration (NOAA), the organization that funded the study. "These reefs stand in stark contrast to declining shallow water reefs in the same area."

Because of the potential of mesophotic reefs to restore depleted fish stocks, local managers are looking carefully at adding protections for these coral ecosystems.

"We recognize the need to extend protections to mesophotic coral ecosystems in Puerto Rico, and the information being provided by this research is key to making that happen," said Ernesto Diaz, director of Puerto Rico's coastal management.