The images are startling — hundreds of dead sea turtles were found floating in the ocean near Puerto Escondido, Mexico. It’s suspected that the olive ridley sea turtles died after being entrapped in a ghost fishing net, but Mexico’s Federal Environmental Protection Agency has launched an investiation and is looking into algae as a potential cause.

When anglers discard their nets into the ocean, the equipment continues to trap and kill marine life. When the turtles were found on Aug. 28, their shells were cracked from the sunlight, leading scientists to believe that the 300-plus turtles had been dead for more than a week.

Olive ridley sea turtles are listed as Vulnerable by the IUCN. Mexican officials say they will speak with local fishermen about the impact of discarded nets on local marine life.