When fishermen returned to the deep reefs of the Gulf of Mexico weeks after BP’s gushing oil well was capped, they started catching grouper and red snapper with large open sores and strange black streaks, lesions they said they’d never seen and promptly blamed on the spill.

Now, two years after the Deepwater Horizon rig exploded and sank, killing 11 men and touching off the worst offshore spill in U.S. history, the latest research into its effects is starting to back up those early reports from the docks: The ailing fish bear hallmarks of diseases tied to petroleum and other pollutants. Those illnesses don’t pose an increased health threat to humans, scientists say, but they could be devastating to prized species and the people who make their living catching them.