BARATARIA BAY, La. - 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 had 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.

There's no saying for sure what's causing the diseases in what's still a relatively small percentage of the fish, because the scientists have no baseline data on sick fish in the gulf before the spill to form a frame of reference. The first comprehensive research may be years from publication. And the gulf is assaulted with all kinds of contaminants every day.

Still, it's clear to fishermen and researchers alike that something's amiss.

"Some of the things I've seen over the past year or so I've never seen before," said Will Patterson, a marine biologist at the University of South Alabama and at the Dauphin Island Sea Lab. "Things like fin rot, large open sores on fish, those were some of the more disturbing types of things we saw. Different changes in pigment, red snapper with large black streaks on them."

All of which has biologists - and many fishermen - worried.

James Cowan, a reef fish expert at Louisiana State University doing long-term sampling for the Louisiana Department of Wildlife and Fisheries, received his first report of fish with what looked like ulcers in November 2010. He began reading up on what scientific literature was available on oil spills and fish.

"There is so much in the literature that links exposure to PAHs [the compounds in oil] to exactly what we are seeing: sicknesses, lesions, and everything else," Cowan said.

Even if oil could be pinpointed as a contaminant, however, it's difficult to definitively tie it to BP's Macondo well. The gulf is littered with natural oil seeps, pipelines, oil wells, and pollution from passing ships.

But in the last year, research has emerged showing deepwater corals, seaweed beds, inshore bait fish, dolphins, and other species were injured by the spill. Portions of a few bays remain closed because of the spill.