Barataria Bay, La. • Two years after the Deepwater Horizon rig exploded and sank in the Gulf of Mexico, touching off the worst offshore spill in U.S. history, research into the disaster's environmental effects is turning up ailing fish that bear hallmarks of diseases tied to petroleum and other pollutants.

Those illnesses don't pose an increased health threat to humans, scientists say, but the problems could be devastating to prized species such as grouper and red snapper, and to 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 from 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.

A recent batch of test results revealed the presence of oil in the bile extracted from fish caught in August 2011, a year after BP's broken well was capped and nearly 15 months after it first blew out on April 20, 2010, leading to the rig explosion that killed 11 men.

"Bile tells you what a fish's last meal was," said Steve Murawski, a marine biologist with the University of South Florida who was chief science adviser for the National Marine Fisheries Service until November 2010 when he began working on oil spill studies for USF. "There was as late as August of last year an oil source out there that some of those animals were consuming."

Bile in red snapper, yellow-edge grouper and a few other species contained on average 125 parts per million of naphthalene, a compound found in crude oil, Murawski said. Scientists expect to find almost none of the toxin in fish captured in the open ocean.

"Those levels are indicative of polluted urban estuaries," he said.

Last summer, a team of scientists led by USF conducted what experts say is the most extensive study yet of sick fish in shallow and deep Gulf waters. Over seven cruises in July and August, the scientists caught about 4,000 fish — from Florida's Dry Tortugas to central Louisiana — using miles-long fishing lines dragged from close to shore out to depths of 600 feet. The work was funded with a federal government grant and help from the Florida Fish and Wildlife Research Institute.

About 3 percent of the fish they caught displayed gashes, ulcers and parasites symptomatic of environmental contamination, according to Murawski, the lead researcher. The number of sick fish rose not only as scientists moved west away from the relatively clean and oil-free waters of Florida but also as they investigate bacteria and parasites that could be the cause of lesions and sicknesses.

Other important differences are that oil and natural gas have been pouring out of fissures from the floor of the Gulf for centuries, and the muddy waters of the Mississippi River flush into the same spots where scientists and fishermen are finding sick fish.

Laboratory work over the past winter on the USF samples indicates the immune systems of the fish were impaired from an unknown environmental stress or contamination. Other researchers say they have come to similar conclusions over the past year.

"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."