Dissolved oil from the Deepwater Horizon spill off Louisiana wafted underwater all the way down to Florida's Sanibel Island, sickening fish along the way, according to a new study from University of South Florida scientists.

An upwelling of cold water from deep in the Gulf of Mexico swept the oil up onto the continental shelf about 80 miles offshore, spreading it far from where it was spewing out of a damaged rig, the study found.

USF scientists used computer modeling to plot the path of the oil, then tied it in with diseased fish by checking their livers for signs of hydrocarbons with a similar chemical signature to Deepwater Horizon oil. The fish livers were trying to screen out the impurities but could not cope with the quantities, leading to immune system problems.

The study, published this week in a scientific journal called Deep-Sea Research II, is not good news for BP, which is battling the U.S. government and other claimants in federal court over how many billions of dollars in damages it owes for pollution caused by all the oil it spilled.

BP spokesman Jason Ryan dismissed the study's findings as not matching the real-life findings of BP and government scientists during the disaster.

"These researchers use a model," Ryan said. "However, the thousands of water and sediment samples collected and analyzed as part of the Deepwater Horizon response refutes the researchers' hypothesis and shows that there was no (Deepwater Horizon) oil in Tampa Bay or the Florida shelf adjacent to it."

He also pointed out that state officials, with funding from BP, have been conducting tests of seafood and so far no samples have exceeded federal safety standards.

"Nothing we do on Deepwater Horizon is free of controversy," USF oceanographer Steve Murawski, one of the study's authors, said Wednesday.

The disaster began with a fiery explosion aboard an offshore drilling rig on April 20, 2010. Eleven rig workers died, and two days later oil began spraying from the sunken rig 5,000 feet beneath the waves. BP was unable to shut off the flow until July 15, 2010, after an estimated 4.9 million barrels had escaped.

Oil that floated to the surface tainted shorelines from Louisiana to the Florida Panhandle, but did not reach farther south. However, the USF study says oil particles swirled down the shelf paralleling the state's coastline to nearly the Fort Myers area.

Although the spill has largely faded from national headlines, some oil is still in the gulf. Weathered particles remain buried in the gulf bottom's sediment and could be there for up to a century.

Meanwhile, scientists continue trying to gauge the impact on fish and other creatures. Previous USF studies have found that the oil killed off millions of amoeba-like creatures that form the basis of the aquatic food chain. Other scientists and commercial fishermen reported finding eyeless shrimp and deformed crabs.

Diseased red snapper and other fish turned up there a few months after BP shut off the flow of oil. The discovery of fish with lesions faded out the following year, suggesting their ailments were tied to an event that had ended.

During the disaster, the only extensive sampling of fish occurred in the vicinity of the oil rig, Murawski said. But later USF scientists took fish samples where oil had gone to the surface and also along the continental shelf where computer modeling by USF oceanographer Bob Weisberg said the upwelling had occurred.

Based on the diseased fish they found there, and Weisberg's studies of the currents, "we conclude that hydrocarbons of Deepwater Horizon origin were likely transported to the (continental shelf) and may even have entered Tampa Bay and contacted the beachfront between Tampa Bay and Sanibel," the study says.

However, Weisberg cautioned against assuming that this oil was part of the underwater plumes found beneath the gulf's surface after BP sprayed an unprecedented amount of chemical dispersant at the spewing wellhead. At this point, there is no evidence of that, he said.

Craig Pittman can be reached at craig@tampabay.com. Follow him on Twitter at @craigtimes.