Whale sharks, the biggest fish in the sea, may be the latest victims of the massive Gulf of Mexico oil spill.

Officials at the National Oceanic and Atmospheric Administration reported this week that four of the polka-dotted creatures, stretching about 12m long, had been spotted swimming alongside oil in search of food.

Since whale sharks are filter feeders - scooping up plankton and small fish with their gaping mouths as they swim just beneath the surface - scientists are concerned they will swallow large amounts of toxic oil and die.

THREATENED: The world's largest fish are in danger from the Gulf of Mexico oil spill.

"The problem is that these are surface feeding animals and if they digest the oil they will sink and we will not know how many are dying," said Dr Eric Hoffmayer, who has studied whales in the northern Gulf for the University of Southern Mississippi.

"I don't think there is any question we're going to lose whale sharks to this oil spill. That's why we need to tag these sharks so that we can determine how they are impacted by the oil," Hoffmayer said.

Hoffmayer spent three days on the Gulf where he and other researchers discovered an extraordinary gathering of more than 100 feeding whale sharks about 145km south of Grand Isle, Louisiana.

The site where they were feeding was about 95km west of BP Plc's blown-out Macondo well off the Louisiana coast and the gathering of whale sharks was among the largest seen in the northern Gulf of Mexico, Hoffmayer said.

In addition to the danger inherent in swallowing oil, it could cause untold harm to the giant but vulnerable fish when they force the water they feed on, after it is sucked into their mouths, to filter out through their gills.

Hoffmayer and a team of marine scientists came up with a plan on Thursday to tag the sharks so they can track their movements and hopefully find out if oil is being digested.

One of the big problems, he said, is that there is no known way of steering the whale sharks away from oil contaminated areas of the Gulf.

Marine scientists in Mississippi are hoping to save other species from the oil, which breached Mississippi's mainland this week for the first time since the April 20 BP well blow-out.

"This is like a forest fire in the Gulf," said Moby Solangi of the Institute for Marine Mammal Studies in Gulfport, Mississippi.

"The oil is spreading so fast through their habitat it is disappearing," Solangi said.

He said he and his team had rescued hundreds of oil-covered sea turtles and are making room at a facility run by the institute for dolphins that may succumb to the oil.

"Now that the oil is here in Mississippi the animals are running and there is nowhere to go," Solangi said.

"The problem we all face now is how to rebuild nature. People have the Red Cross and such to rebuild their lives after a disaster but dolphins, whales, turtles and everything in the Gulf have nothing but their own habitat, which is now in jeopardy."