Federal agencies are responding to sightings by two charter boat captains of a diesel fuel slick first reported Saturday, June 19, about 10 nautical miles off San Diego’s coast.

Capt. Dominic Biagini said he was looking for blue whales at a feeding hotspot near San Clemente Island when he spotted the rainbow-colored slick on Saturday. The Navy-owned island is about 68 miles south of the Orange County coastline.

On Sunday morning, June 20, Biagini, who operates Gone Whale Watching San Diego, posted a video on social media that he said shows dolphins swimming through parts of the slick.

U.S. Coast Guard officials, the agency responsible for federal waters, said they sent up a helicopter between noon and 1 p.m. on Saturday, determining the diesel fuel covered about three nautical miles.

Officials with the Department of Fish and Wildlife, National Oceanic and Atmospheric Administration and the Environmental Protection Agency said they are also monitoring the slick.

Biagini said in a social media post, that when his charter got out to the spot popular with blue whales looking to feed on Saturday, he was surprised to find they had “seemingly vanished. Just 24 hours earlier this zone was full of the feeding giants.”

Heading south, the charter came upon the fuel slick which he said seemed at the time to span an area much larger than the Coast Guard saw, about 50 miles, he said.

Coast Guard Petty Officer Alex Gray said the diesel fuel spill was detected on Saturday about 10 nautical miles off Point Loma and that an estimated 100 gallons of diesel fuel were in the water. (A nautical mile is about a half mile longer than a land mile.)

“It’s unrecoverable,” he said. “It will likely dissipate into the ocean.”

Gray said the slick could have spread out after the Coast Guard’s helicopter got its first look on Saturday, explaining why the charter boat captain measured a bigger field. How big it may have become, he said, would be speculation.

“We don’t know where the pollution came from,” he said, adding it is not expected to reach a coastline.

Coast guard officials will continue to monitor the slick and expect it to dissipate.

While the slick might quickly dissipate, diesel fuel is harmful to wildlife, said Eric Laughlin, a spokesman with the Department of Fish and Wildlife.

“It’s not as bad as thicker petroleum products that can heavily coat birds and other marine mammals,” he said. “Typically, diesel dissipates faster in natural conditions. Sunlight and heat help break up the hydrocarbons. Since diesel is so light, it only takes a small amount to make a large sheen.”

Laughlin said the department is on standby in the event ill or injured animals need assistance.