SOUTHWEST PASS, La.  A couple of weeks ago, enormous orange-brownish strings of something were seen floating out here in the open water near the mouth of the Mississippi.

The water looked like chocolate syrup in some parts and Coca-Cola in others, said Cindy Cruikshank, who has been fishing for 53 of her 59 years. It smelled like an auto-body shop, and it left stains on the hull of her boat. Along with other fishermen and environmentalists around the country, she had no doubts: this was oil.

“I know what I saw,” she said.

Local scientists begged to differ.

“It is not oil; it has none of the properties of oil,” said Ed Overton, an environmental science professor at Louisiana State University, who analyzed two samples of the mysterious substance provided by the Coast Guard.

This was an outbreak of algae, scientists concluded, common in this area at this time of year.

“A lot of the oil, when it weathered, ended up with a consistency that wasn’t too far from this stuff,” Mr. Overton said, though Ms. Cruikshank and many other fishermen who saw the stuff remain unconvinced.