"Rust tide" has returned to the East End for the 11th straight summer, with experts saying the fish-killing algae blooms struck sooner and harder than in the past.

In Sag Harbor, the algae were detected in the highest densities in several years, said Christopher Gobler, a professor at the Stony Brook University School of Marine and Atmospheric Sciences.

Gobler said the rust tide in Sag Harbor exceeded 30,000 cells per milliliter. Marine life can be harmed when levels exceed 500 cells per milliliter.

The algae also have invaded Three Mile Harbor and Accabonac Harbor in East Hampton, but at much lower densities, Gobler said.

Gobler and Richard Amper, executive director of the Long Island Pine Barrens Society, said the early Sag Harbor bloom surprised them.

Rust tide usually first appears in the western Peconic Estuary in mid- to late August and then travels east. But late last week rust tide appeared at "moderate levels" in East Hampton, said Gobler, who initially dismissed that as a blip.

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"However, this week, the rust tide spread to at least three distinct harbors and reached a level in Sag Harbor we have not seen anywhere on Long Island in several years," he said in a statement.

Rust tide, which creates a brown and orange hue, likely will infest Suffolk waters until temperatures cool around mid-September, experts say. Summer storms help kick off the blooms by increasing nitrogen pollution from human waste and fertilizer.

Though it also kills shellfish, rust tide is not toxic to humans, experts said, though swimmers should shun it, the way large fish do.

"We suggest people use common sense; when you see discolored water, it's not a good idea to be recreating in that situation," said Kimberly Reece, a professor at the Virginia Institute of Marine Science in Gloucester Point.

To stamp out rust tide, Amper urged elected officials to enact laws cracking down on nitrogen pollution.

"We need to pass state legislation that will mandate a dramatic reduction in the discharge of nitrogen in the ground water, principally sewage and also fertilizer," he said.