NEW YORK (AP) – The city’s waterfront is getting cleaner, and bothersome river critters not seen in hundreds of years are once again attacking wooden ships and piers.

The waters were once so filthy that early 20th-century sailors could be sure their boats would be safe from such threats – because organisms simply couldn’t survive in the muck.

But scientists are now seeing a resurgence in gribbles, shrimp-like crustaceans that grow to about one-17th of an inch in length and attack wood from the outside, and shipworms, which latch onto the outside of wood and burrow inward, growing up to several feet long as they devour the material.

“As the river gets cleaner, it’s easier for things to live in it,” Chris Martin of the Hudson River Park Trust said of the return of the tiny mollusks and crustaceans. “We don’t make the piers out of wood anymore because of them.”

But many of the region’s older waterside structures remain, and from the South Street Seaport to the Jersey City waterfront, wooden piers have had to be expensively refitted or abandoned entirely.

The city’s floating Waterfront Museum fell victim recently, springing leak after leak, and the holes were getting so bad that they couldn’t be plugged.

Captain David Sharps sent the antique wooden barge upriver for repairs, then discovered another big problem when he went looking for the museum’s dock.

“Lo and behold, there was no dock,” Sharps said, recalling a trip to the Brooklyn pier with his daughters on a sunny summer weekend. “Practically the entire pier had fallen into the water … We had fixed up this old barge and she was basically all dressed up and nowhere to go.”

Years ago, captains would actually park in Brooklyn’s Gowanus Canal – not far from where the museum was docked – to kill off any marine growth on their ships, thanks to water so polluted it could change color daily, said John Waldman, a Queens College professor who has studied the environmental history of New York Harbor.

“It had this legendary smell that you could smell for blocks, even miles,” he said. “The harbor was quite dead.”

With human waste being dumped directly into the waterways, wildlife – including shipworms and gribbles — died off, Waldman said.

By 1900, the harbor’s oysters were smothered. Visitors would gape as methane gas bubbles rose to the surface, produced by waste degrading at the bottom. Sections of the harbor caught fire. In the first decade of the century, civil engineers found 10-foot-tall deposits of human waste at the bottom of the waterway.

Officials soon began treating the sewage, but it wasn’t until the Clean Water Act of 1972 that wildlife began fully returning to the harbor. A recent study in Hudson River Park found that all the species of fish that had existed there in the early 1800s had returned.

Shipworms began returning to the region in full force in the 1990s, according to Mal McLaren, president of McLaren Engineering Group, which repairs wooden structures that have been attacked by the critters.

Earlier this year, scientists found evidence of two shipworms near the wooden supports of the Tappan Zee Bridge. But the water in the area is likely not salty enough to support a shipworm colony that could cause structural damage, experts said.

Gribbles require even higher levels of salinity, meaning they present less of a problem in many New York spots.

McLaren, who recalled once seeing a single pier in Jersey City that carried traces of tens of thousands of shipworms, said weakened wooden pilings can be jacketed in concrete or replaced with plastic. For now, the Hudson River Park Trust and other custodians are taking such steps to maintain many of the original piers along Manhattan’s coastline.

But others, like Captain Sharps, say that sometimes it’s best to simply accept that the invisible critters thriving beneath the water may have won.

After spending more than $200,000 rebuilding his barge and coating its wood in shipworm-resistant tar and plastic, Sharps didn’t bother paying for repairs on his 20-foot-by-100-foot pier in Red Hook. That fallen-in dock has since been abandoned, and the Waterfront Museum now sits at a structure of stone and steel.

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