Paul Sieswerda was supposed to be retired by now. The 77-year-old, who has worked as a curator for the New England and New York aquariums since the late 1960s, was ready to call it quits in 2009.

“But then the humpback whales started showing up,” he says.

For at least a century, humpback whales hadn’t been seen near our shores. But in 2010, several boaters swore to Sieswerda that they’d spotted the 66,000-pound beasts gliding past the five boroughs.

One year later, Sieswerda launched Gotham Whales, a nonprofit that conducts sightseeing tours and also collects data on Atlantic whales. In 2011, just five humpbacks were identified in the waters off New York City. Last year, that number jumped to 209.

“So far this year, we’ve seen 268 whales,” says Sieswerda, a Boston native who lives on Staten Island. “And we’re barely into August.”

The local surge is no accident. Whales are coming because they’re following the menhaden, or bunker fish, which are increasingly attracted to New York’s less-polluted waters.

Cleaner waters have led to a boom in lots of marine life. Last summer, a construction worker at Pier 40 in downtown Manhattan stumbled upon an 8.5-inch oyster in the Hudson River, which weighed 1.3 pounds and was “the size of a small shoe,” according to witnesses who helped measure it.

John Madsen, a geophysicist at the University of Delaware, made a remarkable discovery last June, finding sonar evidence of a 14-foot, 800-pound sturgeon in the Hudson. “We don’t have enough data yet to say this is a population increase,” Madsen tells The Post. “But it’s not an anomaly. It makes me very, very optimistic.”

According to a new report by the city’s Department of Environmental Protection, the harbor is cleaner today than it’s been in nearly 110 years. The last time the waters have been this pure, the Model T had just been introduced to the public and Albert Einstein had just published his theory of relativity.

“This should be bigger news than it is,” says John Waldman, a biologist at Queens College and author of “Heartbeats in the Muck: The History, Sea Life, and Environment of New York Harbor.” “We’ve had a large humpback whale one mile from Times Square. That’s just an astounding victory for the harbor.”

“People make a big deal about Central Park,” he continues. “But in 2019, New York City’s greatest natural amenity is the harbor.”

When British colonists first settled the area in the 17th century, New York Harbor “was a magnificent coastal paradise,” says John Cronin, a former New York commercial fisherman turned conservationist. “Early visitors would write that the sound of waterfowl would keep them awake at night. There were oysters that were a foot long. These are not mythological things. This was the reality here.”

But as the city’s population skyrocketed, from 60,000 in the early 1800s to almost 4 million by the turn of the century, the infrastructure was incapable of handling the onslaught of, well … poop.

“Not a single waste product was treated in any way,” says Waldman. “It went straight into the harbor.”

The raw sewage, accumulating to 10 feet in some parts of the Hudson, choked the water of oxygen, making it uninhabitable for marine life.

“When people think about water pollution in New York, they think about chemical pollution. PCBs, heavy metals or whatever,” says Waldman. “But what really did the harbor in was a torrent of turds.”

Not that industry pollutants didn’t play a role. When Cronin, 69, fished the Hudson for crabs and shad during the early ’80s, “oil slicks along the waterfront were commonplace.”

He was so enraged by what he saw that he joined the Hudson River Fishermen’s Association, a volunteer environmental group that hired him in 1983 to become their first Riverkeeper. He patrolled the rivers on a 25-foot powerboat, tasked with catching industry polluters in the act.

His first big case was against the Exxon oil company, which was rinsing its oil tanks in the Hudson. From his tiny boat, he demanded answers from the tanker’s crew, and they shouted back, “What authority are you?” He had none, not even a badge. But four months later, the company pulled their tankers from the river, according to the Hudson River Fishermen’s Association, and paid $2 million in fines to the state and several environmental groups, including the Hudson River Improvement Fund.

While the Clean Water Act of 1972 often gets the lion’s share of credit for the harbor’s recovery, Cronin says it’s only half the story. It did lead to stricter controls on dumping waste and raw sewage into the harbor, and billions in investments towards the city’s sewer infrastructure, including 14 new wastewater treatment plants.

But the harbor didn’t get clean (or at least cleaner) by government regulations alone. It happened, says Cronin, “when New Yorkers decided they wanted their harbor back.”

New York City was once the oyster capital of the country. When the 17th century English explorer Henry Hudson first sailed on the river that would bear his name, there were 350 square miles of oyster reefs, more than half of the world’s oyster population. Pearl Street in Lower Manhattan was named after the amazing bounty found in the water.

In the 1800s, New Yorkers (especially poor New Yorkers) ate more oysters than meat, with “oyster cellars” selling all-you-can-eat oysters for just 6 cents. The bivalves were hawked on the streets like hot dogs are today, a quick snack for a low price.

Those shellfish disappeared in the early 20th century, thanks to a typhoid fever epidemic blamed on oysters exposed to polluted waters.

Now, Pete Malinowski hopes oysters can make a comeback in New York, if not for eating — we’re decades away from edible New York oysters — then for their purifying effects. A single oyster can cleanse between 30 to 50 gallons of water every day.

Malinowski is executive director of the Billion Oyster Project, which aims to re-establish 1 billion live oysters in the harbor by 2035. Since 2014, they’ve restored about 28 million oysters and 12 reef sites to local waters.

Where once recycled public-school toilets were used to create makeshift reefs, the project has recently been collecting oyster shells donated from over 70 city restaurants, including the Mermaid Inn and the Grand Central Oyster Bar.

The discarded shells are taken to a hatchery on Governors Island, where they’re cleansed of organic matter and implanted with oyster larvae. Every shell can become a new home for up to 20 live oysters, depending on its size.

Malinowski believes the project’s ultimate success is not about the number of oyster reefs they put into the harbor, but the number of people they attract down to the shore. The Billion Oyster Project draws on students from 75 public schools across all five New York City boroughs, as well as hundreds of adult volunteers.

“There are teenagers scuba diving in New York Harbor to restore oyster reefs,” he says. “That’s pretty remarkable, if you think about it. When adults hear about that, they want to get in there too. We learned early on that our only way of being successful is to engage as many of the 8 million people who live here as possible.”

In 1973, folk singer and social activist Pete Seeger called a meeting for concerned New Yorkers to talk about the harbor’s polluted waters and what could be done about it. Just three people attended.

Almost half a century later, they’re showing up in droves. Last summer’s Riverkeeper Sweep attracted more than 2,000 volunteers to remove trash in and around the Hudson. They cleared 37 tons of refuse from the water, including 194 tires, in just a single day.

The harbor is now full of life, and not just marine life. “There are sailing clubs, boat races, water taxis, swimmers everywhere,” Cronin says. “This is something I never imagined I’d see in my lifetime.”

During Cronin’s childhood in Yonkers during the ’50s and ’60s, the Hudson “existed just to separate New York and New Jersey. It was not an asset to the community, it was not an asset to your daily life. It was something to be avoided. But that perception has flipped.”

Fishing in the rivers leading in and out of the harbor, once considered a punchline, is on the rise. Registrations for recreational marine fishing across the state have increased nearly 50 percent since 2014, but marine fishing licenses in New York City have jumped by a jaw-dropping 142.7 percent, according to the New York Department of Environmental Conservation.

Emily Cavanaugh, who’s 15 years old, grew up in New York with parents who warned her to avoid the harbor. But then she joined the SeaWorld & Busch Gardens Youth Advisory Council and began visiting the Hudson for Big City Fishing events.

“I honestly never thought I would be able to fish in the Hudson,” says Cavanaugh, who lives in Midtown East with her parents and brother. “It’s incredibly exciting to be able to fish so close to where I live.”

Meanwhile, the same firm that designed the High Line is planning Manhattan’s first public beach just a block away — on the 5-acre spit of land jutting off Gansevoort Street in the West Village. Due to be completed in 2022, swimming access will be unlikely, due to still-unsafe levels of fecal bacteria in the waters off Manhattan’s west side.

Even though water treatment plants work fine on dry days, it takes just a tenth of an inch of rain every hour to cause raw sewage to overflow into the local waterways.

“This is normally a problem for developing countries,” says Malinowski. “But here in the greatest city in the world, we can be denied access to our greatest natural resource because it rains sometimes and we can’t handle our own waste. As soon as New Yorkers are no longer OK with their sewage flowing into the harbor, we can stop that.”

Complacency may be the biggest danger to the harbor’s future. “Don’t take it for granted,” says Cronin. “As a matter of fact, demand more. Demand more access. Demand more education.”

For Sieswerda, he’d just like a few more New Yorkers to come out on the waters to see the humpbacks with him.

“We have a difficult time filling up the boat,” he says. “It’s kind of disheartening that the people of New York don’t realize that you don’t have to go to Cape Cod or Hawaii or Alaska to see whales. They’re right here in our back yard. All you have to do is get to Rockaway.”

What’s in the harbor?

It’s not just sewage and municipal waste at the bottom of the harbor. Here are just a few strange things hidden in its murky depths, as discovered by the Army Corps of Engineers and the Cultural Research Divers, among others, over the last few decades.