The woman bitten by a shark while swimming alone at Rockaway Beach is believed to have lost  “approximately 20 pounds of flesh” in the rare attack — believed to be the waterfront’s first since 1953, sources told The Post on Tuesday.

Tatyana Koltunyuk, 65, was attacked off the shore near Beach 59th Street shortly before 6 p.m. Monday — losing a large chunk of her left leg, above the knee, to the shark, according to sources and a photo of her injury.

Lifeguards spotted her screaming for help in the water — and pulled her to shore, where she was given first aid.

A tourniquet was also applied to her leg to help stem the bleeding before first responders arrived.

Koltunyuk was taken to Jamaica Hospital, where staff said she was in stable but critical condition Tuesday.

The teeth marks on the victim’s leg wound indicated that she may have been bitten by a bull or Thresher shark – though the possibility of a great white encounter could not be ruled out, a source familiar with the incident said.

“[It’s] definitely a shark bite,” Dr. Gavin Naylor, the Program Director at the Florida Program for Shark Research, told The Post after seeing an image of the victim’s wound.

“Looks as though it was a fairly clean single bite with some force. You can see the spacing between the teeth,” he continued while noting he would need to see the other side of the woman’s leg “to narrow the species down.”

A shark could have been drawn to the busy Queens shoreline while chasing menhaden, or small prey fish that cluster in large schools,  Gary Metzger, the Shark Research and Education Program Field Coordinator for the South Fork Natural History Museum, explained.

“The sharks follow the food. If there were large schools of menhaden close to the beach, it’s quite possible that the sharks were also there, feeding on them,” Metzger said.

“[Schools of menhaden nearby] would certainly increase her chances of having a negative interaction with a shark.”

Naylor also noted that the victim took a risk by swimming alone, away from crowds, and in the evening, which “certainly is not advisable.”

“We advise people not to swim at dawn or dusk, largely because animals will move then, and predators will look for them and chase them down,” Naylor said.

“[At dusk, low tide] probabilities [of shark attack] go from very, very tiny to slightly larger.”

Both Naylor and Metzger said it’s possible the woman was attacked by a blue fish – which has very sharp teeth and strong jaws – but the severity of her injuries made a shark bite more likely.

“Twenty pounds is a lot of flesh… there’s no bony fish that does that,” Naylor said.

Metzger added: “The chances of it being something else [other than a shark] would be fairly small [based on the injuries].”

The shark attack is believed to be the first at Rockaway Beach since 1953. A surfer did report a possible shark bite there in September 2017, but experts said his wound was more likely caused by something else.

“It was in a tear down his leg, that’s not really characteristic of a shark bite,” Naylor said of the injury, which was pictured in local outlets at the time.

NYPD scoured the area with helicopters and drones following Monday evening’s attack but did not spot any sharks.

In the absence of a fresh sighting, Metzger and Naylor said the shape and size of the victim’s injury would be a key indicator of what kind of shark may be responsible for the attack.

“Depending on the puncture wounds, you can get an idea if it was a shark or not, and also the shape of the wound could help to narrow what it would have been,” Metzger explained.

Sometimes, Naylor said, scientists can even narrow down the search by collecting a DNA swab from the injured person’s wound.

In general, Naylor continued, sharks are not interested in targeting people.

 “Look at that animal, it has the opportunity to get anybody by the ankle… but it’s focused on chasing schools of fish! All these people are sort of in the way,” he said, referring to a video from last month that showed hammerhead sharks darting between boaters and swimmers on the Gulf Coast.

“That shows you that sharks are not interested in people,” he concluded.

While the risk of another attack remains relatively low, according to experts, Monday’s incident prompted the city Department of Parks & Recreation to shut down swimming and surfing at the popular seaside spot Tuesday.

“We hope for a full recovery for this swimmer,” the Parks Department said in a statement. 

“Though this was a frightening event, we want to remind New Yorkers that shark bites in Rockaway are extremely rare. We remain vigilant in monitoring the beach and always clear the water when a shark is spotted.”

Koltunyuk, of Astoria, is originally from Odessa, Ukraine, her Facebook profile indicates.

The mother of one lost her husband just a few weeks after the family immigrated to New York in the early 2000s, according to a Huffington Post profile of her daughter from 2017.

Koltunyuk and her daughter are frequently pictured smiling together in social media snaps, many of which pay tribute to their Ukrainian heritage.

Last June, about five months after Russia invaded Ukraine, Koltunyuk posted a photo of herself waving a Ukrainian flag on the steps of the Metropolitan Museum of Art.

“Thank you for your concern but no comment,” her daughter, Dasha Koltunyuk, told reporters outside the hospital where her mom was recovering Tuesday.