A baby seal lying alone on a beach at Mukilteo Lighthouse Park was cause for both adoration and alarm last week.

Passers-by stopped to wonder at the 2-foot-long plump pup that stared back, slept and sometimes rubbed an eye with a flipper. Smiles from observers were common, but so were questions of whether it had been abandoned.

One visitor left a handwritten note on a nearby park bench telling others not to touch it because mother seals often temporarily leave their pups while hunting for food. Officials from the National Marine Fisheries Service's Northwest Region said that was exactly the right advice.

"We generally counsel people to leave seal pups alone," agency spokesman Brian Gorman said. "Nine times out of 10, they haven't been abandoned. The mother has just plunked them on the beach, and she's gone hunting."

Gorman added that people should not crowd around a seal pup because they can scare off the mother seal from coming back for its youngster.

The city's animal-control officer said he gets about five calls a year about seals on the beach, but often by the time officer Paul Delgado gets there, the mother seal has already picked them up.

"The mothers consider the beach as a little day-care center for them," Delgado said. "They sit there and sun themselves and rest while Mom is out there at work. And when they're done, they call them back to the water so they can eat. They're a lot like us."

But if seal pups haven't left the same spot for more than a day or two, the prognosis isn't good. Stranded pups don't have a high survival rate, Gorman said.

A few weeks ago, a malnourished seal was found on Naketa Beach in Mukilteo. Delgado took it to the Progressive Animal Welfare Society, near Lynnwood, which is overseeing its care. In that case, the pup wasn't doing so well and had been left on its own after six to eight weeks of parental care.

Seal pups are mainly born in July, though some are born later, said Stephanie Norman, a veterinarian with the fisheries service.

The pups seen now have likely just been weaned and are learning how to regulate their body temperature and how to hunt on their own. Nearly a quarter of them will die before their first birthday because they are so vulnerable to predators and disease, Norman said.

After hearing about the pup at Mukilteo Lighthouse Park, Delgado went there the next morning, but all he saw was the sign on the bench and an empty beach.

Unless someone saw the mother seal coming for it, it's hard to know whether it followed Mom or returned to Puget Sound on its own.

"I looked all around, but it was gone," Delgado said.

Lisa Chiu: 425-745-7804 or lchiu@seattletimes.com