TODD GARDINER/LONG ISLAND AQUARIUM PHOTO | Jellyfish, such as these moon jellies on display at Long Island Aquarium in Riverhead, are a rare sight in local waters lately.

If someone pens a folk song about the Peconic Bay estuary this summer it could be titled, “Where have all the jellyfish gone?”

And the answer is, nobody knows.

While jellyfish large and small are usually the bane of bathers’ and boaters’ existence in mid-August, there have been few sightings of the gelatinous zooplankton in either the bays or Long Island Sound. That holds true for the large red lion’s mane jellies and the smaller milky white sea nettles, the species seen most often in local waters.

Marine scientists can only guess why.

“It may have something to do with the water temperature or the water temperature over the winter, but we’re not sure,” said Emerson Hasbrouck, director of the Cornell Cooperative Extension marine program in Riverhead. In both cases water temperatures were found to be somewhat higher than normal.

“Water temperature controls a lot of things, so a few degrees can change many things,” including migration and reproduction, Mr. Hasbrouck said.

Salinity levels, which tend not to vary much from year to year, are of less concern, he said.

Mr. Hasbrouck said he’s never before heard of jellyfish numbers so low in his 24 years with the cooperative extension research group.

The marine program has no jellyfish research projects under way, but staffers conducting field work on other studies have noticed the dearth of jellies, Mr. Hasbrouck added. The extension program’s phone lines have not been jammed with calls questioning the jellies’ disappearance.

“Over the years we hear more about when jellyfish are in high numbers, but people don’t usually complain when the numbers are down,” he said.

He’s certainly not complaining.

“It makes it easier for our eelgrass researchers, who dive almost every day,” Mr. Hasbrouck said.

Lifeguard Ryan Farrell of Jamesport said there’s been no need to refill the vinegar container he keeps with his equipment while watching over swimmers at the Southold Town bay beach in New Suffolk. Vinegar is a common treatment for jellyfish stings.

“I’ve seen one or two this season,” he said from his chair Tuesday afternoon. That’s far from the norm.

“In mid-August, you can’t go into the water sometimes,” said Mr. Farrell, who is in his fourth year as a lifeguard.

So far this summer just one swimmer has come seeking treatment for a sting.

“We all have theories why, but it’s anybody’s guess,” Mr. Farrell said.

Nearby, Candi Jacobs of Mattituck and Jackie Rodgers of Cutchogue were catching some sun while their kids splashed about in the shallows earlier this week. They were only too happy that jellyfish stings weren’t high on their list of parental concerns.

“They were here in early June, but then they seemed to dissipate,” Ms. Jacobs said. “I’m not complaining and the kids are not complaining.”

Megan and Christopher Eilers, also of Mattituck, were also relieved the chance was low that a stinging tentacle — perhaps chopped by a passing power boat — would find its way to their 1-year-old son, Chris.

“We were talking about it just the other night,” Ms. Eilers said. “We’ve seen one this year. This is the least I’ve ever seen and we’re really quite happy.”

tkelly@timesreview.com