Traffic on the Maine Turnpike may be down this summer, but that other north-south freeway # the watery one just off the coast # is getting plenty of use by some unlikely visitors.

Boaters and fishermen, in fact, are being warned to steer clear.

Leatherback turtles, the largest reptiles on earth, have been showing up in uncommonly large numbers all along the New England coast this summer, including in the brisk coastal waters of Maine.

So far, seven of the huge turtles have been seen along the Maine coast. Three of the them, including one near Ogunquit Beach last month, were dead. The other four were tangled up in fishing gear but released.

And there are almost certainly more out there or on their way. September is usually the time when turtle sightings # if there are any # typically get reported in Maine.

A record number of sightings of both live and dead leatherback turtles off New England # nearly 100 since June # prompted a federal warning to boaters to keep eyes open for them. Many of the dead turtles have shown up with propeller wounds.

Leatherbacks are more mobile and can tolerate cooler water than other turtles, migrating from the Caribbean to as far north as Canada each summer. Most sightings in this part of the world are around Cape Cod. The turtles are more rare, but not unheard of, in Maine waters.

Last year also was a relatively big one, with seven turtles reported along the Maine coast. Before that, it was typically one or two a summer, at most.

It's not clear how much of the increase here is due to more reporting. But there is clearly something attracting them this summer.

Named for their rubbery shell, adult leatherbacks can grow to 8 feet and 2,000 pounds. A more typical visitor here is about 1,000 pounds and 6 feet long.

What makes their size even more remarkable is that their favorite food is jellyfish, which are as filling as, well, sugar-free Jell-O. You have to eat a lot of jellyfish to weigh 1,000 pounds.

Which seems to explain why they're here.

"We've just had a huge number of jellyfish in coastal waters," said Kara Dodge, a researcher at the University of New Hampshire who has tagged leatherbacks to study their migration.

And why are the jellyfish here? "We just don't really know," she said.

Leatherbacks are protected as an endangered species. In some parts of the world, the turtles and their eggs are taken for food.

In this part of the world, they can get struck by boats, entangled in fishing gear or choked by plastic bags mistaken for jellyfish.

Although all those things are considered a threat to the population, Atlantic leatherbacks have been doing much better than those in the Pacific.

For now, the federal government is simply urging caution along the new England coast. Sightings, especially of entangled turtles, should be reported by calling 1-888-SEA-TURT or going to this UNH Web site.

And keep your eyes open. You'll know them when you see them.

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