This is, it hardly needs saying, the story everyone's been talking about all week. The turtles are coming!

Giant leatherback sea turtles. Dermochelys coriacea, to be formal about it. Those who know about these things include them in an elite group known as "charismatic mega-fauna" – lions, elephants etc – not because they (necessarily) light up a room when they join a party, but because, without wishing to anthropomorphise too much, they're much "nicer" than slugs and jellyfish, and possess a serious wooo! factor.

Children are happy to stare and stare; adults get a frisson at the sheer size, the grace in the water. They are big, up to three metres long. This is one big turtle. This is Turtlezilla. And sightings, normally, are extremely rare.

This summer is different though. The Marine Conservation Society (MCS) has asked the public to log any sighting of a giant leatherback. There have already been an unusually high number; eight in the last fortnight, mainly from around here, by walkers, yachtsmen, surfers. August is the month to watch for them.

"What we do know," according to Peter Richardson of the MCS, "is that something's been happening in the Atlantic which is good for the leatherbacks." They're still on the list of critically endangered species – a colony in Gabon which was huge in the 1970s is now practically extinct. But, in the Atlantic, the leatherback population appears to be starting to thrive.

So here I am in very wet, very Welsh, very west Wales, near the whales. Looking for a turtle, somewhere out at sea, off the coastline near Solva, a coast of such grandeur it would be rejected by film-location scouts as ridiculously overblown unless they were scouting for ridiculously overblown things such as Dr Who or Camelot.

My turtle sighting is, frankly, unlikely to happen. There's a lot of coast to cover for one: there's 180 miles of it, around Pembrokeshire, on a walk which was, incidentally, last week named the third best walk in the world, just after the Appalachian Trail and way above the derisory trudge to Machu Picchu. Plus, the turtles are hard to see. They look, at sea, I'm told, like "a log" which means there are few ways from afar of distinguishing between them and, say, a log. They hardly ever beach, unless injured.

Whether they're actually increasing in numbers off Wales, or people are just starting to become aware of reporting procedures set up by the likes of the MCS is something of a moot point. They've been coming for millions of years; that's nothing new.

There are still dangers to them – long-line trawling in the deep, plastic bags closer to land (because the turtles love to eat jellyfish). But, thanks to successful protection measures in parts of the Caribbean, mainly the US and British Virgin Islands, many more nesting mothers are surviving rather than being killed and eaten by us. ("It's not meant to be very good meat actually, pretty oily," says Richardson. "But when people are hungry …").

The mothers, exhausted through the nesting period on these warm shores, throughout which they're constantly hauling their bulk up the beaches rather than swimming gracefully, lose an astonishing amount of body mass in the process. They need to feed. So begins, as the nature programmes say, the swim east. Travelling at up to 40 miles a day, and stopping for a while, it's now thought, in Mauritania, they eventually hit the waters of the Irish Sea, where one particularly positive development seems to be attracting the leatherback.

Jellyfish. There are more of them than ever. It's complicated, according to Richardson, but the factors would seem to be general raising of sea temperatures, nutrient run-off from coastal pollution and, earlier, the near-removal of herring from the Irish Sea through overfishing (oily fish such as herring like to eat plankton, a good part of which contains baby jellyfish). Thus there has been a bloom in jellyfish, which is not a good thing. The good news, however, is that, as I said, leatherbacks love to eat many jellyfish. So: safer on the hot far beaches; better fed when they arrive here: there's hope, in the Atlantic at least and for the moment, for the leatherback.

Whether they're coming in significantly higher numbers is difficult to pin down. But it's hoped that through public awareness, phoning in a sighting or doing so online, the MCS can be helped in ascertaining, in the UK at least, something closer to the true state of affairs, in order to help plan international measures to save the turtle; it's struggling not just in Gabon but increasingly in south-east Asia. According to Dr Lou Luddington, the MCS's Welsh project officer, who generously jumps into her wellies in her Solva kitchen to lead me around this spectacular coastal path, the public is becoming increasingly observant as well as simply eco-conscious, and slowly aware that there's now a body they can help hugely by keeping their eyes open and making a quick call.

The more I learn, the more complicated becomes the world of marine biology. Sometimes it must seem as if there's only one law and that's the one of unintended consequences. It strikes me, walking the coast later, failing (boo hoo) to see a turtle, that, if Europe ever got unaccountably sensible about fishing, then herring (mmm, with oatmeal) might get a look-in again in the Irish Sea. Which would be good for eating the bad jellyfish. But that would then be bad for the good turtles, who would then … well, who'd be a marine biologist? Their heads must hurt all the time, and not just from the diving.

It also strikes me, though, that, no matter how remote the chance of sightings, there could be worse things to do than take one of the world's three finest walks while helpfully looking out for a prehistoric behemoth. Which, wonderfully, eats jellyfish. We all know the first line of Moby-Dick is "Call me Ishmael". What's less well known – not much more than a rumour actually – is that an early draft of the last line was "O come, turtles, come; and eat the bastarding jellyfish." It would have been a stirring call to flippers.