Image copyright OCEARCH / ROBERT SNOW Image caption Lydia was satellite-tagged off the coast of Florida last year, allowing scientists to track her movements

The first great white shark seen to cross from one side of the Atlantic to the other may be pregnant, says the head of the expedition tracking her.

At the weekend, the satellite-tagged fish, called Lydia, crossed the Mid-Atlantic Ridge, which marks a rough boundary line between east and west.

She has now turned towards the UK, but it is unclear where she will go next.

Chris Fischer told the BBC he thought she was pregnant and heading for birthing grounds in the Mediterranean.

Mr Fischer, who is expedition leader and founding chairman of the Ocearch shark tagging venture, said that if Lydia continued on to Europe or Africa, she was likely to become "more coastal".

If we're going to look after some of these magnificent apex predators - the lions of the oceans - we're all going to have to work together. No one country can do it Chris Fischer, Founding chairman, Ocearch

He told BBC News: "I certainly think that it's possible for Lydia to make it to the UK." But he said that he could not know if that was where she was heading.

Mr Fischer said there had been sightings of great white sharks in UK waters before, but added: "I think these were anecdotal versus a documented presence. So hopefully we'll be able to help with that."

On Sunday (GMT) , Ocearch announced that Lydia had crossed the ridge into the eastern Atlantic. And although it is often argued that the waters of these isles are too cold for the sharks, Mr Fischer cast doubt on this.

"One thing we have learnt just in the last year with sharks in the Atlantic is what we used to think was too cold simply is not," he said.

"Lydia has come over from Nova Scotia [in eastern Canada]... These sharks have the capacity to deal with very cold water temperatures for long periods."

But he said: "If I had to guess, I would guess that Lydia is pregnant, and that she has been out in the open ocean gestating her babies, and that this spring she will lead us to where those baby white sharks are born - the nursery."

Mr Fischer, who has led numerous ocean expeditions, added: "If you forced me to guess where that was, I'd say it was over in the Mediterranean, near Turkey... but that's longball I'm playing. She could turn around right now and head back to Florida."

Image copyright OCEARCH / ROBERT SNOW Image caption Chris Fischer described Lydia as "super-healthy" and reproductively mature

He said that small white sharks had been observed in the Aegean Sea before, but scientists working on the team did not share his view. This was because preliminary analysis of blood samples from Lydia suggested she was not pregnant at the time of her tagging.

But Mr Fischer defended his theory, saying there were still uncertainties over the way that white sharks become pregnant, adding: "The sperm from the male comes in a packet with a shell on it. They can carry it around for a while until a special organ inside them breaks down the shell and they get pregnant.

"We know it's 18 months from when we discover the breeding aggregation to when they lead us to the nursery... What we don't know is how long do they carry that sperm packet and how long is it until their body breaks down the shell."

Media playback is unsupported on your device Media caption Marine biologist Dr Gregory Skomal tells 5 live: "We're as surprised as anyone"

Lydia is now roughly 1,600km (1,000 miles) from the western coasts of Ireland and Britain, and nearly 4,800km from Jacksonville, Florida, where the tracking device was attached to her in March last year.

The young female shark has travelled more than 30,500km since last year.

The Ocearch project was initiated to gather data on the movements, biology and health of sharks for conservation purposes as well as for public safety and education.

The scientists have been using a custom-built 34,000kg (75,000lb) capacity hydraulic platform, operated from their research vessel the M/V Ocearch, to safely lift mature sharks so that researchers can tag and study them.

Image copyright OCEARCH Image caption Rizzilient's satellite tag now shows a location on land, in northern Portugal

Lydia was on the platform for 15 minutes, during which researchers extracted blood for analysis and performed an ultrasound examination, in addition to attaching the tag.

Mr Fischer described Lydia as "super-healthy" at the time of her tagging. "I would say she's just been sexually mature for a short period of time, which would put her in her 20s," he said.

"She looked like she had a bright future ahead of her - but I never would have dreamed she would lead us over to your neighbourhood.

"It just shows that if we're going to look after some of these magnificent apex predators - the lions of the oceans - we're all going to have to work together. No one country can do it."

Indeed, the threats currently facing shark species were illustrated when one of Ocearch's tagged animals - a 5ft-long female mako shark called Rizzilient - was apparently caught by commercial fishermen. The most recent fix for Rizzilient's satellite tag shows her on land, in the coastal city of Povoa de Varzim in northern Portugal.

Millions of sharks die each year as by-catch or through targeted hunting to remove their fins, which are highly prized in parts of Asia for use in shark fin soup and as traditional cures.

The Ocearch project has now tagged nearly 150 sharks, including not just great whites, but mako, hammerhead, tiger sharks and other species. Mr Fischer said one of the project's aims was to move away from the often competitive nature of academic research where data is proprietary.

"One of the things we wanted to do [with Ocearch] is disrupt the whole way research like this normally works," he said.

"The data is totally open-source, this is multi-institution and multidiscipline."

He added: "The great thing about this is that the whole world gets to watch... people feel inspired when they're part of something."

Paul.Rincon-INTERNET@bbc.co.uk and follow me on Twitter