Why are Great Whites leaving US shores?

2017-10-06 11:29
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Boston - The movements of great white sharks in the Pacific and Indian oceans have long been the subject of academic study, but new research is just starting to shed light on the behavior of their Atlantic Ocean counterparts.

READ: Great white critical status could put an end to shark cage diving 

Changing patterns in Great White movement

Researchers in Massachusetts say white sharks appear to venture offshore farther, with more frequency and at greater depths than previously known in the Atlantic.

Some of the 32 sharks tracked between 2009 and 2014 ended up as far east as the Azores, the Portuguese island chain located more than 3 701 kilometers from Cape Cod, where most of the animals were initially outfitted with satellite tags.

They also were found to make frequent deep dives - as far down as 1,127 meters - and spend more time at those dark depths than previous studies in the Atlantic suggest.

The team, which included scientists from the state Division of Marine Fisheries, the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution, had its findings published last week in Marine Ecology Progress Series, a prominent scientific journal.

"Everything we knew previously indicated that the white shark in the Atlantic is more coast-ally-oriented, moving north-to-south and remaining on the continental shelf," explained Gregory Skomal, the study's lead author. "So what we're now describing is this other component, this offshore movement into open ocean."

Skomal says the work has implications for shark conservation efforts since it extends the known habitat for these ancient predators. White sharks are not considered endangered or threatened, but it's illegal to hunt them in US waters.

"You've got US protection within 200 miles of shore, but you have sharks clearly leaving that protection that are vulnerable to harvest," Skomal said. "We need to engage other countries fishing in these waters to talk about putting similar protections in place."

READ: South Africa's great white shark mystery: The full story 

Scientific efforts

The research is exciting because it represents the "first real insights into the movement patterns of white sharks" in the northern part of the Atlantic, says Tobey Curtis, a Massachusetts-based shark researcher for the National Marine Fisheries Service who was not involved with the study.

"Prior to this, we were only able to piece together information about their distribution from widely scattered reports from fishermen, scientists and the public," he said. "Having tracks of individual sharks really helps fill in the gaps, and provides a more complete picture of white shark movements and migrations."

In the image below Captain Brett McBride poses with the crew's first specimen while scientists collect data such as blood, tissue samples and attach a tracking device on the shark. 

Source: Associated Press 

The study seems to hew closely to what's been observed of white sharks in other oceans — that juveniles tend to stay in the relatively shallower waters of the continental shelf where food sources abound, while adults are more apt to venture into open ocean, observes Christopher Lowe, a shark researcher at California State University in Long Beach also not involved with the research.

Indeed, most of the tagged sharks in the Atlantic study generally followed a north-south migration along the Eastern Seaboard. They headed to Newfoundland and New England waters in the summer, then down south to the Carolina's and even the Bahamas for the winter.

Lowe says it remains to be seen what impact continued growth of white shark populations in the Atlantic has on these habits, or whether climate change is playing a role.Another key question is finding out what these sharks are actually doing so far offshore.

Researchers in northern California suggest offshore movements are for mating, a ritual that has never been observed among white sharks. But Skomal and his team believe the animals are more likely foraging — it's just not immediately obvious what they're feeding on.

"That's the great mystery right now," he said.

Great White status in SA

There has been a mysterious decline of Great White shark numbers in South Africa that has been ongoing since May 2017 and is still unresolved. Recently some believe the cause of the decline could be attributed to the May 2017 attack of three great white sharks by orcas, their carcasses washing up on the shore.

During this attack it was found that the sharks were targeted for their livers by the Orcas as it is a nutritional source of food for the species. 

READ: Orca killing spree: SA's iconic shark cage diving under threat 

The Department of Environmental Affairs has labelled the decline a natural phenomena - due to a change in temperatures and the food chain of the animal kingdom. The Atlantic Ocean waters are being affected all across the globe and the reasons remain unclear.

The orca is referred to as 'the wolf of the ocean' and is the only predator that preys on Great White sharks. According to the Marine Dynamics blog post on 5 October 2017 - the Great White sightings in Gansbaai have dramatically decreased but there is still hope that spotting more soon.

On Friday NSRI issued a warning that a whale shark carcas is attracting sharks to Cape St. Francis during the high spring tide, so best to stay out of the waters.

ALERT: Spring tide and shark warning for SA coastlines

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