The great white voyage
Cape Town - Spend a couple of hours swimming at the beach and you get really hungry, right? So spare a thought for a great white shark on a regular non-stop journey of more than 4 000km – and sometimes as much as 22 000km.
Contrary to what one might expect, however, researchers have recently discovered that these sharks don’t spend their travelling time chasing and consuming prey.
Rather, they appear to fatten up in shallow coastal waters in the same way that bears do before they hibernate for winter.
Then, literally and figuratively buoyed by this massive amount of energy they’ve stored in their huge livers, the sharks cruise in an energy-efficient way that allows them to make the most of the stockpiled resources.
New research by scientists at Stanford University and the Monterey Bay Aquarium in California – published in Proceedings of the Royal Society B, the society’s flagship biological research journal – reveals previously unknown details of how great white sharks power themselves and stay buoyant during their marathon non-stop trips.
“We have a glimpse now of how white sharks come in from nutrient-poor areas offshore, feed where elephant seal populations are expanding... and store the energy in their livers so they can move offshore again,” explained researcher Barbara Block, a professor of marine sciences and a senior fellow at the Stanford Woods Institute for the Environment in California.
“It helps us understand how important their near-shore habitats are as fuelling stations for their entire life history.”
Scientists have long known that ocean mammals such as whales and sea lions build up blubber to burn on their long seasonal migrations, but until recently very little was known about how sharks make similar voyages.
They’ve also learnt that great whites migrated from the coastal waters of Baja California to an area halfway to Hawaii known as the White Shark Café, where they spend at least 100 days before returning to Baja.
Through tagging the scientists learnt that on the journey out the sharks swim slowly and dive to about 900m.
The findings about the role of the sharks’ livers emerged from a study initiated by a summer project of Stanford undergraduate Gen Del Raye, which involved first looking at a well-fed juvenile great white at the Monterey Bay Aquarium.
Over time they documented a steady increase in its buoyancy as the shark’s body mass increased. They ascribed this to the addition of stored oils in its liver.
They then looked at detailed records from electronically tagged great whites free-swimming in the eastern Pacific Ocean, including location, depth and water temperature. From this the scientists identified periods of “drift diving” – a common behavior of marine animals in which they passively descend while momentum carries them forward like underwater hang-gliders.
By measuring the rate at which the sharks sank during drift dives, the researchers estimated the amount of oil in their livers, which accounted for up to a quarter of their body weight. A quicker descent meant less oil was present to provide buoyancy, while a slower descent meant more oil.
Sal Jorgensen, a research scientist at the aquarium, said the sharks faced “an interesting dilemma”.
“They carry a huge store of energy in the form of oil in their massive livers, but they also depend on that volume of oil for buoyancy. So, if they draw on those reserves, they become heavier and heavier.”
The data showed that the sharks’ buoyancy consistently decreased over the course of each studied animal’s migration, indicating a gradual but steady depletion of oil in the liver. In other words, they were primarily running on energy stored up before they began their journeys.
Del Raye said the most difficult part of the research was bringing the different sources of data together into a “coherent and robust story”.
Part of that story was the importance of calorie-stocked coastal feeding grounds – not just for mammals such as whales, but also for sharks readying for long-distance migrations, the researchers said.
“Could the same be true for other ocean animals such as sea turtles and a variety of fish? The study may help answer that question too through a novel technological approach that can be applied to ongoing studies of other large marine animals.”
So the new discoveries have potentially broad implications for conservation and the management of coastal waters.
Nicole’s huge 22 000km journey
The most famous long-distance swimming shark was Nicole, a sexually immature great white of about 3.6m who was first sighted and named off Dyer Island, near Gansbaai, in 1999.
She was sighted in this area each successive year between July and December, but her movements during the first six months remained a mystery.
As early as 2001, research had suggested there could be white shark movement across the southern Indian Ocean, because animals in South African waters were genetically very similar to those off Australia. This suggested that at least some of the sharks were long-distance swimmers – and Nicole proved this to be true.
She was fitted with a pop-up satellite tag on November 7, 2003, at Gansbaai. Upon its release on February 28, 2004, it surfaced near Australia, making her 11 000km journey the first trans-oceanic migration of a great white to be officially recorded.
On August 20, the now 3.8m Nicole put the cherry on top by reappearing in South African waters, having swum 22 000-plus kilometres in a little over nine months.
Although she was less than 5m below the surface about 66 percent of the time, she’d spent significant periods in the deep, to 900m, so she may also have been “drift diving”. - Cape Argus
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