Who I amI’ve had a lifelong interest in sharks, but as a genetics scientist I covered a broad range of creatures – until an earnest young student known to have a passion for shark conservation listened to me lecturing about the population genetics of snails. Why, he challenged, was there no similar work on sharks? I replied that population genetic analyses of sharks would be easy; getting the required samples would be almost impossible. The student – his name was Michael Scholl – said I could leave that problem to him. If I could look after the genetics, he’d take care of the samples. That was the start of my professional interest in shark conservation, some 14 years ago.
Where I workWhat’s to stop a great white shark from swimming wherever it likes? The answer is simple – not much! Long before Europeans ‘discovered’ Australia, some white sharks from Down Under had become permanent residents in the Mediterranean. As big fish in a relatively small pond, this population offers unparalleled opportunities to study aspects of white shark biology that are unlikely to be glimpsed in ocean vastness. But our genetic analyses here have raised an alarm, showing that the Mediterranean’s great whites could be the most vulnerable shark population in the world.
Thanks to Save Our Seas Foundation (SOSF) funding, we are able to ask where, when and how this story started, what makes the Mediterranean white sharks so vulnerable, and what can the genetic analysis of long-dead museum specimens tell us about their future?
Our story starts 450,000 years ago in an ocean far, far away from the Mediterranean, where the warming climate of a wickedly turbulent interglacial was playing havoc with normally stable oceanic currents. One current in particular was affected, the Agulhas. Having transported warm equatorial water southwards along the east African coast, at the southern tip of the continent this powerful current locks horns with the strong, cold Benguela Current from Antarctica. The resulting clash makes the seas around the south coast of Africa among the roughest – and most productive – in the world.
When the Agulhas is strong it occasionally pushes the Benguela aside and a great ring of warm water drifts into the Atlantic Ocean. As this warm eddy collapses, the flora and fauna it carries are precipitated into the cooler waters, where most expire – but some may survive. The Agulhas Current is stronger during interglacials and the larger and more frequent eddies it produces then penetrate further into the Atlantic.
Two additional factors are crucial to this story. Recent research indicates that Indian Ocean white sharks off South Africa may regularly migrate to Australia, with their Australian counterparts making the reverse journey. Secondly, we have shown that female sharks return to their nursery areas to pup, which has the effect of associating female-transmitted genetic markers (mitochondria) with a pupping location.
Our genetic analyses suggest that the Mediterranean’s white sharks are more similar to those of the South Pacific than to any northern hemisphere population. They may have originated from a migrating Pacific individual that followed an Agulhas ring, making a navigational blunder of epic proportions. If, after the eddy collapsed, the shark continued north while trying to turn east, it may have wandered far beyond its normal range, even to latitudes where white sharks seldom venture.
The warm, salty and deep eastern Mediterranean basin is likely to have provided many cues for a lost Pacific shark. It would have been well stocked with marine mammals, swordfish and tuna, all potential prey, and during later glaciations it would have remained warmer and more ice-free than its western counterpart. If a female white shark gave birth in this Pacific proxy, the pups and their descendants would be inexorably tied to the Mediterranean.
What I doSince my professional interest in sharks began, my group has focused on exploring how population genetics can be used to help conserve them. By integrating genetic tools with other approaches to better understand how sharks use the oceans, we are able to explore, and hopefully suggest ways to diminish, human impacts on them and their environment.
Our genetic analyses suggest that Mediterranean white sharks could be more vulnerable than ever suspected. But they are rarely glimpsed, so would they be missed if they disappeared? Probably, because as top predators they play a pivotal role in stabilising ecosystems, preventing loss of biodiversity and species imbalance, and maintaining commercial fisheries.
The stock in the eastern Mediterranean is dangerously homogenous, an indication that it was founded from one or a few individuals. It seems unlikely that white sharks would have entered the Mediterranean from the east more recently than 450,000 years ago, as a route via Suez would have taken them through hypersaline lakes and temperatures at, and possibly beyond, their tolerances. Also, white sharks are not recorded from the highly trafficked northern Red Sea.
That the Mediterranean became the world for these Pacific descendants, with perhaps few or no more white sharks joining them, has been borne out by our work. Further contemporary samples, together with historical material from Mediterranean museums, suggest that white sharks from this area have maintained a dangerously limited genetic repertoire over the past 150 years, and in that time have even lost some of the little variation they once had.
This is worrying, as genetic diversity is the engine of evolution, allowing organisms to evolve to survive disease, climate change, pollution and other natural and anthropogenic disturbances. In effect, Mediterranean white sharks face evolutionary bankruptcy – and at a time when their sea is one of the busiest, most exploited and most polluted in the world.
For this reason SOSF is funding us to use historical and contemporary material to determine this population’s current genetic diversity and quantify its historical decline. This will give us a better understanding of what lies in store for other white shark populations; what happens to white sharks in the Mediterranean tomorrow might well happen to populations elsewhere the day after.