Great white sharks split into separate populations a long time ago, and have only socialized within those groups ever since.
Three distinct population groups were formed up to about 200,000 years ago, according to a new paper in the journal Current Biology, with sharks only hanging out and breeding with other sharks of their own type.
The groups are located in the southern Pacific Ocean and Indian Ocean, the northern Pacific Ocean, and the North Atlantic and Mediterranean, and appear to be kept separate by ocean currents.
"At the end of the Penultimate Ice Age—between 100,000 to 200,000 years ago—white shark populations appear to have divided into three discrete lineages which seldom interbreed," study co-author Catherine Jones, a researcher at the University of Aberdeen, said in a statement. "This separation can still be observed today as modern major ocean currents seem to act as boundaries of the areas occupied by each group."
The researchers describe in the paper how they analyzed the genomes of sharks from around the world, discovering that they rarely interbreed.
"If all the white sharks belonged to a single large, integrated global population, it would not matter so much if they vanished from a specific area. But if white sharks comprise separate genetically distinct populations, such as in the Mediterranean, and this were lost, unique genetic diversity would also be lost," Jones said.
Great white sharks can grow up to 20 feet in length and weigh over 4,000 pounds, and are found in oceans worldwide, particularly in coastal temperate waters. They are listed as "vulnerable" on the IUCN Red List of Threatened Species and "critically endangered" in Europe due to the numerous threats faced by the species, with their worldwide populations having dropped to around 63 percent of their 1970s numbers.
They are often accidentally caught as bycatch in commercial fishing operations targeting other species, which can lead to injury or death. Additionally, marine pollution, including plastic waste and chemical contaminants, affects the health of great white sharks and their prey, and overfishing of fish populations, which are the primary prey for great whites, can lead to reduced food availability.
These impacts are exacerbated by the fact that great whites have a slow reproductive rate, only reaching sexual maturity at around 15 years of age, and producing very few offspring. This means that their populations are slow to recover from declines.
"Successful conservation requires recognition of management units, but this has been unavailable for white sharks, which were suspected to exist as a single global population," study co-author Les Noble, a professor of aquatic biosciences at Nord University in Norway, said in the statement.
"Although not widely appreciated, man's future is increasingly bound up with that of white sharks. As top predators they play a crucial role in maintaining the health and diversity of local marine ecosystems, which are responsible for 20 percent of protein in our diet. In the last 50 years, white shark abundance has almost halved, and it is recognized as critically endangered in Europe."
This discovery has implications for great white shark conservation, as it means that the number of individuals available to mate with an at-risk shark is limited to their population, and therefore is even smaller than we thought.
"The lack of interbreeding suggests offspring from cross-lineage matings may not be viable. Should recent forecasts of changes in strength and direction of major ocean currents over the next 50 years be accurate, breakdown of these geographic boundaries between lineages may allow more unproductive matings, further endangering white shark populations and the diversity, health and productivity of ocean ecosystems," study co-author Galice Hoarau, a professor of molecular ecology at Nord University, said in the statement.
The researchers hope that their discovery will help conservation efforts for these sharks in the future.
Dr. Jones said: "It has long been considered that sharks lost from one area would be compensated for with the movement and matings of sharks from other locations. Our findings suggest this is clearly unlikely and this, together with bycatch fishing, depletion of their food reserves, pollution, and poaching, could put this iconic apex predator in real danger of extinction."
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