Geneticists discover a diversity lifeline for the endangered shortfin mako, the fastest shark in the sea

‘There’s some good news here,’ begins Professor Mahmood Shivji, the director of the Save Our Seas Foundation Shark Research Center (SOSF-SRC) and Guy Harvey Research Institute, Nova Southeastern University. ‘Despite decades of heavy overfishing, shortfin mako sharks in the Atlantic Ocean still show (relatively) high levels of genetic diversity.’

Overfishing is driving sharks to the edge of existence. Recent scientific assessments have shown that more than one-third of sharks, rays and chimaeras are threatened with extinction from a host of threats, including habitat loss and degradation, deep-sea mining and pollution. But by far the greatest concern lies with the unsustainable global demand for the meat, fins and other products from sharks: two-thirds of all these threatened species are at risk from overfishing alone.

Genetic diversity is a lifeline for imperilled species. The greater the diversity in a population, the higher their ‘resilience’; in other words, the more variation there is in a population, the better the chances that it can adapt to changes or survive a catastrophe.

‘What that means for shortfin mako sharks is that if we can prevent further erosion of this genetic diversity by urgently curbing overfishing, we have more hope for this species to retain the resilience needed for its populations to adapt to our fast-changing climate and survive,’ Professor Shivji explains.

Shortfin mako sharks are a conservation priority

In today’s blur of media headlines, where drama, intrigue and despair all clash for the highest views and most shares, it can be hard to find a good-news story.

And a good-news story the tale of the sea’s fastest shark is certainly not – until now. Large, wide-ranging predators that encounter fisheries across their range, shortfin mako sharks have been overfished for their lucrative fins and meat. They’ve made conservation headlines for all the most concerning reasons, reaching Endangered status on the International Union for the Conservation of Nature’s (IUCN) Red List of Threatened Species and being included on Appendix II of the Convention on Trade in Endangered Species of Wild Fauna and Flora (CITES).

Halting the shortfin mako shark’s decline is important for the health of our oceans; we should care about the conservation of this species, and not only because it has evolved to be impressively fast. Recent reviews of the ecological role of sharks in our ocean ecosystems show that large predators tend to play an outsized role in how the ecosystems function. Some big sharks influence their prey’s behaviour, or that of their competitors. The shortfin mako shark has evolved to outpace its torpedo-like tuna prey and other swift swimmers like swordfish and mackerel.

However, shortfin mako sharks in the Atlantic Ocean are in especially dire straits, but few data on their genetic status have been available to guide conservation and management efforts. Populations are currently managed as two presumed separate populations (or stocks), and assessments indicate that northern Atlantic mako sharks are overfished. Indeed, independent scientific surveys using data from satellite tags deployed on shortfin mako sharks suggest that fishing mortality may be 10 times higher than estimates from some of the previous fisheries models. This raises the question: is the current fisheries management strategy, which is based on two stocks that are presumed to not be genetically mixing, backed by scientific evidence?

Shortfin mako sharks differ genetically across hemispheres

The general assumption for wide-ranging, open-ocean sharks like shortfin makos is that they mix freely, with few physical (or genetic) barriers. And based on a high-resolution genomics assessment of the nuclear and mitochondrial DNA of shortfin mako sharks throughout the Atlantic, on one level that is exactly what the team of researchers led by Dr Andrea Bernard (scientist at the SOSF-SRC and the lead author) and Professor Shivji have published in the paper ‘Connections across open water: A bi-organelle, genomics-scale assessment of Atlantic-wide population dynamics in a pelagic, endangered apex predator shark Isurus oxyrinchus’ in the journal Environmental Applications.

Nuclear DNA in vertebrates is inherited from both parents, and the researchers’ data suggest that male shortfin mako sharks are indeed ranging and reproductively spreading their genes across the Atlantic. ‘Female mako sharks, which get much larger than males, are also very capable of making these large-scale journeys. So there was no reason to think that these females were not also spreading their mitochondrial DNA widely by giving birth across the Atlantic,’ says Professor Shivji.

‘But when we look at the mitochondrial DNA – the genetic material inherited only from mothers – we see a contrasting picture.’

The results from sequencing entire mitochondrial genomes from shortfin mako sharks across the Atlantic show that there is in fact genetic structure for northern and southern hemisphere populations. That’s scientific-speak telling us that the populations in each hemisphere differ and are genetically distinct from each other. In fact, the results suggest that although female shortfin mako sharks may well be as wide-ranging as their male counterparts, they return to key sites in one hemisphere to pup. And if we’re to protect this important genetic diversity, the management of two distinct Atlantic populations – the northern Atlantic and southern Atlantic shortfin mako sharks – is now backed by this high-resolution genetic information.

A major focus of the SOSF-SRC research is the application of modern molecular genetic techniques to investigate trade-related issues in elasmobranchs. Photo by Justin Gilligan | © Save Our Seas Foundation

There’s hope for shortfin mako sharks – but only if we halt overfishing

The science is novel, offering insights at the highest possible genetic resolution for mako sharks, and it confirms that the assumption of two Atlantic stocks and current fisheries management on this basis is correct. But where in all of this is the good news story again?

‘We were very surprised to see this result,’ says Professor Shivji of the relatively high levels of genetic diversity that have been maintained across the shortfin mako shark populations in the Atlantic. ‘Typically, in most of the species we study, we see pretty low diversity.’

Indeed, the critically endangered great hammerhead shark stands out in melancholy contrast to the shortfin mako. Another species being fished to the edge of existence, the great hammerhead’s vulnerability to being tipped into extinction is higher because it lacks the diversity to adapt to our rapidly changing climate, even if we were to combat overfishing.

Predicting how species will respond to rising temperatures and changing ocean chemistry is complex and under continuously evolving scientific scrutiny. The relatively high levels of genetic diversity still present in the shortfin mako shark provide optimism that this magnificent large ocean predator has a good chance to adapt and survive in our changing world – if the current overfishing of it comes to an end.