Shortfin mako sharks (Isurus oxyrinchus) are the fastest sharks in the sea, but they’re failing to outpace the very thing driving them toward extinction: overfishing. Global demand for their meat and valuable fins has placed this predator on the International Union for Conservation of Nature’s endangered list and on Appendix II of the Convention on Trade in Endangered Species of Wild Fauna and Flora. The situation is particularly dire in the Atlantic, where populations are managed as two separate ‘stocks.’ Fishery-based assessments indicate that northern Atlantic makos are overfished, but independent satellite-tagging studies suggest that actual fishing mortality may be ten times higher than previous estimates. With mounting pressure from international fisheries, scientists are questioning whether shortfin makos still have the genetic resilience to adapt… and whether the current management strategy of treating them as two populations is scientifically sound.
A new study led by Dr. Andrea Bernard and Professor Mahmood Shivji from the Save Our Seas Foundation Shark Research Center (SOSF-SRC) and Guy Harvey Institute at Nova Southeastern University provides crucial answers to these questions. Published in Evolutionary Applications, the work represents the first full-genome sequencing of mitochondrial DNA and high-resolution scans of nuclear DNA from shortfin makos across nearly the entire Atlantic range. These genetic assessments have revealed a surprising (and what some say is a hopeful) finding: despite decades of heavy fishing pressure, shortfin makos still exhibit relatively high genetic diversity. This is very important, because genetic diversity helps species adapt to environmental changes and survive catastrophic events. Shivji explains that while overfishing is the biggest threat to sharks worldwide, many species also face additional dangers from habitat loss, deep-sea mining, pollution, and climate change. The fact that shortfin makos have maintained genetic diversity means they may still have the ability to adapt—but only if overfishing is halted before their populations are pushed past the point of recovery.
“Typically, in most of the exploited shark species we study we see pretty low diversity,” says Shivji. “We were rather surprised, but also pleased, to see that the genetic health of shortfin makos does not appear to have been severely compromised—yet—by the population reductions caused by overfishing. […]
That means that if we can prevent further erosion of this genetic diversity in shortfin mako sharks 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.”
The research team also found that shortfin makos, known for their vast oceanic migrations, mix freely across the Atlantic. High-resolution scans of nuclear DNA (inherited from both parents) confirm that male makos do, in fact, range widely and spread their genes across the Atlantic. However, the mitochondrial DNA — passed down only through mothers — tells a different story. It reveals that female shortfin makos exhibit a strong matrilineal structure, meaning that while they may travel extensively, they return to specific areas within their hemisphere to give birth. This confirms that the northern and southern Atlantic populations are genetically distinct, and reinforces the need to manage them as separate stocks.
“With increasing loss of species genetic diversity and assessments and monitoring of genetic diversity now called for by the United Nations Convention on Biological Diversity as important activities to inform national policy actions for conserving and restoring species, the genetic diversity data we report here for mako sharks provide a baseline for assessing, and future monitoring, of the genetic health of this iconic species throughout its global distribution,” the team concludes in their work. While the genetic health of shortfin makos provides a rare opportunity to safeguard their future, their resilience is not infinite. Without immediate, science-based fisheries management and stronger international protections, even their remaining genetic diversity won’t be enough to save them. The challenge now is ensuring that conservation measures keep pace with science before it’s too late for one of the ocean’s most iconic predators.
