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Jumping the Shark: The Decline of the North Atlantic’s Shortfin Mako

  • Conservation scientists have recommended a total fishing ban on shortfin mako sharks (Isurus oxyrinchus) to allow the species’ population in the North Atlantic to recover from decades of overfishing.
  • The shortfin mako has a slow breeding cycle, making overfishing particularly deletirious and recovery a slow process.
  • A decision on whether to ban fishing of the species will be made in October by nations that are party to the International Commission for the Conservation of Atlantic Tunas (ICCAT), but past decision have often veered away from scientific recommendations.

With a reported top speed of 74 kilometers per hour (46 miles per hour), the shortfin mako (Isurus oxyrinchus) is one of the fastest creatures in the sea. It can jump up to 9 meters (30 feet) out of the water; its unique skin has inspired developments in the aeronautics industry; and with one of the largest brain-to-body ratios of any shark species, it may be quite intelligent.

But the mako’s prowess hasn’t stopped it from being overexploited. A new report by the Shark Species Group for the International Commission for the Conservation of Atlantic Tunas (ICCAT) warns that the North Atlantic population of shortfin mako is dangerously depleted, and calls for a ban on fishing.

“The projections show that there is a long lag time between when management measures are implemented and when stock size starts to rebuild,” reads the report, for which 17 scientists served as rapporteurs. “This fact emphasizes the importance of taking immediate action to reduce fishing mortality and rebuild the stock.”

As the body responsible for the conservation of tuna and tuna-like species within the Atlantic Ocean, the ICCAT also manages the shortfin mako. Its 48 member states receive scientific reports on stock abundance and use these to set catch limits and decide other conservation measures.

But conservation groups including the WWF and the U.S. National Oceanic and Atmospheric Administration (NOAA) have criticized the ICCAT for prioritizing the short-term financial interests of its member countries over the conservation needs of the species that fall under its umbrella.

The shortfin mako may be an example of such short-sightedness. The Shark Species Group first recommended a ban on North Atlantic mako fishing in 2017, when a new, integrated stock assessment model combined shark biology, catch history and a range of other data to show a worrying decline in mako abundance. Even before then, stock assessments based on computer simulations showed the species was vulnerable to fisheries. Assessments of the Northwest Atlantic population from 1992 and 2005 found the population has declined by 38 percent to 48 percent.

Instead of moving forward with a ban, the ICCAT’s response in 2017 was to order the release of any shortfin mako landed alive. But the ruling can be superseded by a country’s own regulations. The measure also failed to take high post-release mako mortality into account. A 2015 study found that even healthy shortfin had a 30 percent mortality rate on re-release.

Ironically, it’s the mako’s very speed and strength that have made it a popular target for sport fishermen; hooking a mako guarantees an exhilarating chase. Their meat is highly sought after from Spain to Latin America to the Caribbean; and it is one of the 10 species most represented in Hong Kong’s fin markets.

While the North Atlantic population is the best-studied, the shortfin mako has a global range.

“We cannot be certain about shortfin mako numbers worldwide as our data is limited; in many areas the sharks are not monitored and fisheries are poorly regulated,” said Fran Cabada, a marine biologist with the Zoological Society of London’s Evolutionarily Distinct and Globally Endangered (EDGE) program. The shortfin mako is in EDGE’s top 50 list of sharks and rays, meaning it is considered to both represent a wealth of evolutionary history and be in need of immediate conservation efforts.

“These active, mid-water sharks are a key part of the open ocean ecosystem,” Cabada said. “Not only because they predate a number of other species, but also because they act as vehicles of nutrients between surface and deeper waters through their vertical movement.”

Hunting at depths down to 150 meters (nearly 500 feet), the shortfin mako is a formidable predator. Cephalopods make up the bulk of its diet, but it also feeds on sea turtles, porpoises, seabirds and many species of large fish. Swordfish are frequent prey, although the shortfin mako is not always the victor in these encounters. On the beaches of Sicily, dead mako have been found with broken swordfish bills lodged in their heads and gills.

This isn’t the only way that swordfish get the better of the mako; both are targets of the same efficient fishing methods, but the two species have very different life histories, with female swordfish producing several million eggs every year.

The mako, meanwhile, may swim fast, but reproduces slowly. A female shortfin mako doesn’t reach sexual maturity until her late teens, and will produce only four to 12 pups every three years

While swordfish overfishing could be reversed fairly quickly, this isn’t the case for the shortfin mako, according to researchers. The ICCAT report warns that even if governments enact a total ban on North Atlantic shortfin mako, it will at least take until 2045 for the mako population to recover.

Shortfin mako shark in the North Atlantic at Condor Bank, Azores. Image by Patrick Doll via Wikimedia Commons (CC BY-SA 3.0).

This report is only the first step in a much longer process: formal advice to the ICCAT Commission won’t be agreed until the Standing Committee on Research and Statistics meets in October.

“The ICCAT recommendations, if properly implemented, should reduce by-catch mortality and increase the reliable data, ultimately improving our understanding of shortfin mako biology and our assessment of their numbers,” Cabada said. “However, as with other shark species, implementation of these actions is essential throughout their entire range.”

After years of delay and indecision in the face of scientific bodies’ recommendations, Spain, the ICCAT party with the highest shortfin mako take, is to host this year’s meeting. It remains to be seen whether this time the evidence is sufficient to inspire action. And, in one final twist of irony, studies have shown that shortfin mako flesh contains high levels of heavy metals, well beyond the limits for safe consumption.

The shortfin mako’s situation may be unsustainable and, quite literally, toxic.

Banner image: Shortfin mako shark (Isurus oxyrinchus). Image by Mark Conlin/SWFSC Large Pelagics Program via Wikimedia Commons (Public domain).

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