The Pacific Sleeper Shark: New Insights Into A Mysterious Marine Predator

The Pacific Sleeper Shark: New Insights Into A Mysterious Marine Predator

Despite being one of the largest predatory fish in the ocean and having a range that spans throughout the Pacific, little is known about the Pacific sleeper shark. In an effort to tackle the most pressing gaps in our knowledge – and thus how the species can be conserved – a team of scientists has stepped in to create a “one-stop shop” of information about the sluggish shark species.

Pacific sleeper sharks are closely related to Greenland sharks, to the point that they can interbreed, and from what we do know or have theorized about them, have similar characteristics. They can reach up to 14.1 feet (4.3 meters) in length, though it’s speculated that adults living in deeper parts of the ocean might even reach 23 feet (7 meters). It’s also suspected they are similarly long-lived.

However, unlike their relations, they are notoriously understudied, which the study authors attribute to their lack of commercial value compared to Greenland sharks, the inaccessibility of their habitats, and the safety and logistics in capturing and handling such a large animal. It’s also rare to spot an adult Pacific sleeper.

Without a significant body of knowledge in hand, it’s difficult to know how to best manage and conserve the sharks, which are currently listed as decreasing in number and near threatened on the IUCN Red List.

“We don’t have a lot to go on for managing this species. Right now they are managed based on historical catch. That’s not ideal, especially for a highly vulnerable species,” said study co-author Cindy Tribuzio in a statement. “The more information we can gather, the better we can develop and apply alternative methods that will do a better job of assessing this and other data-limited species.”

And so the team began a deep dive into the scientific literature on the mysterious shark species, borrowing bits of information about their close relations along the way – and made several new discoveries that could inform conservation.

One of the most significant findings came from genetic studies, which suggested there might actually be more Pacific sleeper sharks than previously thought, thanks to some sharks in disguise.

“We used to think there were three large species in the genus: Pacific sleeper shark, southern sleeper shark, and Greenland shark,” said lead author Beth Matta. “Now we know the southern sleeper shark is not genetically distinct.”

They also uncovered a plethora of evidence about the potential lifespan of the sharks, supporting the existing theory that they can reach an impressive age (though maybe not as old as the 400-year-old Greenland sharks). 

A study using radiocarbon dating to analyze an eye lens, for example, found they had a growth rate that was twice as fast as Greenland sharks, but still significantly slower than many other fish. Another catch and analysis of an immature female shark suggested it was at least 35 years old. “That indicates not only extreme longevity, but also delayed maturity,” Tribuzio explained.

Close up of a shark eye lens white, but slightly see-through) on a blue background.

If you didn’t know what the lens of a Pacific sleeper shark’s eye looked like, well now you do.

Image credit: NOAA Fisheries

With the findings from the study, the team hopes to not only have highlighted the remaining gaps in scientific knowledge, but also use what is known to inform better management of the species.

“With the information we compiled in this study, we were able to demonstrate the need to prioritize Pacific sleeper shark assessment efforts,” said Tribuzio. “And that we need to think out of the box on how to manage this species given its vulnerability and challenges to assessing it.”

For example, the evidence suggesting that Pacific sleeper sharks have slow growth, delayed maturity, and long lifespans could make them more susceptible to overfishing – providing a target for management.

“The more we learn about these sharks, the more we care about keeping them around into the future,” added Matta. “And there is so much more to learn.”

The study is published in Polar Biology

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