Shark species that may be world’s ‘largest predatory fish’ is a rare sight, NOAA says

A mysterious shark that may count as the world’s largest predatory fish appears to be in decline off the U.S., prompting a rush to gather as much information as possible about the secretive species, NOAA Fisheries says.

So little is known about Pacific sleeper sharks that experts don’t know how many exist, the extent of their movements or how they reproduce, according to a newly published study.

“Adult Pacific sleeper sharks are rarely encountered. No pregnant female has ever been retained,” NOAA Fisheries reported in an April 15 news release.

“This has led scientists to believe that mature sharks may live in abyssal habitats, 3000-6000 meters deep (9,842 feet to 19,585 feet). Very large sharks — up to an estimated 23 feet — have been caught on submersible cameras at great depths. None larger than 14 feet has ever been measured from fishing or survey vessels.”

Even larger examples of nearly 25 feet have been caught outside of Alaska’s waters, NOAA says.

Pacific sleeper sharks are most often encountered as bycatch in the nets of Alaska’s commercial fisheries, and few survive the trauma, experts say. They’re considered “the most vulnerable of all managed fish stocks in Alaska waters,” yet no management plan exists.

“We still know little about even its most basic biology,” NOAA Fisheries research biologist Beth Matta said in the release. “We can’t manage what we don’t understand.”

The new NOAA Fisheries study, published April 14, is a first step toward finding answers. It includes everything known or suspected about the species, including details found in century-old writings, officials said.

Among the known details: Pacific sleeper sharks are slow to grow, which means they mature later and have a long lifespan.

Just how long they live is not yet known, but a 35-year-old female studied by scientists proved to be immature, indicating “not only extreme longevity, but also delayed maturity,” study coauthor Cindy Tribuzio said in the NOAA release.

The study has also revealed trends: Fewer Pacific sleeper sharks have been found off Alaska and Taiwan, while more have been seen in Russian waters, officials said.

There is growing evidence of a “critical nursery habitat for Pacific sleeper sharks in the Bering Sea,” where aggregations of younger sibling sharks have been discovered, officials said.

As for what they eat, it’s suspected smaller ones scavenge “on whatever falls to the bottom of the sea,” while larger Pacific sleeper sharks may prey on marine mammals, including Steller sea lions that reach 11 feet in length and 2,500 pounds.

“With the information we compiled in this study, we were able to demonstrate the need to prioritize Pacific sleeper shark assessment efforts,” Tribuzio said.

“And that we need to think out of the box on how to manage this species given its vulnerability and challenges to assessing it.”

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