Federal researchers have just returned from an expedition to study the biodiversity and mechanisms of an unusually rich deep-sea ecosystem off the coast of Hawaii island.

Scientists with the National Oceanic and Atmospheric Administration said in a telephone interview Thursday that the abundance of sea life sampled in a particular stretch of water off the Big Island points to a thriving deep-sea habitat, but they aren’t exactly sure why. The area, about a mile off the south shore, was full of fish including sawtooth eels, dragonfish and many other deep-sea creatures.

Much of the ocean surrounding Hawaii is among the least productive water in the Pacific, said the expedition’s lead researcher, Jamison Gove, a NOAA oceanographer.

“Yet we know that Hawaii is this biological hot spot,” he said. “So there’s kind of this paradox: How can you have so much productivity around Hawaii yet the surrounding ocean waters are literally a barren ocean landscape?”

Part of the mission’s purpose was to pinpoint why the islands, and this location in particular, are so rich in marine diversity, Gove said.

They took samples of the area from depths of about 1,500 to 2,000 feet using large trawling nets. They are now assessing those samples in hopes of better understanding potential management and policy needs around the region. They also hope the research will advance understanding of the overall ocean ecosystem, especially the largely unknown and unexplored deep-sea areas.

Jack Kittinger, senior director of the Hawaii program at Conservation International, said that the Kona Coast is “such a gift,” full of spectacular life. Some areas of the world’s oceans simply have more life than others, he said, and a combination of factors, such as currents, water temperature and undersea topography, likely all play a role.

“We really have to do a good job of managing these special, amazing places, and Kona is absolutely one of them,” Kittinger said. “If there’s one (hot spot) in Kona, there’s probably dozens and dozens of them in other places, including in Hawaii. We just haven’t stuck anything down there to find them yet.”

It will take the researchers up to a year or more to draw their conclusions. But they believe part of the reason for such a rich habitat in this location is the way the seafloor dramatically rises as it reaches the island, bringing nutrients up and creating food for a wide range of sea life.

The federal research team was joined by scientists from Bangor University in North Wales, United Kingdom, and the University of Hawaii.

Another recent expedition by Conservation International and the University of Hawaii was conducted farther off the coast of the Big Island at a group of seamounts, active and dormant underwater volcanoes similar to the Hawaiian Islands that never reach the surface. The seamounts, like the area studied off the coast of the Big Island, were also rich in marine diversity, likely for many of the same reasons.