A new study on eating habits of the Hawaiian monk seal has scientists reeling, and could have serious implications for how the endangered animals are managed.

Laboratory study on seal fat shows that the seals don't eat much lobster, while most scientists thought they did.

Adult seals also don't eat all that much on the shallow reefs, and scientists thought they did.

And a great deal of their foraging is done in extremely deep, dark water, which scientists never knew.

Among significant items in their diet are two specific kinds of deep-water snapper, the gindai and ehu, said Charles Littnan, a foraging ecologist with the National Marine Fisheries Service. He conducted the study with with Sara Iverson of Dalhousie University in Nova Scotia and National Marine Fisheries Services biologist Frank Parrish.

The researchers, whose paper is still being reviewed and won't be published until next year, said it's probably too early to make management decisions based on the results, but one issue seems inescapable: If seals depend a great deal on snappers, what is the effect of heavy fishing pressure on those species.

Parrish said the results of the study were a shock, a "radical redirection" in what marine scientists have believed about seals. In large part, that was because scientists believed what they saw — seals eating reef fish, shallow water eels, an occasional lobster, octopus and the like. But they never knew about seals' secret life, which was the time they spend out of human sight, in the dark depths hundreds of feet below the surface.

Now it turns out they do most of their foraging for food down there.

"Seals spend their time often foraging at (300 to 900 feet) or more, and we can't see what they're eating," Littnan said. To get at the diet information, they began using a technique called quantitative fatty acid signature analysis. It is an extension of the old saying, "you are what you eat."

"In the marine environment, you have very complex long chain fatty acids that are unique to the marine environment, and each prey species has a unique fingerprint. The seals eat those fish and lay down all these fingerprints," Littnan said.

Scientific teams from 1998 to 2002 collected fat samples from 248 seals, mainly in the Northwestern Hawaiian Islands but also in the main islands. They also collected tissue from every creature they thought seals might eat, building a library of 186 prey species.

"We have done a very good job of sampling shallow, slope and deep-water species, although there are a few species we would like to add in," Iverson said.

During the past decade, the decline in the Northwestern Hawaiian Islands lobster population — probably from a combination of fishing pressure and environmental change — has been linked to the decline in seals. That link may now be broken, based on the fatty acid studies, Littnan said.

"Lobsters and crustaceans in general don't appear to be very important to monk seals. The lobsters don't really show up. There are other species of lobster out there that are still in relatively high abundance ... but they're not eating them," he said.

In fact, seals seem to eat a lot of what is found hundreds of feet down, and often far from their home reefs. They are capable of diving to 1,500 feet or more, Parrish said.

"We were surprised that a lot of these deep-water slopes are very important to monk seals," Littnan said.

"We didn't expect these findings. We're working at the edge of our knowledge," Parrish said.

When the researchers did their analysis, they found that four deep-water prey items were dominant, Parrish said. First, the snappers, found at 600 to 1,100 feet; boarfish, which look like a kind of butterflyfish and live 600 feet and deeper; duckbill flathead, a fish that perches on the bottom deeper than 600 feet; and squid, which are often seen in deep ocean surveys lying on the bottom.

"What we see is that the seal is pretty much of a bottomfish feeder. They go along and kind of trap the fish against the bottom," Parrish said.

The other major item in the seal diet is the one researchers expected, coral reef fish from 150 feet and shallower, like surgeonfish.

The most common shallow water crustacean in their diet isn't a lobster but a box crab, which Parrish said "sits in sand, and can occur shallow enough that it would pinch your foot."

The research shows that young seals also feed largely in the deep water. Only newly weaned pups are primarily feeding on the shallow reefs.

"We didn't sample them, because the signature that would have shown up is their mothers' milk," Iverson said.

Still, the health of the shallow ecosystem is important because there have been problems with malnourished seal pups, particularly at French Frigate Shoals in the Northwestern Hawaiian Islands. Managers of the Northwestern Hawaiian Islands Marine National Monument still must deal with shallow-water prey availability issues.

But for older pups and adult seals, scientists need to do a lot of thinking, Littnan said.

"The biggest take-home message is that our idea of what monk seals do has changed dramatically in the last eight years or so.

"We've evolved in our appreciation of how important deeper areas are," he said.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.