NEWPORT, Ore. - The eastern gray whales that commonly appear along the West Coast of the United States seemingly have recovered from over-hunting with new protective guidelines established in the 1970s. Their counterparts across the ocean - western gray whales - have not fared as well.

Some scientists believe that a lack of prey may be a limiting factor in the recovery of western gray whales, which number fewer than 200 in their feeding area near Russia's Sakhalin Island. For years, researchers were unable to assess the growth of whale prey in the region because of the remote location, inaccessible conditions of winter ice cover, and the rugged weather that prevented winter sampling.

However, researchers from Russia and the United States studied an inch-long crustacean, Ampelisca eschrichtii, an amphipod that is a favorite food of the western gray whale, in samples that were collected from the Sakhalin Shelf between late spring and early fall over six years between 2002 and 2013. The research team found enough information in the limited samples to assess the missing winter-life history of these amphipods and to document their great abundance and production.

Their results were published this week in the journal PLOS ONE.

"The Sakhalin Shelf could be the richest gray whale feeding area in the world," said John Chapman, a co-author who works at Oregon State University's Hatfield Marine Science Center in Newport, Oregon. "But this discovery includes some surprises, still surrounded by mystery."

One such mystery was the discovery that Ampelisca eschrichtii are simply too abundant to be threatened by over-consumption by western gray whales. If that is the case, the researchers say, why aren't western gray whales rebounding like their eastern gray counterparts when food is plentiful and protections are in place?

"That's really the enigma," Chapman said. "Access to prey could be limited by an unsuitable benthic community or by unsuitable sediments. The whales' benefits from the rich food source could also be limited by the distance and energetic costs of their trans-Pacific migration to reach it."

Previous research by Russian and U.S. scientists - including Bruce Mate at Oregon State - documented the extraordinary migration of several western gray whales across the Pacific Ocean and down the coast of the Americas all the way to breeding grounds of Baja Mexico.

"Such extreme migration between the feeding grounds on the Sakhalin shelf and the breeding grounds in Baja California and back may be too energetically costly to pay for the trip," Chapman said.

The researchers say their study of Ampelisca eschrichtii documented low frequency of brooding females, a lack of early-stage juveniles and the lack of growth in individuals found in the late spring and summer samples of the study.

"These results indicate that Ampelisca eschrichtii grow and reproduce primarily in winter, under the ice," Chapman said. "This is certainly significant because other Arctic Ampelisca species might similarly depend on winter ice formation to grow and reproduce."

Unlike western gray whales, some eastern grays thrive along the West Coast of the Americas on a varied diet that includes mysid shrimp and other crustaceans; they are not dependent on winter ice for their abundance.

However, whales on both sides of the north Pacific Ocean depend in varying degrees on the Arctic species of Ampelisca to survive.

"The whales can't get to this prey until the ice recedes each summer," Chapman pointed out. "But if the ice-free areas expand too far, or persist too long, the production of these crustaceans could decrease significantly.

"Ice could be the gray whales' 'golden goose,' and if it dies, there might be fewer golden eggs for gray whales everywhere."