Sea Lions and 'junk food' ocean

So what really triggered the mysterious and devastating crash of Steller sea lions in Alaska?

Not munching by killer whales. Not illegal kills or competition by fishermen. Not strange diseases nor insidious poison. It was, at least at first, a vast regional warming of the sea, a “regime shift” that triggered cascading changes in the fish available for sea lion dinner.

When a cold ocean dominated by fat-laced goodies of herring and capelin transformed in the late 1970s into a warmer marine world dominated by less nutritious pollock and flatfish, Steller sea lions along 1,000 miles of coast just couldn’t eat enough of the right food to grow to adulthood, stay healthy and make babies.

This is the famous ‘junk food hypothesis,’ and a team of scientists now says the controversial notion remains the best explanation of the crash. They outline their findings in a new comprehensive review of the causes, research and observations of the Steller sea lion’s stunning decline in western Alaska waters.

Led by Andrew Trites, a major investigator of sea lion biology, some 30 authors contributed to “Bottom-up forcing and the decline of Steller sea lions (Eumetopias jubatus) in Alaska: assessing the ocean climate hypothesis,” published in January by the journal Fisheries Oceanography.

An easy-to-read summary posted online by the Marine Mammal Research Consortium spells it out:

Nearly 30 years ago, an abrupt change in ocean conditions swept through the North Pacific Ocean, affecting everything from sea-surface temperatures to fisheries. The so-called regime shift of 1976-77 was a natural event in the ocean’s climate cycle, but its impacts on Alaska’s marine ecosystems are still felt today.

According to a recent study by a team of 30 leading scientists, this single climate event may be the missing link that ties together the various theories behind the decline in western Alaska’s Steller sea lion populations, and the curious success of populations to the east.

One of the most remarkable threads of evidence comes from Herb Maschner, an archaeologist based at Idaho State University archaeologist who has uncovered dramatic shifts in human populations and available food in a 10-year study of ancient Aleut civilization. The clues emerged from the trash heaps outside former Alaska Peninsula settlements — suggesting eras when sea lions and other marine mammals virtually disappeared from the diet.

Sorting out the original causes of the sea lion crash could potentially impact management of Alaska’s $2 billion commercial fishery, restricted from fishing near certain critical sea lion haulouts and rookeries. The western population of sea lions has been listed as endangered since 1997.

The western population of Steller sea lions — from the Kenai Peninsula and Kodiak to the tip of the Aleutian Chain — plunged by more than 75 percent in the 1970s and 1980s. The decline slowed in the 1990s and, by the early 2000s, may have begun a slow rebound. Counts at certain rookeries and haul-outs found the first increases in a quarter century in 2002 and 2004.

The 2006 stock assessment used pup production figures for an estimate of 44,780 animals, about 30 percent of the population in the 1960s.

Beginning with studies of captive sea lions, Trites has long favored the “junk food hypothesis” as a driver of the declines. But other studies argue that the answer has always been more complicated, with the causes of the original crash different from the factors that now slow the recovery.

Confounding information emerged, for instance, with studies that found plenty of forage fish for young sea lions off Kodiak, even as their numbers failed to rebound. And what about Southeast Alaska, where the same regime shift occurred, yet sea lions thrived? (The Trites paper addresses this issue.)

One of the most comprehensive looks was released in 2002 by the National Academies Press: The Decline of the Steller Sea Lion in Alaskan Waters: Untangling Food Webs and Fishing Nets. (Order the book)

“The causes of the decline of the western stock have been the subject of much speculation and debate despite numerous analyses and many detailed reports,” the report says in its executive summary. “There is no widely accepted answer to the question of why the Steller sea lion population is declining.”