The answer to the eternal mystery of what makes up a Filet-O-Fish sandwich turns out to involve an ugly creature from the sunless depths of the Pacific and a cautionary tale of diminishing returns.

The world’s insatiable appetite for fish, with its disastrous effects on populations of favorites like red snapper, monkfish and tuna, has driven commercial fleets to deeper waters in search of creatures unlikely to star on the Food Network.

One of the most popular is the hoki, or whiptail, a bug-eyed specimen found far down in the waters around New Zealand and transformed into a major export. McDonald’s alone has recently used roughly 15 million pounds of it each year.

The hoki may be exceedingly unattractive, but when its flesh reaches the consumer, it’s just fish — cut into filets and sticks or rolled into sushi — moist, slightly sweet and very tasty. Better yet, the hoki fishery was thought to be sustainable, providing New Zealand with a reliable major export for years to come.

But that is turning out to be doubtful at best, and arguments over managing this resource are flaring not only between commercial interests and conservationists, but also among the environmental agencies most directly involved in monitoring and regulating the catch.

‘Major concerns’

A lot of money is at stake, as well as questions about the effectiveness of global rules and guidelines meant to limit the effects of industrial fishing.

Without formally acknowledging that hoki are being overfished, the New Zealand government has slashed the allowable catch in steps, from about 275,000 tons in 2000 and 2001 to about 100,000 tons in 2007 and 2008 — a decline of nearly two-thirds.

The scientific jury is still out, but critics warn that the hoki fishery, at a minimum, is fast losing its image as a showpiece of oceanic sustainability.

“We have major concerns,” said Peter Trott, the fisheries program manager in Australia for the World Wildlife Fund, which closely monitors the New Zealand fishery.

The problems, he said, include population declines, ecosystem damage and the accidental killing of skates and sharks. Trott added that New Zealand hoki managers let industry “get as much as it can from the resource without alarm bells ringing.”

The hoki lives in inky darkness about a half-mile down and grows to lengths of more than 4 feet, its body ending in a sinuous tail of great length, much like its cousin, the rattail. Large eyes give the fish a startled look.

Scientists say its fate represents a cautionary tale much like that of its heavily harvested forerunner, orange roughy. That deep fish reproduces slowly and lives more than 100 years. Around New Zealand, catches fell steeply in the early 1990s under the pressures of industrial fishing, in which factory trawlers work around the clock hauling in huge nets with big winches.

Hoki rose commercially as orange roughy fell. Its shorter life span (up to 25 years) and quicker pace of reproduction seemed to promise sustainable harvests. And its dense spawning aggregations, from June to September, made colossal hauls relatively easy.

As a result, the New Zealand Ministry of Fisheries set very high quotas — roughly 275,000 tons a year from 1996 to 2001. Dozens of factory trawlers plied the deep waters, and dealers shipped frozen blocks and fillets of the deep fish around the globe.

Moreover, the fishery won certification in March 2001 from the Marine Stewardship Council, a private fisheries assessment group in London, which called it sustainable and well managed. The group’s blue label quickly became a draw for restaurant fish buyers.

“Most Americans have no clue that hoki is often what they’re eating in fried-fish sandwiches,” SeaFood Business, an industry magazine, reported in April 2001. It said chain restaurants using hoki included McDonald’s, Denny’s and Long John Silver’s.

Ominous signs of overfishing — mainly drops in hoki spawns — came soon thereafter. Criticism from ecological groups soared. The stewardship council promotes hoki as sustainable “in spite of falling fish stocks and the annual killing of hundreds of protected seals, albatross and petrels,” the Royal Forest and Bird Protection Society of New Zealand said in May 2004.

When the stewardship council had to decide whether to recertify the hoki fishery as sustainable and well-managed, the World Wildlife Fund, a Washington-based group that helped found the council, weighed in as strongly opposed.

“The impacts of bottom trawling by the hoki fishery must be reduced,” the fund said in one of its lengthy written objections.

The wildlife fund was overruled, and the council recertified the fishery in October 2007. At the same time, the New Zealand ministry cut the quota still further, reducing the allowable commercial catch from roughly 110,000 tons to about 100,000 tons.

Some cutbacks

Some restaurants cut back on hoki amid the declines and the controversy.

Last year, Yum Brands, which owns Long John Silver’s, issued a corporate responsibility report that cited its purchases of New Zealand hoki as praiseworthy because the fishery was “certified as sustainable.”

Now, Ben Golden, a Yum Brands spokesman, said hoki was “not on the menu.”

Denny’s said it served hoki only in its New Zealand restaurants.

Gary Johnson, McDonald’s senior director of global purchasing, said hoki use was down recently to about 11 million pounds annually from roughly 15 million pounds — a drop of about 25 percent. “It could go up if the quota goes up,” he said in an interview. He noted that McDonald’s also used other whitefish, in addition to hoki, for its Filet-O-Fish sandwiches.

Johnson called the diminishing quotas a sign not of strain on fish stocks but of good management.

“Everything we’ve seen and heard,” he said, “suggests the fishery is starting to come back.”

The Ministry of Fisheries agreed and said hoki critics had worked hard at ignoring progress. “If you look at the current state of the fishery, it’s apparent that the string of management actions that we’ve taken, which came at severe economic impact, have been effective,” said Aoife Martin, manager of deepwater fisheries. “It’s recovered.”

But the Blue Ocean Institute, a conservation group in East Norwich, N.Y., that scores seafood for ecological impact on a scale from green to red, still gives New Zealand hoki an unfavorable orange rating. The fish’s overall abundance has fallen, the group says, and the fishery “takes significant quantities of seabirds and fur seals.”

Trott of the wildlife fund was more pointed. He called the fishery’s management “driven by short-term gains at the expense of long-term rewards” — a characterization the ministry strongly rejects.

But he, too, held out the prospect of a turnaround that would raise the hoki’s abundance off New Zealand and significantly reduce levels of ecological damage and accidental killing.

“We are currently working with both industry and government to rectify all these issues,” he said. “Our hope is that we will see great change and willingness by industry and, importantly, government to improve the situation dramatically.”