The answer to the 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 now into a major export. McDonald's alone has recently used roughly 15 million pounds of it each year.

The hoki might be ugly, 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. The hoki fishery was thought to be sustainable, providing New Zealand with a reliable major export for years.

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

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.

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

"We have major concerns," said Peter Trott, the fisheries program manager in Australia for the World Wildlife Fund.

Gary Johnson, McDonald's global purchasing chief, said hoki use was down recently to about 11 million pounds annually from roughly 15 million pounds.

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."

And Trott held out the prospect of a turnaround that would raise the hoki's abundance off New Zealand and significantly reduce levels of ecological damage.