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, whose bounty, it seems, is not limitless.

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 at one time 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.