So this is what is used in a filet-o-fish

always wondered but never really wanted to know exactly - some things are better left unsaid

http://www.nytimes.com/2009/09/10/science/10fish.html?no_interstitial

"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."

I don't really feel like registering with the NYT in order to read, but I have to assume they're talking about Alaskan Pollock?

hoki  http://www.nowpublic.com/environment/hoki-fish-orange-roughy-filet-o-fish-and-crashing-fishery