For several years I had an article posted to the web, God's Food Laws in Today's World. I incorrectly stated that halibut was kosher. The article is now fixed.
Halibut is a delicious species of fish found off the east and west coasts of North America. Alas, it is delicious in the same way that shrimp, pork, lobster, and oysters are. Halibut is a bottom-feeding, carnivorous fish comparable to flounder.
For a fish to be kosher, or legal to eat under the Law of God, the Torah, it must have fins and scales. The fins and scales must be easily visible to the naked eye, under the "could a naked man in the Garden of Eden do this" rule. Halibut definitely have fins, but they lack scales.
A simple web search will tell you that halibut has scales. However, a deeper search reveals that halibut have tiny scales, not visible to the naked eye with a simple glance. And the "scales" are hidden under a layer of skin, so you wouldn't notice them when you scrape a knife along the skin. When a fish has good scales, if you scrape a knife along the skin, the scales will pop off and fly all ove the place in a very noticeable way.
To get this information I spoke to "Will" at Aero Trading in Prince Rupert, which processes a lot of Pacific Halibut. At his suggestion, I then called the International Pacific Halibut Commission in at the University of Washington.
Canada produces Pacific halibut (hippoglossus stenolepeis) and Atlantic halibut (hippoglossus hippoglossus), which are both not kosher. A photo on the web of a California halibut (paralichthys californicus) does seem to have large, easily visible scales. The Halibut Commission recommended I call them back when one of their experts on California halibut is in. California halibut don't exist any farther north than California, and I don't know how commercially important they are. It is safe to assume that any halibut being sold in stores is Pacific or Atlantic halibut unless the sign says otherwise.
While doing my research, it was alleged that "Canfisco" plant in Prince Rupert, the Canadian Fish Company, is processing some type of halibut and calling it "kosher". Even though they call it kosher, if it went into the fish packing plant without fins and scales, it isn't kosher.
The article is now also updated with better information about tuna, mackerel, sailfish, and flying fish. Previously I had thought these fish also were kosher. Flying fish has a type of gooey slime over its skin that wards off predators. It isn't good for humans to handle flying fish for very long, the goo affects us as well. Why do we want to eat them again?
Keep Torah, keep healthy!