Being a fish unique to the west coast of North America only (Basically from Anchorage, Alaska down to La Paz, Mexico), it's a very common fish in seafood markets. However, the majority of people buying it falsely believe that it is a type of cod-fish.
Not many people know that there's also a fish called Ling (Common Ling) that's fished and eaten in Northeastern Atlantic waters and is common in Iceland, British Isles and the Norse coast. It's flesh is similar to cod and is usually interchangeable in most recipes.
Cod, on the other hand, is a very common place fish found along the coastlines of northeastern US, around the Maritimes in Canada, around Greenland, Iceland, British Isles, Spain, Portugal, the English Channel, Norway, Sweden, Denmark, Netherlands and Germany. Surrounding countries periodically get Cod imported. There's also Pacific cod, which is three times the size of Atlantic Cod and found around the northwestern parts of the US and along the BC/Alaska coastline.
Then we come to Lingcod. Not very close to Cod-fish or Ling, it received its name simply because it resembled them.
It's very high in Protein, but also very high in Cholesterol (30% and 12% daily intake/100grams respectively) and is considered a sub-par fish for healthy eating.

Basically what I'm trying to say here is that if you're buying Lingcod, it's not a cod-fish, nor is it ling. It's a totally separate family and should be treated as such.