Cods - family Gadidae - are one of the most commercially important families of fish in the sea. As such, they have been fished to the brink of disaster. The book above is a fascinating study of the historical relationship between western civilization and the codfish.
The only freshwater representative of the cod family is the Burbot.
Cods are easily identified: they all have three separate soft-rayed dorsal fins and two separate soft-rayed anal fins. They also have a sensory barbel on the chin, although in some species this is so reduced as to be virtually absent.
Our two commonest "Hakes" differ from cods above in having the three dorsal and two anal fins merged into two and one, respectively. The Red Hake or Ling Urophycis chuss is easily identified by its pelvic fins, which are formed into long white split streamers. It grows to 20" and 6 lbs. Ling are very common on just about all dive sites.
These two codfishes have similar-sounding names, looks, and habits. Both are much more likely to be found schooling in open water around the upper reaches of offshore wrecks and reefs than other bottom-hugging cod types.
These fishes take the fin-merging of the Hakes one step further, merging all the unpaired fins and the tail into a single continuous margin. You can find tiny inch-long baby Cusk Brosme brosme under rocks and debris. Adults are found in coastal waters, in depths 60 ft and below, and grow to 39" and 27 lbs.