The skin is a big thing with any fish and should always be left on IMO. Unfortunately, fish mongers often see that very differently and buying cod with skin on is tough, with the exception of black cod (sablefish or butter fish in Hawaii) and sometimes lingcod (a Hexigrammid, not really a cod).
Skin does everything from holding it together when getting it off the racks to regulating brine uptake, to being pretty for presentation.
Fortunately we catch most of our cod and get to leave the skin on.
If you are dealing with fresh wormy fish, just put the fillets in a glass dish spread out in a single layer. Then sprinkle non-iodized salt on them lightly... Perhaps a touch more heavily than you would if salting them to eat. Put them in the frig overnight, covered with plastic. The next morning have the least squeamish person you can find rinse them quickly under ice cold water. The worms will have crawled out of the fillets and be decorating the dish. It can be an amazing sight... in a special kind of way.
I agree with Wade on keeping the brine simple on cod. I always use a hypertonic brine these days because I have found control is perfect where the dry brines can vary quite a bit depending on how wet the fish is. This is one of many reasons why I NEVER rinse fish in freshwater while cleaning. I wash the outsides well before cutting, but do not ever freeze in water or, as many do drop fillets in a bucket of freshwater to rinse before packaging.
All of the different true cod species are treated the same way in smoking and cookin; the black cod and lingcod another.
Black cod is simply the most amazing, best fish there is, IMO. The pieces should be cut small as the fish is too rich to believe. BTW, Costco here currently has black cod, headed and gutted, at $7.99... an incredible deal IMO&E. The skin stays on.
The brine is one cup salt, two cups brown sugar per gallon of water. I let this cool before brining, or ice it if rushed for time. Again, 90 minutes only for brining. The pellicle is not a big deal because of the methods that follow, but I do air dry on racks before smoking.
Place the black cod racks in the smoker as cool as possible. Usually 80 is about as low as I can easily maintain.
Run lots of smoke for about an hour. Black cod is rich enough to handle any wood, but it is easy to wash over the butter flavors in the fish. I use a mix that always has cherry, a little mesquite, hickory, and white oak. I will often add other woods.
This is strictly a flavoring step and temperature of the fish is unimportant.
After an hour cook a piece in the microwave... just put it on a plate and zap it until the first bubbles show up on the surface. It will seem too moist, but that is just the way black cod is and the oils prevents serious drying. What I look for is adequate smoke. Having been hanging around the smoker for a while will have fatigued your nose a bit and the smoke will probably seem light. Usually I am satisfied with it and take it out now...
But sometimes it does seem much too weak and I will give it a bit more.
I immediately vacuum pack the pieces in small packages of one to three pieces and freeze. I get to do this because my typical batch runs 50 pounds and it is not just for a single meal.
Sometimes we find them cruising a small bay on Afognak Island and fish for them at night with surface plugs. They are smaller, feeder fish, but still outstanding. Larger fish require fishing at 600 feet or deeper. I will stick to the little guuys... ;)
Anyway, after removing from the freezer or frig if you prepared them for a single meal simply cook them very hot and fast on the grill, under the broiler, or in the microwave. Just get it hot to the point you see the first bubbles on the surface and it is done.
Because it is so rich four ounces is a big piece.
It can be broken up into a fish dip with cream cheese, or just about anything you would do with smoked salmon.
I will now add more in a followup post to break this up a bit.