Salmon quality is dependent on how the fish is taken care of. There are are three things you must do for the high quality salmon. You must bleed, you must gut/gill, and you must chill to close to 32 degrees. For longevity in the freezer, all three are essential. If your fish tastes like fish in six months, it wasn't handled properly. Blood is one of the biggest factors in fish taste and quality. Getting the blood out requires bleeding, scraping the kidney, and getting it cold. As a commercial salmon troller, I must do all of these things to be able to sell a fish and have it be considered premium quality. In the video above, I swung a small king aboard to measure it. I usually stun it in the water but kings must be 28 inches to keep in our fishery. It was over 28 inches, and even at just 7.5lbs when I sold it yesterday, it was worth some decent money at $10.50 a pound. Troll caught fish in the winter command a premium price and are flown around the United States to the finest restaurants.
The first step when you get a salmon aboard is to cut either it's gills or the artery behind the gills. I cut the artery because it's much easier to get the gills out if the gills are in one piece. You will immediately see blood flowing. It takes about 5 minutes for the bleeding to stop. It is a good idea to have a bleed box in your boat to keep your boat cleaner. But, if you have a self bailing boat, many bleed it on deck and rinse the blood away.
Then, I remove the gills. Give removal is important when keeping your fish on ice for multiple days, as bacteria will grow if you keep them in. For day charters and regular sportfishing, you can leave them in but taking them out just takes a seconds and makes removal of the guts easier.
Next, cut the belly from the anus to about an inch below the throat.
Carefully cut around the throat inside the belly cavity to pull the guts, heart, and membrane out. It should all come out at once if you do it correctly. Getting the guts out helps prevent belly burn from acid in the belly, allows the salmon to cool quickly in ice, gives you access to the kidney, and prevents the guts from contaminating salmon flesh while at the fillet table. If you have ever observed a fish cleaning table at a public dock where filleting is taking place when the salmon is not gutted, you will notice fish feces, guts, and acid making contact with the meat you eat. It simply is impossible to fillet a salmon with the guts intact and not have the knife draw the stomach juices into the fillet. Rinsing only does so much and if you blast your salmon fillet with fresh water at high pressure to get stomach juices off of it it kills the quality as the fresh water will expand when frozen.
Once you have that done, you will see the kidney, at the base of the belly cavity. It is dark red. Some call it a "blood line". Make a cut in it, and scrape it out. A knife with a spoon on it was make for this task. This will also allow the salmon to completely bleed out. You can take the back of the spoon and rub the flesh inside of the gut cavity at the veins you see and see more blood come out. The less blood left in that salmon will mean the longer it will last in the freezer.
Once you get that done, rinse it all clean. With practice, a good knife, and place to clean the salmon, it should take about 20 seconds per coho, and 40 seconds per king. But, you need to do it a bunch to get your time down to that. Take your time, learn the proper cuts, and soon you will be a pro at it.
The last step is to get that fish on ice. Keeping it submerged in 45 degree water isn't ideal with a self draining belly tank. Belly tanks that fill at rest and drain when running are popular because they are easy to keep clean, but easy doesn't always mean the best quality. They are more suited for halibut and bottom fish. You want sufficient ice so that when you pull that fish out to fillet it, it burns your hands because the fish is so cold. 32-35 degrees is about right. It firms the flesh up and gets out the very last of the blood.
If you are serious about fish quality, your goal is that your ice slurry be clear at the end of the day even with salmon submerged in it. That means that you properly bled and cleaned your fish. Even after a three day commercial troll trip, my slurry is still clear.
I have taken people fishing who have fished for years. I will go through my routine for fish care and pack the fish on ice. The feedback is always the same... "That was the best salmon I have ever eaten".
Take lots of ice, and take care of your salmon. Your friends and family will thank you when you pull that fish out of the freezer and feed it to them.