Getting from here…
One of the delights of catching your own fish is that when treated right it is a food unlike anything you can get anywhere else. Fish caught, cleaned, and frozen or refrigerated within minutes of when it was swimming is like no other fish you have ever eaten.
The route to good quality fish starts as soon as you hook it. You want to bring it to the boat as quickly as possible. Fish that struggle for long periods of time build up lactic acid in their blood, which affects the texture of the meat, and in extreme cases can turn it to mush. When the fish is alongside the boat, land it with a net if possible to avoid damaging the flesh with a gaff hook. If it is a fish large enough to require gaffing to land safely, aim for the head. Not only will it avoid damaging the meat, some fish (most notably mahi-mahi) tend of rip off a gaff that is buried in the side of the fish.
Now we have a fish on deck that might still be alive and struggling. In the case of large and toothy creatures like wahoo, king mackerel, and bluefish they can be extremely dangerous until subdued. The teeth are incredibly sharp and can cause wounds that are deep and dangerous. Other fish can have spines and other defensive weapons that can seriously injure an angler. Any large fish thrashing around with hooks dangling is a obvious hazard that needs to be respected. A couple hard blows to the top of the head, just behind the eyes will stun the creature. A less violent alternative is to squirt a few teaspoons of alcohol onto the fish’s gills. (The cheapest vodka you can find works great) Either in the mouth, or under the gill plate, which ever is more accessible.
A couple important things need to happen now fairly quickly. Many fish will benefit greatly from bleeding. Getting the blood out of the body removes the lactic acid that has built up during the fight. The body temperature of some fish (especially tuna) can rise dramatically during the fight, and draining the blood helps cool off the meat. For some fish, leaving the blood in the meat produces an “off” taste, or even one of ammonia. Not every kind fish needs bleeding, but it will never hurt, so just do it. You want to start the bleed before the heart stops pumping, so don’t wait.
On each side of the fish, using a short, sharp knife, make a cut behind each pectoral fin. It doesn’t have to be very deep, the artery you are looking for is near the surface. Blood should come out in large amounts. The next cut is at the base of the gills. Not the red gills themselves, but the artery that carries the blood to them. Cut under the gill plate, near the base of the gills. Again, blood should flow freely. If not, try again. Some fish, grouper, snappers, and other very white meat fish, hold surprisingly little blood. But in the case of tuna, be prepared for more blood than you thought the fish could hold!
If you can, run salt water from your raw water hose over the fish while it is bleeding. That will help cool it, and makes less of a mess. If the local shark population allows, you can hang the fish in the water while it bleeds out with a rope around its tail.
Once blood stops flowing, clean the fish promptly. We get the fish into meal-sized portions as quickly as we can. No matter if you are freezing or just keeping on ice, you need to keep the fish isolated from air and water, and keep any fish drippings from contaminating other foods. There really is no substitute for a quality vacuum sealer. We have tried inexpensive ones, and they don’t work well, or don’t work for long. For us, a mid-priced FoodSaver model has worked well onboard. In the absence of a vacuum sealer, heavy duty zipper-close bags are your best bet. Squeeze as much air out as possible, and close them up tight. If you are fortunate enough to have a freezer on board, get the fish in as quickly as possible.
We have found that different fish have very different lifespans in the freezer. Most last 6 to 12 months without noticeable degradation. We did find that skipjack tuna, delicious when fresh were noticeably less palatable after 6 weeks frozen, so eat them quick!