Fishing, while lost at sea, requires patients and practice. If there is no bait available a piece of leather or rubber can be used. At least until a fish is caught and can be reused as bait for the next catch. "I attached a hook to a wire leader, which I tied to a line... I carefully worked the hook into a flat piece of hiding." (pg. 178)
If you do not succeed at fishing right away do not give up "This loss did not strike me as a terrible blow." (pg. 178)
-A GUIDE TO FISHING WHEN LOST AT SEA-
If you have a hard time killing the fish at first, it won't be long before you are "gleefully bludgeoning to death" the fish you caught
After you catch your fish you must prepare them for you and your companion to eat. You must "break its neck, sight unseen" (pg. 183) you can "cover the fishes head with a blanket" if you desire. If you are feeding your wild animal you do not have to descale your fish, but if it is for you to eat, you can choose to descale it or not. It wont harm you.
Once you are happy with the amount of fish you have caught and dried, feed yourself and your companion(s). You can "keep what you can eat and give the rest to" your companion. Feed yourself until you feel full, there will be more fish for a later fishing session, even sea turtles will become available to you.
If you find that you have more fish than you and your companion can eat, you can dry and save "strips of dorado, flying fish," or whatever you catch for later meals.