You can’t put a mackerel back together again once it is out of the water. It may be kicking against the light and air and trying to thump its way back to the sea but for all that wild energy and fight its is done for.
Half the life will have gone from it in the effort made to get off the hook. You can’t see them there in the black of the water but as you are pulling them in with the orange line falling in a pile in the bottom of the boat they will be swimming in every direction the power in their tail can take them to get away from the cruel sharp piece of metal.
Once they are out of the water they will bump against the side of the boat and they are then in your hand. Each knock and touch scrapes away or disturbs their fine scales and if they went back into the water then the sleek ease by which they move will have been lost.
In the water each fish is one small part of a great shoal that moves through the water as one. If a fish is taken from that and spat out in the air before falling back in the sea again how will it ever find its place back in the shoal which by then will have moved on.
Once you have the fish on the hook it is done for and you might as well keep it.