The idea of killing fish bother some. The idea of farming fish bothers me more. But when it comes to long-line fishing or fishing in general, I think most people have no idea about how to preserve a fish once you land it. You must consider that a fish’s metabolism is largely influenced by oxygen and the temperature of the environment.
Almost immediately after death, most enzymic activities are still functioning but, without oxygen being supplied by blood pumped from the heart, the heat-producing glycogen ceases to recycle and consequently it accumulates as lactic acid. Referred to as glycolysis, the ATP continues to break down, acting on the myosin and actin to produce actomyosin. This continuously contracts the muscles until all the ATP is used up, at which point rigor mortis results.
The metabolism of fish is similar to that of other animals in that the oxidation of glycogen is used to produce heat or energy for the body and muscles, and energy for muscle contraction is gained from adenosine tryphosphate (ATP). When fish are cruising in the water their body’s biochemical metabolism is normal, with enough oxygen being supplied by blood circulation.
Maintaining the freshness of fish is simply a matter of preventing these enzymic activities and keeping as much as possible of the original amount of glycogen and ATP in the muscle.
Temperature, technique and experience is what counts, as well as understanding that spiking fish is one way to help maintain the fish’s integrity before it arrives to the market for sale. The iki-jime method is used to destroy and isolate the nervous system from other organs. In particular, its object is to destroy the part of the hind-brain, which is responsible for most of the autonomic reactions.