The following excerpt is from Umami: Unlocking the Secrets of the Fifth Taste by Ole G. Mouritsen and Klavs Styrbaek:
Ikijime, which means to terminate while alive, is a 350 year old Japanese technique for killing fish. It has the effect of delaying the onset of rigor mortis, thereby ensuring that the taste of the fish is of the highest quality and that there is least damage to, and discoloration of, the flesh. The fish dies humanely and unstressed, which preserves and releases more of the savory substances that bring out umami.
The traditional method is as follows. With a heavy knife, a cut is made in the head on the dorsal side of the live fish, slightly above and behind the eyes, severing the main artery and the elongated medulla, which is the lowest part of the brain stem. This is the part of the brain that controls movement. A second cut is made where the tail is attached to the body. Then the fish is plunged into an ice slurry in order to allow it to bleed out. The muscles of the fish relax in the ice cold water while the heart continues to pump, but the fish has ceased to struggle for its life and is unstressed.
The final, definitive step is to shut down completely the autonomic nervous system, which continues to send messages to the muscles to contract. It is destroyed by inserting a long, very thin metal spike along the length of the fish through the neural canal of the spinal column. At this point, the fish relaxes totally and all movement ceases.
The blood that remains in the muscles retracts into the entrails of the fish, which are removed under running water so that blood and digestive fluids do not spill onto the flesh. The head, tail, gills, and fins are cut off and the fish is wrapped in paper or cloths to absorb any blood that might still seep out. At this point, the fish can be filleted for cooking, sliced for sashimi, or allowed to age for one or two days in the refrigerator.
Surprisingly, a really fresh fish is not always the one that tastes best. Allowing the fish to age generally brings out a greater range of taste impressions and more umami because taste substances are released into the muscles. At the same time, the ongoing enzymatic breakdown of the muscle fibers at low temperatures leads to a softer, more pleasing texture and a much better mouthfeel. In the case of flatfish, for example, this is partly due to the release of inosinate, which interacts synergistically with the glutamate content. Naturally, the determining factor is whether the fish has been killed by ikijime so that the fillet is perfect, with no traces of blood or digestive fluids. There should also be no signs of tissue damage caused by the trauma or rough handling that are characteristic of less skillful ways of slaughtering and bleeding out the fish. Fish that have been killed by ikijime and then allowed to age are known in Japanese by the term nojime.
At first sight it might appear that ikijime is a brutal technique, but there is no doubt that if it is car ried out professionally it is a very humane way of killing a fish and causes it the least suffering. At the same time, it allows the fish to be used to the best advantage, with more taste and higher gastronomic value.