Ikejime vs. just icing your fish
Dropping a fish straight on ice slows spoilage, but it does nothing about the stress response that already burned through the flesh. Ikejime kills the fish instantly first, so icing preserves a much better starting product.
What icing alone does and misses
Putting a fish on ice slows bacterial spoilage and is far better than letting it warm up. But a fish that thrashed and slowly suffocated has already flooded its muscle with lactic acid and burned through its ATP. Icing cannot undo that damage, it only slows what comes after.
What ikejime adds
Ikejime stops the stress response at the moment of death by destroying the brain instantly. Bleeding then removes blood that carries off-flavors, and the ice slurry chills the fish just like plain icing would. You end up icing a much better product than you would have otherwise.
The bottom line
Ikejime and icing are not competitors, they are partners. The spike and bleed protect quality, and the ice slurry locks it in. For quality table fish the difference in texture, flavor, and shelf life is easy to taste.
FAQ
Is ikejime worth it if I already ice my fish?
Yes. Icing is a good habit, but it cannot reverse the lactic acid buildup and ATP depletion from a stressful death. Ikejime prevents that damage before you ice the fish.
Does ikejime replace putting fish on ice?
No. Ikejime ends with the fish going straight into an ice slurry. The chilling step is part of the method, not a substitute for it.