This post is like a repository of information. Thank you.Hello there - just new to this but couldn't resist the sashimi question as have spent a good many years in the tuna industry.
The quality of sashimi story starts when you hook the fish. Well at least the things that you have some control over. If you take 20 min to land your fish then the biochemistry of the flesh will be changed compared to one you land in a minute. The more energy left in the flesh at dispatch the better as there is more fuel to enhance the flavour so if you want it for sashimi get it in quickly. The dispatch method makes less difference as if you took 20mis to catch it a few seconds less to kill it will make little diffence. That said best to be quick clean and humane as not doing that is a good way to ruin your appetite. After that dropping the core temperature of the fish is important - for the tunas this is best achieved by gilling and gutting along the lines already shown in this post string, and immersing the carcass in an ice slurry (you can use a salt water one but be careful not to partially freeze your fish as salt slurry can get below zero degrees C). You can then have shasimi for about six ( six ice days) where the flesh is kept at or below zero degrees C. With tunas the whole carcass is ok in the slurry until you butcher it - then you have to keep the water off the flesh With other fish i would get it out of contact with the water in the slurry within a couple of hours. If you eat your sashimi on day one you will find it a bit bland - by 24-36 hours after dispatch (in ice slurry) it should be really tasty with the flesh flavour enhancers at their peak. After that small losses in flavour will gradually become apparent and muscle structure changes will make the texture less ' crisp'.
So: land yor sashimi quickly;drop the core temp quickly; kept it on ice if you want to have sashimi for up to a week; on ice your sashimi will be best for these species (and most) after 24-36 hours; make sure your sashimi doesn't get water logged in your ice slurry. Finally if you look at you soy sauce bottle you will see inosine mono phosphate (IMP) that's the thing that peaks in fish muscle when it is at its best (sweet and meaty).