As soon as you have caught the kahawai, iki the fish and bleed. Then gill and gut and wash out any blood or gut contents thoroughly with salt water only. Do not use fresh water on salt water fish at any stage of preparation.
Ice the fish down in salt ice or an salt ice slurry till you are back home then paper towel dry the fish and place in your chiller hanging by the tail(use a meat hook or cooking twine or similar). Your chiller MUST be well ventilated (good internal fans) and sitting at around 4 degrees Celsius. The air flow must be able to circulate around the hanging fish. Do not overload the chiller with fish.
Hang for 7 to 10 days.
Remove the fish and fillet. Use the upper part of the fillet and shoulder as sashimi sliced in generous pieces.
Serve with a spoonful of Colmans Mustard in the centre of the plate as a replacement for wasabi and white soy on the side. Use standard soy if white soy is not available.