The fish are knocked out and then injected with the die, much like tattooing. This is how ever quite stress full to the fish and many die from this process, i dont know numbers but i understand that around half the fish die from the process but as the sell for 3-4 times they price this practice will happen.
I believe that some states in the US has banned these fish and in the UK it's a big "No No" to have or sell these fish.