Who knows what fish suffer? We know only that they do. Many of you are transposing what you think it would be like as a person into what it would be like as a fish. Cold may not affect them the same way, as they are cold blooded. When I worked the boats, we just slid them into the icy brine tank, where they died pretty fast at about -2. There is some evidence that consciousness continues a time after even decapitation, though that might just be a bit of inertia and bodily processes grinding to a halt at different rates. Throwing into concrete may just stun the fish, which then dies of anoxia, same as any asphyxiation.
As to what is cruel and what isn't, your guess is as good as anyone's. The iki-jimi thing is mainly to stop the fish thrashing and thus not bruise the flesh. Throat slitting would improve the flavour in certain species, but as to speed and cruelty? We can't say. I think most of these methods are about preserving eating qualities of fish, not to alleviate suffering, though they may help.
In humans there are some methods of death that are though of as "good" ones and some as "bad", but for fish, we don't really know.
Given the lack of any empirical evidence, use what method you think best.
