science

I remember there was a study done on how to best slaughter swine (pigs).

The methods that were investigated included: a mechanical hit on the head, suffocation in CO2, and some other measures.

What was found was not only that the suffocation method caused significant stress in the animals, but also that the meat collected this way tasted way worse than meat collected through other slaughtering methods.

this could be relevant in this case: if fish suffocate slowly to death, meat producers might have a financial incentive to change that, to be able to sell better-quality meat, possibly at a higher price. anyways, it would make for good advertisement. that is why meat-producers (fish-producers) should take this seriously.