Tanks saturated with carbon dioxide have been used to make fish unconscious. Then their gills are cut with a knife so that the fish bleed out before they are further processed. This is no longer considered a humane method of slaughter. Methods that induce much less physiological stress are electrical or percussive stunning and this has led to the phasing out of the carbon dioxide slaughter method in Europe.
How To Harvest Humanely
There are multiple ways of killing your fish once the time comes. Some of these methods are considered inhumane, while others are more acceptable. Since there are no clear rules on legality with regards to the “inhumane” methods, the choice is left to the individual. Here are both, the frowned-upon and the accepted methods:
AIR ASPHYXIATION. This amounts to suffocation in the open air. The process can take upwards of 15 minutes to induce death, although unconsciousness typically sets in sooner.
ICE BATHS / CHILLING. Farmed fish are sometimes chilled on ice or submerged in near-freezing water. The purpose is to dampen muscle movements by the fish and to delay the onset of post-death decay. However, it does not necessarily reduce sensibility to pain; indeed, the chilling process has been shown to elevate cortisol. In addition, reduced body temperature extends the time before fish lose consciousness.
EXSANGUINATION WITHOUT STUNNING. This is a process in which fish are taken up from water, held still, and cut so as to cause bleeding. According to references in Yue, this can leave fish writhing for an average of four minutes, and some catfish still responded to noxious stimuli after more than 15 minutes.
ELECTRIC STUNNING. This can be humane when a proper current, duration, conductivity, and temperature are present. One advantage is that in-water stunning allows fish to be rendered unconscious without stressful handling or displacement. However, improper stunning may not induce insensibility long enough to prevent the fish from enduring exsanguination while conscious. It’s unknown whether the optimal stunning parameters that researchers have determined in studies are used by the industry in practice.