242. This method involves putting the fish in water which has been pre-treated with a gas, normally gaseous carbon dioxide, until movement ceases and then severing the gill arches. Carbon dioxide exposure must be followed by exsanguination or percussive killing if recovery is to be prevented. Care must be taken that all fish in the batch have progressed from immobility into insensibility before bleeding takes place. After severing the gill arches fish are placed in seawater to bleed out.
243. Some concern has been expressed in relation to the aversive behaviour observed in fish when exposed to carbon dioxide treated water. However, it has also been demonstrated that it is possible to expose salmon to carbon dioxide in sea water without producing an unacceptable response. It is essential that carbon dioxide concentrations are maintained at the correct levels, which can be identified by measuring pH. The fish must remain in the carbon dioxide treated water for sufficient time to become insensible. We understand that research is underway using inert gas mixtures which may be less aversive to the fish and look forward to seeing published results.