Ultra-low temperature stunning

Summary

The purpose of this project was to test and evaluate if ultra-cold temperature (supercooled brine) can act as a method to stun fish prior to slaughtering. Today the salmon industry can choose between electrical stunning or a mechanical blow to the head. Both methods are fast, but the error rate can be high, and this has led to an increasing demand for optimization or new alternatives. Brines of salts can withstand the freezing temperature beyond waters normal freezing point. A saturated NaCl-brine can theoretically withstand freezing lower than -21 C, but for practical purposes it is possible to keep such a brine at around -18 C. Experiments on small salmon (< 500 g) show that the fish seemingly calm down when it enters supercooled brine. Handling prior to the treatment clearly induced a stress response, but this stress response seemed to be pacified shortly after entering the supercooled brine. This preliminary study show that a 30 s treatment might render the fish drowsy or totally inactive after a very short time, but whether the fish lose consciousness or how long this takes, is still unknown. If the fish experience stress by entering the brine, an acute escape response would be expected. This was not observed and furthermore, the stress hormone (cortisol) level was not elevated after ultra-cold treatment. The aquaculture industry operates under very strict demands, and stunning is to be carried into effect very rapidly (< 0.5 s). Ultra-cold temperature cannot meet this requirement, and thus, will not be an alternative to the salmon industry under these requirements. Nevertheless, some aspects of this sedation method seem to be very promising, and future work might benefit by focusing on wild caught species which are offered no sedation prior to slaughtering.