The feasibility of using gas mixture to stun seabream (Sparus aurata) before slaughtering in aquaculture production

Abstract

Current European Union regulation explicitly states that farmed fish should be spared any avoidable pain, distress or suffering at the time of slaughter. It has been shown that fish suffer when they are killed in an ice slurry, the most common method of killing farmed fish in the Mediterranean. Thus, it is necessary to find a method of slaughtering Mediterranean fish that is, (1) efficient in inducing unconsciousness with minimal pain and distress, (2) practical to be applied to a large group of animals at the same time, and (3) feasible to be used at sea. The present study assesses the welfare of Gilthead seabream (Sparus aurata) stunned by two different gas mixtures authorised for stunning other farmed species.

To achieve this objective, commercial sized seabream were stunned and/or sacrificed under different protocols: a) killed directly in ice slurry, b) exposed to a mixture of 30% CO2 + 70% N2, and then moved to ice slurry and c) exposed to a mixture of 40% CO2 + 30% N2 + 30% O2 and then moved to ice slurry. Electroencephalograms (EEG) were recorded to evaluate the state of consciousness of seabream during stunning, while blood and brains were sampled to obtain acute stress indicators and relative gene expression, respectively. Additionally, dead fish were kept for in situ meat quality evaluation.

When exposed to the gas mixtures, fish lost balance at 1 min 23 s ± 31 s with CO2 + N2 and 1 min 12 s ± 32 s, with CO2 + N2 + O2, respectively. Cortisol, lactate and glucose levels were significantly lower in all fish exposed to gas prior to ice slurry than in fish slaughtered directly in ice slurry (p < 0.05). Electroencephalogram records indicated that fish started to lose consciousness when they lost balance and sank to the bottom of the tank. No differences were found in the meat quality (pH and rigor mortis) among the three treatments.

Altogether, the study concludes that the use of carbon dioxide together with nitrogen prior to immersion in ice slurry is more humane than ice slurry alone.