Introduction
Harvesting and slaughter are the final steps in the production cycle for all farm animals used for human consumption. During the process, a large number of actions can affect fish welfare. The key point of humane slaughter is to ensure that the animals become unconscious rapidly and that this state lasts until death. Slaughter is generally a two-step process, stunning and killing. In industrial seabass and seabream aquaculture, the most commonly used slaughter method is suffocation and immersion in ice water (ice-slurry), where the animal remains conscious for a long period (several minutes) during which there are indicators of distress (inferred through physiological and behavioural responses) before death (EFSA 2009). Regarding seabass, there are diverse studies comparing different stunning and killing methods, with a strong component on flesh quality assessment (e.g. Papaharisis et al. 2019; Simitzis et al. 2014; Zampacavallo et al. 2015; Acerete et al. 2009; Tulli et al. 2015). Most of these studies pointed towards the necessity to improve the knowledge of slaughtering methods to reduce stress and suffering of fish. Thus, the goal of this study was to assess levels of unconsciousness and cardiac response of seabass exposed to different slaughtering procedures.
Material and Methods
This study was carried out in the experimental facilities of IRTA located in La Rápita (Spain). European seabass of about 1 kg in weight were randomly selected, and surgically implanted with bio-loggers capable to measure heart rates and internal temperature (DST milli HRT, 13 mm × 39.5 mm, 11.8 g, Star-Oddi®, Iceland, www.star-oddi.com). For the surgery, we followed the steps developed by Mignucci et al. (2021). Right after the surgery the fish was placed in a quarantine tank and monitored until full recovery. All tagged fish were kept in rearing tanks for several weeks before the slaughtering experiment. The experimental approach consisted on a combination of pre-slaughtering procedures and slaughtering techniques in order to assess the cardiac response through heart rate variations before and after the slaughtering moment.
Level of unconsciousness through observational indicators (turnover, prick, fin reaction, opercular movement) were assessed on three slaughtering techniques: overdoses by anaesthesia (with 2-phenoxyethanol), ice-slurry (common method applied at farms, water temperature: 0-1º C) and Ikejime (japanese immediate slaughter technique). In addition, fasting (seven days) and crowding (three hours) were separately applied on some tagged seabass as pre-slaughtering procedures and combined with slaughtering techniques. These combinations resulted on seven groups: 1) Anaesthesia overdoses, 2) Ice-slurry, 3) Ikejime, 4) Fasting + Ice-slurry, 5) Fasting + Ikejime, 6) Crowding + Ice-slurry, 7) Crowding + Ikejime. Those groups with pre-slaughtering processes were done in duplicate at different temperatures/seasons (winter and summer time). Bio-loggers intensively recorded heart-rates and internal temperature 30 minutes before and 60 minutes after the slaughtering.
Results and Discussion
Observed indicators of consciousness differed among assessed slaughtering techniques. Individuals euthanized by ice-slurry took significantly longer to stop moving their fins and stop breathing than individuals euthanized by overdose of anaesthesia or ikejime. Regarding the bio-loggers, mean heart-rate values and patterns differed during the various combinations. The time until the heart rate signal becomes undetectable varies depending on the slaughter technique. Heart rate of seabass individuals slaughtered using ikejime showed a high peak instantly after slaughter, but heart rate dropped immediately, stopping after 30 minutes. For seabass slaughtered with anaesthesia, heart rate values decreased gradually to basal values, being undetectable after 35-40 minutes after slaughtering. Regarding seabass individuals slaughtered in ice-slurry, the heart rate dropped under basal value, but was detectable up to 45-50 minutes after the start of slaughtering. It is worth to mention that in this latter case, the internal temperature of tagged seabass individuals decreased gradually (not immediately) when exposed to such cold conditions. If we add the effects of the pre-slaughter methods (fasting and crowding) on the cardiac response after slaughtering, it can be seen that the time when heart rate can be detected is reduced in both cases, being furthermore distinct at different seasons (winter and summer). Therefore, in this study we demonstrate the effects of the different slaughtering methods on the behavioural, mental and cardiac response of seabass, providing relevant information for the future development of more humane techniques.
Acknowledgements
We would like to thank the staff of IRTA, and the PhD students Irene Moro-Martinez and Joaquim Tomàs-Ferrer from LIMIA-IRFAP, for their help and assistance during the process of the study. This experiments are part of WELLSTUN project: “Improving the slaughter process of farmed fish: welfare and product quality indicators”, financed by the National Plans of Aquaculture, Spanish Ministry Agriculture, Fisheries and Food.
References
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