Background
The custom slaughter method in the aquaculture practice for the Mediterranean fish species includes captivation of the population by the use of a net and direct immersion in tanks filled with ice-water. The slaughter tank usually contains ice flakes and sea water in a ratio ranging from 2:1 to 3:1 (optimum). The temperature in the slaughter tank is theoretically 0ºC (but practically fluctuates from 0.8 to –1.0ºC). (Urbieta and Ginés, 2000; van de Vis et al., 2003). Fish remain in the slaughter tank until all fish movement ceases.
The fish are actually killed by a combination of asphyxiation and hypothermia. Brain function loss in gilthead sea bream, measured as loss of self-initiated behaviour or measured as loss of VERsis in both cases achieved after 5 minutes. Unpublished trials with optimal ice-water conditions in Greek marine cage farms indicated loss of VERs in less than 2 minutes.
Poli et al. (2005) mentioned a long stunning period, reaching 20-40 minutes for gilthead sea bream and 20 min for sea bass. The traditional slaughter method has been questioned during the last years due to animal welfare issues (Poli et al., 2005).
The effect of the slaughter methods on post-mortem fish quality is another aspect that has been brought into discussion.
Some observations indicate that loss of brain function resulting from cooling is reversible; transfer of fish from ice water to water at normal temperatures, immediately after loss of VERs, results into quick recovery of brain function (Robb and Kestin, 2002). Although the custom slaughter method has been strongly criticized, especially for most fish species, some authors support that in warm Mediterranean species it does not seem to be particularly stressful, especially in comparison to asphyxiation. This argument is based on the observation that live chilled sea bass did not show any violent reaction.
Alternative killing methods have been tested for the Mediterranean fish species and these included asphyxiation in air, percussive stunning by pneumatic gun, electrical stunning, head-only electrical stunning (electrical requirement >200mA) and knocking (van de Vis et al., 2003; Poli et al., 2005). Also some combinations have been tested like ice-water saturated with N2 or with a mixture of 40% N2 and 60% CO2, CO2 narcosis, and knocking (Poli et al., 2005). Comparative study in post mortem quality by evaluation of organoleptic characteristics gave a clear advantage to the traditional ice-killing.
Electrical stunning has been tested for gilthead sea bream (Van de Vis et al., 2003) and very recently for sea bass (Knowles et al., 2007; Lamooij et al., 2008). Electrical stunning is considered as a more humane method as it is used to anaesthetize fish in order that they not suffer. However, in current marine cage farming conditions, the fish will have to be concentrated in a container before the electrical stunning application, invoking an increase in blood cortisol levels, irrespective of the stunning or slaughter method applied.
The indications so far have shown no significant differences between the quality characteristics of electrically stunned and traditionally ice-killed fish, except the lower pH exhibited by the former. However, accelerated rigor mortis onset and resolution has been mentioned for the fish received electricity (Knowles et al., 2007) and this is a major disadvantage for the marketing of Mediterranean fish species.
Resolution
The Mediterranean Committee of the Federation of European Aquaculture Producers is supportive of an effort to establish an optimized Slaughter method for Mediterranean Marine fish:
- The traditional slaughter method with the application of ice-water mixtures in warm Mediterranean species does not seem to be particularly stressful when optimal conditions are followed.
- Due to the indicative nature of the existing research, an improved monitoring and understanding of the welfare implications and the post mortem quality impacts of stunning or slaughter methods on Mediterranean marine fish is required.
- Investigation of the optimal stunning and slaughter conditions at a farm level was not addressed in the existing studies that refer to experimental level application or has been tested on individual fish. Slaughter methods at a marine cage farm level are more demanding in various aspects. This has to be examined in order to establish optimal application conditions in terms of fish welfare, operator safety and fish quality. Differences in harvesting sizes, species, and production sites must be taken into account when considering optimal slaughter methods.
In this context, the representatives of Mediterranean Aquaculture ask to be consulted by the Commission during the entire process of drawing up the Impact Assessment.
SLAUGHTER METHODS FOR MEDITERRANEAN MARINE FISH