SEAFOODplus: Harvesting common carp

Scientific Alert Note

Considering animal welfare when harvesting fish from aquaculture

SEAFOODplus is investigating how to harvest fish from aquaculture with two purposes. One is considering animal welfare and one is considering improved product quality. As a part of project 5.2 ETHIQUAL methods have been developed with common carp where the fish is rendered unconscious without any suffering prior to killing, while optimal product quality is retained. Results are shown from experiments where the fish are crowded in canals that are close to ponds. To achieve this, the ponds are drained into the canals. Then a worker is catching the fish with a hand net. The next steps in the procedure are asphyxia for 30 min, blow on the head that is applied manually, deheading and filleting.

Authors: Hans van de Vis¹, Henryk Bialowas², Maciej Pilarczyk², and Bert Lambooij³

¹ Institute for Marine Resources and Ecosystem Studies, Wageningen University and Research Centre, The Netherlands Institute for Fisheries Research, IJmuiden, The Netherlands

² Institute of Ichthyobiology and Aquaculture of the Polish Academy of Sciences, Golysz, Poland

³Animal Sciences Group, Wageningen University and Research Centre, Lelystad, The Netherlands

: Harvesting carp at a fish farm where the fish is being carefully caught with a net after crowding in a canal close to the pond

For humane slaughter fish should be rendered unconscious without any avoidable suffering prior to killing. Therefore, an experimental method, which consists of rendering farmed carp (Cyprinus carpio) unconscious immediately by applying electricity in combination with chilling of the stunned animals was evaluated and compared to the current method, which comprises 30 min asphyxia followed by a blow on the head.

The current method for slaughtering of carp consists of 30 min asphyxia at room temperature followed by a manually applied blow on the head. The current slaughter method starts with asphyxia to exhaust the carps to facilitate the application of the blow on the head. It is known that asphyxia is stressful and this can be avoided by rendering the fish unconscious and insensible immediately by electricity. In addition, a manually applied blow is in practice often inaccurate or insufficiently hard to result in immediate loss of consciousness and sensibility.

Therefore, the objective of our study was evaluation of electrical stunning in combination with chilling and to compare this to current slaughter method of farmed carp (Cyprinus carpio L).

: EEG and ECG traces prior to stunning (A) and after electrical stunning for 1 second (B)

Welfare aspects

The effectiveness of electrical stunning followed by chilling was assessed by registration of EEGs in combination with behavioural observations. Effects of experimental and industrial slaughter method were assessed by analysis of colour and pH of fillets.

Registration of the electrical activity in the brains of carp revealed that consciousness was lost immediately by applying a current density of at least 0.14 A/dm² 50 Hz a.c. for 1 second in water of 200 µS/cm. Recovery of carp could be prevented by applying electricity for 5 seconds in combination with chilling of the stunned carp in flake ice or a slurry of ice and water. Behavioural observations showed that the capacity of the fish to adapt the position of its eyes as response to changes in body posture (i.e. the so-called eye roll) was not lost in all fishes after electrical stunning. This clearly shows that the eye roll cannot be used as sound indicator for loss of consciousness and sensibility in carp. It is clear that registration of EEGs is needed for assessment of electrical stunning.

Product quality

Average pH values over a seven-day period of post mortem storage of carps slaughtered by the experimental and industrial method are presented in the figure below. The flesh of carps killed by the experimental method had significantly higher pH values during the entire storage period, compared to the batch slaughtered by the industrial method.

In earlier experiments with eel and turbot, application of electricity resulted in fillets with lower lightness and increased redness. When working with carps the experimental slaughter method did not result in any of these effects.

It is thus concluded that the experimental slaughtering method applied is suitable for humane slaughter of carp while retaining a high product quality.

: Figure showing pH changes in carp slaughtered by electricity followed by chilling or asphyxia followed by a blow on the head