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Development of prototype equipment for humane slaughter of farmed fish in industry

Final Report Summary - STUNFISHFIRST (Development of prototype equipment for humane slaughter of farmed fish in industry)

The overall objective of the project STUNFISHFIRST was to develop prototype equipment for humane slaughter of the selected farmed fish species, namely eel (Anguilla anguilla), nile tilapia (Oreochromis niloticus), sea bass (Dicentrarchus labrax) and turbot (Psetta maxima). Humane slaughter consists of stunning (rendering unconscious without avoidable stress prior to killing). Feasibility criteria rule out individual handling of the concerned species. The aim was therefore to develop prototype equipment for electrical stunning, as this can be applied to batches of fish.

In order to achieve the overall objective, the following sub-objectives were envisaged for the project:
- to model the physical processes involved in electrical stunning of fish to predict parameters for electrical stunning of various fish species;
- to characterise requirements for electrical stunning of the selected species with respect to effectiveness of stunning and product quality;
- to design and build prototype equipment on basis of the established requirements;
- to study power saving techniques to facilitate implementation of electrical stunning at small SMEs.
To assess prototype equipment with respect to effectiveness of stunning, product quality and operational characteristics required by the SMEs which farm the selected species.

To achieve the overall objective experimental work was performed in three work packages. A fourth work package was needed for overall coordination. Work package 1 was finished in the first year of the project.

Modelling of electrical stunning
Electric field strengths in water required to eel, tilapia, sea bass and turbot were investigated experimentally. The results show how the electric field depends on water conductivity. The electric fields in water required to generate immediate insensibility in the fishing following exposure duration of 1 s were identified. A mathematical model of the electric field in the stunning tank was constructed and shown to produce results consistent with the experimental results. This simulation indicates that the electric field that occurs in the fish will vary little with variation of the fish position in the tank and fish size but that there may be a significant variation in the electric field experienced by individual fish in a high density cluster of fish if water conductivity is below 50 microS/cm or greater than 500 microS/cm.

Technical file on characteristics demanded by SMEs
Characteristics of prototype equipment, as required by the SMEs and end-users, were established for all fish species. The characteristics concern product quality, required throughput (slaughter rate), prices, ease of use and safety for workers in the company and compatibility with existing processing lines.

Conditions to provoke immediate loss of consciousness without recovery, using available laboratory-scale equipment
At the end of the first year of the project, we decided to use a pulsed square wave current only for stunning of all selected fish species in the project, as it was observed for tilapia that power could be saved by a factor of 1.9 compared to applying a 50 Hz sinusoidal current. Similarly, to our results obtained with a 50 Hz sinusoidal current EEG measurements revealed that immediate loss of consciousness and insensibility can be provoked, provided that sufficient current is passed through the brains of each fish. For eel we assumed that the same current density of the square wave current had to applied as for tilapia, as it was observed during assessment of electrical stunning by applying a 50 Hz sinusoidal current that current densities needed for instantaneous stunning of eel and tilapia were the same. However, EEG data are lacking.

In the project, stunning of sea bass and turbot in water fresh water (1000 microS/cm conductivity) was also investigated, in order to study the possibility to save power to a greater extent. Power can be saved as the conductivity of the fresh water used is 48 to 53 times lower, compared to sea water. As EEG registrations are lacking for stunning of sea bass and turbot in fresh water (1000 microS/cm conductivity) no firm conclusions can be drawn about the immediate loss of consciousness and sensibility in sea bass and turbot by using the pipeline stunners. Conditions to achieve an instantaneous stun in sea bass and turbot in seawater, which is their natural environment at the farms, were established by using EEG data.

EEG registrations revealed that electrical stunning of tilapia followed by gill cutting only resulted in signs of recovery during bleeding for three out of five tilapia. Recovery can be prevented by bleeding the stunned tilapia in a mixture of crushed ice and water. For eel, it appeared that the electrical stun in combination with desliming in a saturated aqueous Ca(OH)2 solution could prevent recovery, as judged from the EEG registrations. EEG registrations in both sea bass and turbot revealed that an electrical stun followed by chilling of the unconscious and insensible fish in ice slurry is sufficient to prevent recovery during for instance gutting and filleting. For turbot, we observed that a temperature increase during gutting of the animal should be prevented, as the animal may still recover. Therefore, gutting should be performed immediately after taking the chilled fish from the tank or the cold chain should not be interrupted for turbot.

Assessment of stunning by observation of behaviour
Results obtained from behavioural measures have to be interpreted with caution. Therefore, the use of EEG recordings as well as evoked responses on the EEG is recommended for an unequivocal assessment of the level of brain function in fish. Nevertheless, EEG recordings cannot be carried under all circumstances and therefore observation of behaviour and responses to administered stimuli is used to assess the prototype stunners. In case fish are able to swim immediately in coordinated way post stunning, there is little doubt that these animals are still conscious.

Electrical stunning and carcass damage
Downgrading in eel, tilapia, sea bass and turbot was not observed when a pulsed square wave alternating current was used. The was not observed either when the selected fish species were stunned by a 50 Hs sinusoidal current.

Study power saving techniques
During the first year of the project it became clear that power had to be saved, so that existing three phase mains, ranging from 16 to 32 A, which are available at the companies of the STUNFISHFIRST consortium, may provide sufficient electrical power needed for stunning of the selected fish species. Our experiments showed that the use of a pulsed square wave alternating current resulted in a reduction of needed power by a factor 1.9 for tilapia, compared to the use of a 50 Hz sinusoidal current for stunning of the fish species.

Design and build prototype equipment
In order to generate a square wave alternating current of we designed and built three amplifiers. Three prototypes for continuous stunning of all fish species were developed in the last year. A pipeline stunner was designed for use with sea bass, one continuous stunner for use with turbot, and one open pipeline stunner for use with tilapia and eel. The open pipeline stunner was not closed, as this facilitated assessment of stunning in the peak stun and maintenance stun area. For eel, sea bass and turbo we also construct a tank for stunning of the species in batches, as this was needed to tune the prototype to needs of some of the SMEs.

Assessment of prototype equipment by observation of behaviour at industrial partners
The electrically stunned fish species emerged from the stunner immobile and showed no escape behaviour as they dropped into the ice slurry of the harvest bin or the aqueous salt solution that was used to deslime eels. Fish that were taken out of the stunning tank were also motionless.

Assessment of prototype equipment by analysis of product quality at industrial partners
In the continuous stunners the water is reused. Thus, food safety is an issue. In the pipeline stunner that was tested with sea bass no microbial growth was detected due to the low temperature of the water. However, more detailed analysis is necessary for drawing firm conclusions. Microbiological analysis of water in the open pipeline stunner, which was reused for 6 hours at room temperature, showed that the total plate count was increased by a factor of 20. Coliforms, which were analysed as indicator for pathogens, were not detected. For the open pipe line stunner more detailed analysis is required for conclusions about food safety.

For all fish species downgrading could be prevented. Product quality was similar to that obtained by current industrial methods. Slaughter rates for eel, tilapia, sea bass and turbot that were in accordance with the wishes of the SMEs (1 to 4 tonners/hour) were obtained.

In conclusion, we can state that all fish species can be stunned instantaneously in the prototype stunners. Recovery during killing can be prevented. Carcass downgrading can be prevented in all species. By using the prototypes slaughter rates can be obtained that were in accordance with the demands of the industrial partners in STUNFISHFIRST.

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