In order to shed some light on this debate, the European Food Safety Authority recently asked its Panel on Biological Hazards to deliver a scientific opinion on food safety considerations concerning species-specific welfare aspects of the main systems of stunning and killing of farmed fish.
Where such an analysis of animal livestock may be able to go into greater detail, the wide variety of farmed fish and methods used to stun and slaughter them makes this discussion much more complex. Whilst it was not possible to evaluate all the different types of farmed fish, evaluating multiple species was important to get an idea of how biological differences may affect the results. The panel decided to limit their scope to eight species of fish, releasing a total of seven scientific opinions.
The fish analysed were:
- Atlantic Salmon (Salmo salar);
- Rainbow trout (Oncorhynchus mykiss);
- European eel (Anguilla anguilla):
- Gilthead seabream (Sparus aurata);
- European seabass (Dicentrarchus labrax);
- European turbot (Psetta maxima);
- European Carp (Cyprinus carpio); and
- Farmed tuna (Thunnus spp.)
Conditions at Slaughter
The scientific opinions only accounted for biological risks to food safety, influenced by conditions at the place of slaughter, pre-slaughter practices and the stun/kill operations adopted. It was believed that high welfare practices should result in less infections and therefore good food safety.
One of the reasons for this is due to the link between animal stress and the resulting susceptibility of microbial infection, which has been well documented in many farmed animals. Stress can have an immediate detrimental affect on animals and suppress the immune system leaving them more susceptible to microbial infection. The handling of fish, quality of feed, water temperature/quality and crowding levels that the fish are exposed to all have an effect on the level of stress that they experience. Consequently, these factors are also believed to affect the safety of fish as human food.
The aquatic environment in which the fish are kept before slaughter significantly affects the chance of microbial infection. The environment may have indigenous microbes that are harmful for humans, or it may be contaminated by faeces from nearby sewage outlets, or by manure and fertilisers from animal farms.
Once the a fish becomes contaminated by these microbes, an incision or wound made during the stun and slaughter phases can take the bacteria inside the flesh of the fish where it can further spread before human ingestion.
Post Mortem Dangers
How a pathogen may develop inside the fish once the fish has been slaughtered was of key interest to the scientists involved in the study. They knew that the different methods of stunning/slaughter and the welfare of the fish throughout this operation would have significant effects on the stress of the fish. Consequently, this would also effect how the flesh changes post mortem and how safe the fish would be for consumption.
Immediately after fish death, a series of changes develop in muscle. Post-mortem chemical changes involve glycolysis, enzymatic activity, nucleotide catabolism, and their overall consequences in pH drop and increase in concentration of free non-protein nitrogen (NPN) compounds.
These post-mortem changes have a direct relationship to quality and spoilage of the meat, but they can also affect food safety levels. The level of glycolysis inside the fish at the time of slaughter has a direct effect on the level of NPN and high levels of NPN will support bacterial growth.
Depending on glycogen reserves, rigor mortis starts some time after death. This can occur immediately when fish are starved or severely stressed. According to the Panel on Biological Hazards, severe, prolonged stress at slaughter time can deplete muscular energy reserves, which disrupts lactic acid production and alters final muscular pH. In this situation rigor mortis starts immediately or shortly after death.
In case of acute stress, more lactic acid will be produced, therefore the pH value will decrease dramatically, says the Panel. Theoretically, high levels of welfare before and during the time of slaughter should reduce the risks associated with post-mortem changes.
Conclusions
The panel concluded that, based on general principles of food hygiene, some methods of stunning and slaughter could lead to an increased risk of microbial infection. During this period, measures used to increase fish welfare and reduce stress will have a positive affect on food safety.
Specific analysis of the different fish types and the methods of slaughter is still largely unknown, says the panel, adding: "a definitive assessment of the food safety risks associated with different stunning and killing methods for fish is not possible at this time."
| Main stunning and killing methods used in farmed fish and the potential food safety implications | ||
|---|---|---|
| Method | Used in fish species* | Potential negative Food Safety Considerations |
| Electrical | ||
| Electrical stunning | Salmon, trout, sea bass/bream, turbot, Carp. | Unlikely if done out of water. If performed under immersion microbiological quality of water should be taken into account. |
| Whole body electrical stunning in water with desliming and evisceration | Eel | Unlikely. Microbiological quality of water should be taken into account. Immediate evisceration would be beneficial for hygiene of fish. |
| Experimental electrical stunning | Eel | Unlikely. Microbiological quality of water should be taken into account. |
| Mechanical | ||
| Percussive stunning | Salmon, trout, turbot, carp | Unlikely |
| Captive needle method | Eel | Spread of microbial contamination from wound to tissues is theoretically possible. Also microbial cross-contamination of fish through handling or via the needle is theoretically possible. |
| Shooting under water with power head, or from outside the water | Tuna | Spread of microbial contamination from wound to tissues is theoretically possible. |
| Coring or spiking | Tuna | Spread of microbial contamination from wound to tissues is theoretically possible. Also microbial cross-contamination of fish through handling, or via the needle is theoretically possible. |
| Asphyxia | ||
| Asphyxia in ice / ice slurry | Salmon, trout, sea bass/bream, turbot | Unlikely. Microbiological quality of water should be taken into account. Some biochemical post-mortem changes in tissues can result from stress responses. |
| Asphyxia in air | Sea bass/bream, Carp | Unlikely. Some biochemical post-mortem changes in tissues can result from stress responses. |
| Exsanguination | Salmon, trout, turbot | Microbial cross-contamination of fish through handling, or via utensils is theoretically possible. |
| Decapitation / neck cut with evisceration | Eel | Microbial cross-contamination of fish through handling, or via utensils is theoretically possible. |
| Pharmacological methods | Salmon, trout | Unlikely microbiological negative effects. Potential chemical residues (outside the scope of this document) |
| Carbon dioxide | Salmon, trout, sea bass/bream | Unlikely. Potentially beneficial effects in microbiological quality of water, and decrease in pH in blood. |
| Other combined methods | ||
| Live chilling combined with carbon dioxide sedation | Salmon, trout, sea bass/bream | Unlikely. Microbiological quality of water should be taken into account. Some biochemical post-mortem changes in tissues can result from stress responses. |
| Unlikely. Microbiological quality of water should be taken into account. Some biochemical post-mortem changes in tissues can result from stress responses. | Eel | Microbiological quality of water and salt should be taken into account. Possible contamination with halotolerant pathogenic microorganisms |
| Ammonia, washing and evisceration | Eel | Unlikely. Potential beneficial effect for microbiological quality of water. |
| Immobilization by exposure to ice (and salt), washing and evisceration | Eel | Unlikely. Microbiological quality of water should be taken into account. Some biochemical post-mortem changes in tissues can result from stress responses. |
| Chilling and freezing | Eel | Unlikely. Microbiological quality of water should be taken into account. Some biochemical post-mortem changes in tissues can result from stress responses. |
|
Source: European Food Safety Authority
*Animal Health And Welfare Opinions |
||
August 2009
