Factors Affecting Shrimp Welfare in Aquaculture

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Executive Summary

While many environmental conditions and farming practices could plausibly affect the welfare of shrimps, little research has been done to assess which factors most affect shrimp welfare. 

 

This report aims to assess the importance of various factors for the welfare of farmed shrimps, with a particular focus on Litopenaeus vannamei (also known as Penaeus vannamei, or whiteleg shrimp), due to the scale and intensity of farming (~171-405 billion globally per annum). Where evidence is scarce, we extend our research to other shrimps, other decapods, or even other aquatic animals. Further research into the most significant factors and practices affecting farmed shrimp welfare is needed. 

 

Conclusions from our review are summarised below: 

 

Eyestalk Ablation: Shrimps demonstrate aversive behavioural responses to eyestalk ablation, and applying anaesthesia before ablation has therapeutic effects. We believe this is strongly indicative that eyestalk ablation is a welfare concern. 

 

Disease: Infectious diseases cause significant mortality events. This is likely to both cause suffering prior to death and increase the total number of shrimps who are farmed and experience suffering. 

 

Stunning and Slaughter: Current slaughter practices (asphyxiation or immersion in ice slurry) are likely to be inhumane. While evidence on the optimal slaughter method for shrimps is limited, electrical stunning appears to be the most promising method to effectively stun and kill shrimps. 

Stocking Density: There is strong experimental evidence to suggest that reductions in stocking density indirectly improve welfare by improving water quality, reducing disease, and increasing survival. There is also some tentative evidence that stocking density directly impacts shrimp behaviour and measurable stress biomarkers (e.g. serotonin). 

 

Environmental Enrichment (EE): Environmental enrichments (e.g. feeding methods that mimic natural behaviours, hiding sites, different tank shapes and colours, plants, substrates, and sediments) probably improve shrimp survival, but there is little evidence on their impact on shrimp stress or behaviour. There is moderately strong evidence that physical enrichment (such as physical structure, plant, and substrate) improves welfare for aquatic animals, including crustaceans. 

 

Transport and Handling: Poor transport and handling practices are likely to lead to physical injury and stress, although research is limited on the welfare effects of current shrimp farming practices. 

 

Food: While some decapods appear resilient to lack of food, inadequate nutrition leads to risk of non-infectious disease and may lead to aggressive and abnormal behaviour. 

 

Water Quality 

 

  • Dissolved Oxygen (DO): Several studies indicate that insufficient DO levels increase mortality, which is likely to be preceded by suffering due to poor physical condition. Optimal DO levels may also help prevent aggressive behaviour. 

  • Un-ionised ammonia: High concentrations of un-ionised ammonia (NH3) are toxic for shrimps and are harmful for shrimp welfare. High un-ionised ammonia hinders the immune response and leads to high mortality rates.

  • pH: Deviations from optimal pH have detrimental effects on health, immunity, and susceptibility to disease. Sudden fluctuations (within the optimal pH range) may also be harmful based on our conversations with farmers and aquaculture specialists, though we were unable to find consistent supporting evidence in the academic literature. 

  • Temperature: Shrimps are adaptive to fluctuations in water temperature. However, high temperatures are highly likely to be harmful for welfare, as the water retains less oxygen, toxic ammonia increases, and survival worsens. While small deviations below optimal temperature may be less harmful, very low temperatures also harm physical health and lead to mortality. 

  • Salinity: While it is likely that extreme salinity levels or fluctuations are harmful for welfare and survival, the experimental evidence does not consistently demonstrate harmful effects for small deviations from the recommended salinity range. 

 

The table below outlines how confident we are that a particular welfare factor is important for shrimp welfare. We judge a welfare factor to be important if small to medium improvements in this factor would reduce harm to the animal (1). Our confidence levels range from very high to low (2). Higher confidence on this scale indicates that there is stronger evidence in the importance of this factor for shrimp welfare. Lower confidence on this scale does not necessarily imply that the welfare factor is less important, rather that the evidence is either limited or mixed. 

(1) This refers to small to medium improvements from a realistic level of living conditions, i.e. the conditions typically seen on semi-intensive shrimp farms, such as a stocking density of 25-35 shrimps/m2. Since extreme levels of any welfare factor are likely to be both extremely harmful and atypical on farms, it is less informative to consider improvements from extreme levels. 

(2)  In practice, we assigned at least medium confidence to all the welfare factors we investigated. 

Note that our report only covers our confidence in the importance of each factor, not the extent of the problem on current shrimp farms. 

Welfare Factor

Eyestalk Ablation

Disease

Stunning and Slaughter

Stocking Density

Environmental Enrichment

Transport and Handling

Food

Dissolved Oxygen

Un-ionised Ammonia

pH

Temperature

Salinity

Confidence

Very High

Very High

Very High*

High

High

High

High

Very High

Very High

High**

High/Medium***

Medium

* We have medium confidence that existing methods of stunning before slaughter, such as electrical stunning, would reduce suffering compared to current practices. Note also that the slaughter process only lasts for a small proportion of the animal’s life, which should be taken into consideration in determining the most important factors for shrimp welfare. 

** We have high confidence that deviations from optimal pH are harmful for welfare, and high confidence that sudden fluctuations (within the optimal pH range) are also harmful. 

*** We have high confidence that deviations above optimal temperature are harmful for welfare, but only medium confidence that deviations below optimal temperature are harmful.

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