Shrimp Welfare Project (SWP) produced this report to guide our decision making on funding for further research into shrimp welfare and on which interventions to allocate our resources. We are cross-posting this on the forum because we think it may be useful to share the complexity of understanding the needs of beneficiaries who cannot communicate with us. We also hope it will be useful for other organisations working on shrimp welfare, and it’s also hopefully an interesting read! 

The report was written by Lucas Lewit-Mendes, with detailed feedback provided by Sasha Saugh and Aaron Boddy. We are thankful for and build on the work and feedback of other NGOs, including Charity Entrepreneurship, Rethink Priorities, Aquatic Life Institute, Fish Welfare Initiative, Compassion in World Farming and Crustacean Compassion. All errors and shortcomings are our own.

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) (Mood and Brooke, 2019). 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. 

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

* 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. 

Measuring shrimp welfare

Measurement of shrimp welfare is a major challenge due to limited knowledge about the pain, suffering, and behaviour of shrimps. Animal welfare is typically characterised by three factors: the animal’s emotional state (including the absence of negative experiences), ability to express natural behaviours, and normal physiological functioning (Jerez-Cepa & Ruiz-Jarabo, 2021, p.1). Alternatively, under the “Five Domains Model”, welfare is characterised by the animal’s nutritional, environmental, health, behavioural, and mental needs (Mellor & Reid, 1994). 

Physiological biomarkers, often measured through haemolymph sampling, may be indicators of stress and welfare (Albalat et al., 2022, pp.8-9). Examples of potential biomarkers include immune measures (e.g. total haemocyte counts) and stress parameters (e.g. lactate). Decapods also possess neuromodulators, such as serotonin, which mediate responses to stressful situations (Fossat et al., 2014; Birch et al., 2021, p.49) - thus high levels of serotonin may be interpreted as an indicator of stress (Bacqué-Cazenave et al., 2017). 

To measure physical health, recent studies have looked at the gut microbiome of shrimps, while other approaches include pathogen screening, observing physical condition (e.g. physical damage, discolouration), and observing self-protective behaviour such as wound-tending (Albalat et al., 2022, pp.7-10). Survival rates are a clear indicator of welfare for two reasons. Increased mortality, holding constant genetics and environmental conditions, is a signal of poorer welfare prior to death. Higher mortality also increases the total number of animals who are farmed and may experience suffering. 

Physiological biomarkers and physical health should be also assessed alongside observations of behaviour, such as lethargy and aggression. For example, well-managed high-density systems may allow shrimps to stay healthy and grow well, but not express their natural behaviours, harming welfare.

What matters for shrimp welfare?

Eyestalk Ablation 

We have very high confidence that eyestalk ablation is harmful for shrimp welfare. This conclusion is based primarily on evidence of aversive behaviour in response to ablation and the therapeutic effects of anaesthesia. Our standards require farms to not buy fry from ablated broodstock.^[3] 

Eyestalk ablation involves crushing or cutting off at least one of the eyestalks of female shrimps, and is typically used to induce rapid maturation and spawning in L. vannamei. Eyestalk ablation may lead to numerous negative outcomes, such as physical trauma, stress, physiological imbalance, reproductive exhaustion, weight loss, high broodstock mortality, and reduced disease robustness among offspring. As a result, some maturation facilities in Brazil, Colombia, Ecuador, México, and Thailand no longer use eyestalk ablation (Albalat et al., 2022, p.4). 

A Review of the Evidence of Sentience in Cephalopod Molluscs and Decapod Crustaceans concludes that eyestalk ablation is a severe welfare risk (Birch et al., 2021, p.76). One study found that shrimps have a recoil reaction to ablation (Taylor et al., 2004). In another study, ablated shrimps were more likely to flick their tails and rub the area of the wound (Diarte-Plata et al., 2012). Furthermore, when the wound was covered or anaesthetic administered, the shrimps reduced these responses, which suggests ablation had been causing pain or distress. 

Recent studies report a statistically significant harmful effect of eyestalk ablation on broodstock mortality (Zacarias et al., 2019; de Menezes et al., 2019). Ablated shrimps require additional energy as a result of increased moulting frequency during a period of heightened nutritional demand (associated with egg production) (Zacarias et al., 2019, p.464). This may in turn lead to exhaustion-related stress (Racotta et al., 2003, p.119; Palacios et al., 1999). 

Ablation may also exacerbate welfare concerns under stressful conditions. Zacarias et al. (2019) provide tentative evidence that the offspring of ablated females are less tolerant to  salinity stress during early development (though the sample size was too small to draw concrete conclusions). In addition, eyestalk ablation may lead to increased disease susceptibility among offspring. In a more recent study, Zacarias et al. (2021) found that the offspring of ablated shrimps were less resilient to pathogen infection. After experimental infection with Vibrio parahaemolyticus, the offspring of ablated shrimps had almost half the survival rate compared to the offspring of non-ablated shrimps. They also had a slightly lower survival rate after infection with white spot syndrome virus, but the difference between groups was not statistically significant. 

In the Appendix, we examine the effects of eyestalk ablation on aquaculture outcomes. 

Disease 

We have very high confidence that diseases are harmful for shrimp welfare due to their significant adverse impacts on shrimp health and mortality. Our standards require farms to implement various health and disease management protocols regarding disinfection, biosecurity, hygiene, and pond preparation. 

Infectious diseases are common threats for L. vannamei, including viral pathogens such as white spot syndrome virus, yellow head virus, and infectious myonecrosis virus, which have significant health effects (Albalat et al., 2022, pp.4-6).^[4] Viral disease outbreaks typically lead to mass mortality events with mortality rates between 40% and 100% (Arulmoorthy et al., 2020). Shrimps also suffer from bacterial and fungal diseases (Albalat et al., 2022, p.4). 

In the Appendix, we outline emerging preventative tools against disease outbreaks. 

Stunning and Slaughter

We have very high confidence that current slaughter practices are harmful for shrimp welfare.^[5] We have medium confidence that existing stunning methods, such as electrical stunning, would reduce suffering compared to current practices.

Shrimps are typically slaughtered by either asphyxiation (suffocation) or immersion in ice slurry (chilling) (Weis 2022, at 2min,15sec), which are likely to be inhumane slaughter methods. A Review of the Evidence of Sentience in Cephalopod Molluscs and Decapod Crustaceans has no confidence that chilling is effective at rendering decapods unconscious (Birch et al., 2021, p.71). In contrast, the Review has medium confidence for electrical stunning. However, evidence on shrimps specifically is more limited. 

One study of shrimps (Weineck et al., 2018) found that chilling in an ice slurry decreased heart rate in L. vannamei, but stunning was reversible after returning to warm water. Sensory responses were eliminated after 30 seconds of exposure to ice slurry. However, chilling may paralyse shrimps without anaesthetising them (Birch et al. 2021, p.73). In practice, tight packing or insufficient ice layering may cause some shrimps to die from asphyxiation because they have little contact with ice. This makes it challenging to extrapolate from experimental studies to aquaculture. Ice slurry could also cause pain from osmotic shock, as salinity drops when ice melts (AHAW 2005, p.104). 

Weineck et al. (2018) also reported that heart rate decreased after electrical stunning, but it increased again with irregularity within 1-2 minutes of electrical stun. Neither electrical stunning nor ice slurry caused detectable differences in neuromodulators serotonin and octopamine. Both methods induced a tail flipping response, although the authors suggest this may be reflexive rather than indicating pain or stress. 

Pharmacological anaesthesia, such as eugenol, is also an alternative stunning method. While eugenol has been shown to immobilise P. monodon (tiger shrimp) (Cai et al., 2012), more research is needed to assess consciousness during anaesthesia (Birch et al., 2021, p.72). 

Electrical stunning induces a “seizure-like” state in lobsters (Fregin and Bickmeyer, 2016), but this does not guarantee total anaesthesia or absence of pain (Birch et al., 2021, p.72). The welfare implications of electrical stunning may depend on the quality and species suitability of the stunning equipment (Birch et al., 2021, p.72). Even if shrimps are effectively stunned, they still need to be killed humanely. Crustastun (an electrical stunning machine) is designed to stun and kill large decapods, but electrical stunning is less established for smaller decapods (Birch et al., 2021, p.71). 

However, Tesco recently introduced electrical stunning into 80% of its Hilton Seafood supply chain for L. vannamei. A case study by Compassion in World Farming (n.d.) provides some indication that shrimps were killed by the electrical stun. At a commercially viable stunning voltage of 60-75 volts, only 2-3% of shrimps still had leg movement after electrical stunning.  Furthermore, of those who had no leg movement, less than 2% continued to demonstrate heart and gill bailer activity (which indicate neuronal and respiratory function). A “significant proportion” of shrimps were irrecoverable for at least 10 minutes after being stunned and returned to water, though the precise proportion is not reported. 

While electrical stunning appears to be the most promising way to humanely stun and kill shrimps, more research is needed to determine whether any shrimps retain consciousness during the electrical stun, or subsequently regain consciousness prior to slaughter. 

In the Appendix, we outline further evidence on the effectiveness of stunning techniques for other decapods. 

Stocking Density 

We have high confidence that reducing stocking density from current levels would be beneficial for shrimp welfare. Stocking density probably affects welfare through indirect mechanisms (e.g. improved water quality, reduced disease and mortality), while there is also some evidence that stocking density directly impacts shrimp behaviour and measurable stress biomarkers (e.g. serotonin). 

In intensive farm systems, stocking density might be a relevant hindrance to welfare, as shrimps have limited space to turn around or crawl (Waldhorn, forthcoming). On the other hand, stocking density may not be optimal at extremely low levels (6 shrimps/m2), at which dominance hierarchies become more prominent, and feed consumption diminishes due to the absence of social cues (Bardera et al., 2020). Studies of fish have also shown some negative effects of extremely low stocking densities, such as aggression and poor feeding response (Saraiva et al., 2022). 

For semi-intensive systems, which represent a significant portion of total production volumes worldwide, SWP tentatively recommends a stocking density of 6-15 shrimps per m². We currently estimate a ~70% likelihood that the optimal density for welfare (or “golden stocking density”) lies within this range on the typical semi-intensive farm. However, the golden stocking density on each individual farm may be higher or lower depending on other environmental factors, such as feeding, water quality, pond design, pond maintenance, shrimp size, and shrimp life stage (Saraiva et al., 2022). We intend to conduct further research into how the optimal density varies across different environments. We hope this research will allow us to develop an algorithm for estimating optimal density as a function of several environmental factors, and to subsequently publish our findings in an updated version of this report. In the absence of this research, we believe 6-15 shrimps per m² is a reasonable approximation for the optimal range on a typical semi-intensive farm.

Optimal stocking density may be important to prevent social stress in decapods (Birch et al., 2021, p.67). Da Costa et al. (2016) found that shrimps at lower density (50 shrimps/m² instead of 75 or 100) distanced themselves from each other, indicating a preference for a less crowded environment. Shrimps also moved less frequently at lower stocking density, which may be associated with reduced stress. Moreover, shrimps at lower density were able to access feed more easily. Perceived stressors, even if the threat is not real, can trigger physiological stress responses and overload decapods’ immune systems, which may lead to death of the animal (Jerez-Cepa & Ruiz-Jarabo 2021, p.5). 

Overcrowding is also likely to lead to aggressive behaviour (Abdussamad & Thampy, 1994), which may further induce anxiety, as well as causing physical injury and cannibalism (Romano & Zeng, 2017). In a study of crayfish, social aggression increased serotonin levels, which signify a response to anxiety (Bacqué-Cazenave et al., 2017), and it is reasonable to expect similar experiences for other decapods. 

Gao et al. (2017) found that lower stocking density decreased the risk of disease outbreak, while improving antioxidant capability and stress resistance ability.^[6] However, in another study, under a white spot syndrome virus challenge, stocking density did not affect the severity of virus infection (Apún-Molina et al., 2017).

Experimental evidence consistently suggests that lower stocking density increases survival rates, which could be explained by reduced stress (Da Costa et al., 2016; Jerez-Cepa & Ruiz-Jarabo, 2021, p.5), less disease (Gao et al., 2017), or improved water quality (see below). Since these results reflect survival under controlled experimental conditions (e.g. controlled water quality), the impact of stocking density on disease and mortality may be even stronger under typical pond conditions.

Experimental studies have demonstrated several positive effects of lower stocking density on water quality^[7], which may in turn lead to improved health and reduced stress (see section “Water Quality”). Our conclusions from these experimental studies are outlined below:^[8] 

• Dissolved oxygen (DO): Lower stocking density probably improves DO levels, potentially reducing the frequency and/or intensity of hypoxia, thereby reducing extended discomfort and mortality. Some studies have shown higher DO levels at lower density (Kumar & Krishna, 2015; Mena-Herrera et al., 2006; Krishna et al., 2015), although others have shown no conclusive impact (Durairaj et al. 2018; Samadan et al. 2018). In Sookying et al. (2011), average DO did not improve at lower densities, but dangerously low DO readings (below 2.5 mg/L) were significantly less common. 
• Un-ionised ammonia: Un-ionised ammonia (NH₃) is toxic for shrimps and harms welfare. One study found that un-ionised ammonia levels fell as density was reduced (Da Costa et al., 2016), but effects were negligible in other studies (Durairaj et al., 2018; Krishna et al., 2015).
• Water hardness:  Lower density may slightly improve hardness (Krishna et al., 2015). 

Other abiotic factors^[9] are optimally within a certain range, including pH (7.8 to 8.2) and water turbidity (40-45 cm).^[10] Therefore, the initial level of pH or turbidity determines whether the effects of stocking density are beneficial or harmful for shrimp health. 

• pH: Reducing density may slightly increase pH (Kumar & Krishna, 2015; Mena-Herrera et al., 2006; Krishna et al., 2015), though Durairaj et al. (2018) found the opposite effect. 
• Water turbidity: Lower density appears to reduce turbidity (Sookying et al., 2011; Tierney et al., 2020). 

The effects of stocking density are negligible for:

• Temperature (Kumar & Krishna, 2015; Sookying et al., 2011; Mena-Herrera et al., 2006; Krishna et al., 2015; Samadan et al., 2018)
• Salinity (Kumar & Krishna, 2015; Sookying et al., 2011; Mena-Herrera et al., 2006; Krishna et al., 2015; Samadan et al., 2018; Durairaj et al., 2018) 

Evidence is insufficient to determine the direction of the effects of stocking density on: 

• Nitrite: (Krishna et al., 2015; Samadan et al., 2018; Arambul-Muñoz et al., 2019). 
• Total ammonia nitrogen (TAN) (Sookying et al. 2011). 
• Alkalinity (Krishna et al., 2015; Suwoyo & Hendrajat 2021)
• Water transparency (Mena-Herrera et al., 2006, Krishna et al., 2015, Samadan et al., 2018)

Environmental Enrichment

Environmental enrichment (EE) includes the use of feeding methods that mimic natural behaviours, hiding sites, different tank shapes and colours, plants, substrates, and sediments. We have high confidence that EE is beneficial for shrimp welfare. While there has been little research on the impact of EE on shrimp stress or behaviour, there is strong evidence that substrate use improves survival. Moreover, physical enrichment (such as physical structure, plant, and substrate) improves various welfare measures for aquatic animals including crustaceans. 

EE aims to increase complexity within the animal’s environment, leading to increased social interaction, less abnormal behaviour, and reduced captivity-related stress (Arechavala-Lopez et al., 2022). In the wild, decapods spend much of their time sheltering in the dark, and should therefore be given access to dark environments (Birch et al., 2021, p.70). For L. vannamei maturation tanks, tanks with dark backgrounds and rounded shape are recommended (FAO, 2003, p.22). 

There is moderate evidence that substrates and sediments enrich the environment and improve survival rates for shrimps. Substrates may enrich the environment by giving shrimps a place to shelter and additional surface area for refuge, such as a rolled packing filler (The Fish Site, 2022). In one study, substrates (elastic packing fillers) improved water circulation, total ammonia nitrogen (TAN), and survival (Huang et al., 2022). However, in two experiments of super-intensive nursery tank systems, artificial substrates did not have a statistically significant impact on survival (Moss & Moss, 2007; Tierney et al., 2020), although they slightly improved dissolved oxygen levels in the latter study. In a biofloc nursery, a polyester-type substrate had no statistically significant effects on water quality, but reduced sludge production (Legarda et al., 2018). 

Sediments on the bottom of a tank, such as sand, can provide a source of nutrients and a place to burrow, although Bratvold and Browdy (2001a, p.82) suggest L. vannamei may exhibit limited burrowing behaviour. In one study, Bratvold and Browdy (2001b) found that while a sandy tank bottom was beneficial, it was particularly effective in combination with an artificial substrate (AquaMat). This combined treatment improved TAN, nitrate, and survival (although nitrite was worsened) relative to a) just sand; or b) no treatment. Results were similar between the sand and no treatment groups, but the no-treatment group had higher nitrate and required more maintenance to reach minimum pH. Similarly, among other decapods, complex substrates such as pebbles, seaweed, and shells reduce aggressive and cannibalistic behaviour more than sand (Romano & Zeng, 2017, p.50). 

Across all aquatic animals, a meta-analysis of 147 studies found that EE (specifically physical enrichment, such as physical structure, plant, and substrate) has a moderately positive effect on welfare (Zhang et al., 2021). Moreover, when the sample is limited to crustaceans, the effect on welfare remains statistically significant. Welfare measures include aggression and other behaviours (sheltering, foraging, abnormal behaviour), survival, and stress and disease resistance. Physical enrichment likely improves welfare through the mechanism of reduced aggression, which is commonly linked to stress levels, since physical enrichment makes it easier to shelter and reduce contact with other individuals (Zhang et al., 2021, pp. 10-12). Sheltering may also reduce population density in open areas, creating familiarity between individuals when stocking density is low enough. 

Transport and Handling 

We have high confidence that inappropriate transport and handling is harmful for shrimp welfare. Although research is limited for shrimps, poor transport and handling practices are likely to pose a significant risk to their physical and mental health. 

Careful handling of decapods is important to reduce physical injury and stress (Birch et al., 2021, p.65-66). Shrimps caught by trawling are particularly at risk of injury. Imposing maximum packing weights for live transport would help prevent the crushing of shrimps at the bottom of containers and prevent hypoxia for shrimps awaiting loading into large containers (Birch et al., 2021, p.78).

Food

We have high confidence that inadequate food and nutrition is harmful for shrimp welfare. While some decapods may be resilient to insufficient food intake, inadequate nutrition can lead to non-infectious disease and behavioural abnormalities. 

Inadequate nutrition can lead to non-infectious diseases in L. vannamei such as soft shell syndrome (Raja et al., 2015). Furthermore, access to sufficient food is likely to reduce incidence of aggressive and cannibalistic behaviour (Romano & Zeng, 2017, p.42). Overfeeding, on the other hand, may lead to the build-up of toxic ammonia (Alune, 2020) and increased turbidity (Kathyayani et al., 2019, p.177). 

Evidence from some decapods suggests that they may be resilient to starvation. Studies of lobsters and crabs found that lack of food did not reduce weight or increase mortality, as long as temperatures were cool. However, lack of food still has gradual physiological effects (Birch et al., 2021, p.70). In addition, one study of L. vannamei indicated that starved shrimps are less connected to and cooperate less with other shrimps (Dai et al., 2018). 

Water Quality 

Perturbations in pond abiotic factors (e.g. dissolved oxygen, un-ionised ammonia, pH, temperature, salinity) can lead to sub-optimal water quality and prolonged stress. As a result, shrimps may experience immune dysfunction and poor health, harming welfare (Albalat et al., 2022. p.7). A recent systematic review on white spot disease found that sub-optimal water quality is associated with stress and weakened disease resistance (Millard et al., 2021).

Dissolved Oxygen (DO)

We have very high confidence that below optimal DO is harmful for shrimp welfare, primarily based on its clear effects on mortality. Our standards recommend DO levels between 5 and 8 mg/L.

Hypoxia (lack of oxygen) leads to the build-up of lactate, which is painful in humans, and may also be painful in decapods (Birch et al., 2021, p.69). De la Vega et al. (2007, p.136) found that hypoxia in P. monodon led to reduced feeding and red colouration, which likely indicates poor welfare. 

Moreover, maintenance of optimal DO levels between 5 and 8 mg/L is crucial for shrimp survival (McGraw et al., 2001; Boyd & Hanson, 2010; Nonwachai et al., 2011; Duan et al., 2013). Optimal DO reduces susceptibility to infectious disease (Le Moullac & Haffner, 2000, p.125), and oxygen is required to reduce the build-up of toxic ammonia (Alune 2020). Sufficient oxygen may also help prevent cannibalism (Duan et al., 2013). 

Un-ionised Ammonia

We have very high confidence that 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. Our standards recommend un-ionised ammonia of <0.05 mg/L. 

High concentrations of un-ionised ammonia are highly toxic for shrimps (Kuhn et al., 2011) and may damage shrimps’ gills, hepatopancreas, and possibly gut lining (Alune, 2020). Shrimps exposed to high un-ionised ammonia may experience hyperactivity, convulsions, lethargy, and coma, often leading to death (Boyd & Tucker, 1988, pp.134-136). 

In one study, higher un-ionised ammonia concentration in water (ranging from 0-20 mg/L) hindered aspects of the immune response and reduced survival in the face of a V. alginolyticus pathogen challenge (Liu & Chen, 2004). Frias-Espericueta et al. (2000) also reported that higher un-ionised ammonia (ranging from 10-20 mg/L) had a highly detrimental effect on survival. Furthermore, in a study of P. stylirostris, higher un-ionised ammonia (ranging from 0-3 mg/L) reduced immune defence capabilities against disease (Le Moullac & Haffner, 2000, p.126-7). 

Nitrifying bacteria convert ammonia into the less toxic nitrite (NO₂) and nitrate (NO₃). Although nitrite is typically considered less toxic than un-ionised ammonia (Alune 2020), it is still highly toxic (Kuhn et al., 2011). In fact, Valencia‑Castañeda et al. (2018) found that nitrite was more toxic than ammonia in low-salinity water. In Gross et al. (2007), increasing nitrite concentration from 4 to 8 mg/L substantially reduced survival, while an increase from 0 and 4 mg/L had no effect. 

Nitrite is subsequently converted into nitrate. Nitrate is less toxic than un-ionised ammonia or nitrite, but shrimp health and survival still deteriorates at high concentrations (Kuhn et al., 2011). High nitrate may damage shrimps’ gills and hepatopancreas, reducing survival (Furtado et al., 2015). However, higher nitrate may also help control the accumulation of hydrogen sulphide, which is toxic for shrimps (Torun et al., 2020). 

pH

We have high confidence that deviations from optimal pH are harmful for shrimp welfare due to detrimental effects on health, immunity, and susceptibility to disease. Our standards recommend a pH of 7.8 to 8.2. We also have high confidence that sudden fluctuations (within the optimal pH range) are harmful. While we have not found consistent supporting evidence in the academic literature, our conversations with farmers and aquaculture specialists suggest sudden pH fluctuations are a major concern for shrimp health.^[11] 

High pH, in particular, appears to reduce survival. As pH exceeds the optimal range, toxic ammonia builds up, depressing immunity and increasing disease susceptibility (Kubitza, 2017). Outside the optimal range, at pH of 6.5 or 9.5, the health of the animal is weakened (Yu et al., 2020). Furtado et al., 2015 found that survival dramatically fell when pH was outside a range of 4.5 to 9.5. However, in a study with a super-intensive, zero water exchange, high aeration system, the effect of pH on survival was minimal (Wasieleski Jr et al., 2006, slide 16). 

While low pH is typically also harmful to shrimp survival, gradual reductions in pH may provide resistance against disease outbreaks. Han et al., 2018 gradually adjusted pH from 8.2 (control pH) to either 9.64-9.81 (gradual-high pH) or 6.65-6.78 (gradual-low pH). The gradual-high pH group had substantially higher mortality than the control group, while the gradual-low pH group only had slightly higher mortality than the control group. In addition, under a V. parahaemolyticus bacteria challenge, gradual-low pH actually enhanced disease resistance, which reduced mortality relative to the other experimental groups. 

Studies of catfish indicate that sudden changes in pH can cause significant stress and mortality for aquatic animals, even if the changes occur within the optimal pH range (Banrie 2013). Our consultation with farmers and aquaculture specialists suggest this is also the case for shrimps. However, Boyd (2017) argues that daily fluctuations from the optimal range may not be highly detrimental for shrimps, despite the concerns of many aquaculturists. We would therefore like to see further research on this issue. 

Temperature

We have high confidence that high temperatures are harmful for shrimp welfare, as the water retains less oxygen, toxic ammonia increases, and survival worsens. We have medium confidence that low temperatures are harmful, since shrimps may be adaptive to small deviations below optimal temperature. Our standards recommend temperatures of 28-30°C. 

L. vannamei are ectotherms, which means extreme temperatures have large impacts on shrimp physiology (Albalat et al., 2022, p.7). Non-optimal temperatures may also impede the immune response of shrimps (Le Moullac & Haffner, 2000, p.125). However, crustaceans are known to have some adaptive abilities when exposed to unfamiliar temperatures (Lagerspetz & Vainio, 2006, p.248-9), and Wang et al. (2019) found that L. vannamei can self-regulate in response to temperature fluctuations. 

High water temperatures are more likely to be a significant welfare concern than low temperatures. At above-optimal temperatures, the water retains less oxygen, leading to higher risk of hypoxia (Kungvankij & Chua, 1986; Albalat et al., 2022, p.7). Moreover, higher temperatures raise the proportion of total ammonia nitrogen that is toxic (Kir & Kumlu, 2006, p.231). Shrimp survival may also be weakest at high temperatures. In Ponce-Palafox et al. (1997), survival was lowest at higher temperatures (30-35°C up from 20-25°C), particularly when salinity was low. De la Vega et al. (2007, p.136) found that hyperthermia in P. monodon led to reduced feeding and red colouration, which likely indicates poor welfare. 

Shrimps may be able to acclimatise to small deviations below optimal temperature, although very low temperatures are harmful for survival (Kumlu et al., 2010). In Yu et al. (2010), swim speed, which is one possible indicator of physical health, improved as temperature increased from 17 to 29°C. Contact with ice or icy water is stressful for decapods and can be fatal (Birch et al., 2021, p.68). 

Temperature may be an important factor influencing disease outbreaks, as white spot disease thrives at temperatures between 25 and 28°C (Millard et al., 2021). In contrast, during chronic infection, as shrimps become afflicted with disease, a higher temperature (33°C) was found to increase stress and accelerate disease progression. 

Salinity 

We have medium confidence that deviations from optimal salinity are harmful for shrimp welfare. While it is likely that extreme salinity levels or fluctuations are harmful, the experimental evidence is mixed for small deviations from the recommended salinity range.

L. vannamei are typically able to survive salinities between 0.5‰ and 45‰ (Millard et al., 2021). Our standards recommend 10‰ to 20‰ to ensure shrimps are healthy. Large fluctuations in salinity within a short period, rather than the level of salinity, may play a larger role in influencing disease susceptibility (Millard et al., 2021). This is particularly concerning in the face of climate change, which is expected to increase the occurrence of extreme weather events. 

Salinity may also affect welfare through its interactions with other water quality parameters. As salinity increases, dissolved oxygen becomes less soluble (thereby increasing the risk of low dissolved oxygen and hypoxia), but the toxic component of total ammonia nitrogen is diminished (Boyd & Tucker 1988, p.94). In a recent study, increasing salinity from very low levels (1‰ to 3‰) reduced the toxicity of ammonia, nitrite, and nitrate (Valencia‑Castañeda et al., 2018). Similarly, Lin and Chen (2003) found that nitrite became less toxic as salinity increased from 15‰ to 35‰. 

Experimental evidence suggests higher salinities may be preferable for survival. In a super-intensive system without water exchange, increasing salinity from to 32‰ (from 4‰ or 16‰) improved survival (Maicá et al., 2014). Similarly, at high temperatures (30-35°C), increasing salinity (50‰ up from 20‰) improved survival (Ponce-Palafox et al., 1997). However, Jaffer et al. (2019) found no statistically significant effects of salinity (between 1‰ and 25‰) on survival. Decamp et al. (2003) reported higher survival at higher salinity (18‰ or 36‰ up from 9‰), but the effect was not statistically significant. 

However, low-salinity water may be beneficial in some cases, as Ching et al. (2014) showed that higher salinity (30‰ up from 2‰ or 5‰) may facilitate halophytic bacterial growth and increase the risk of disease outbreak. 

Other Water Quality Parameters

Other water quality parameters may impact shrimp welfare, but were not the focus of our research due to a less extensive evidence base. These include, but are not limited to: 

Total ammonia nitrogen (TAN): High levels of TAN, which consists of both toxic un-ionised ammonia and non-toxic ionised ammonium (NH4+), restrict the maximum biomass of shrimps in a pond before oxygen needs to be replenished. This may lead to hypoxia and a stressful environment for shrimps (Alune, 2020). Higher TAN has also been shown to hinder survival (Rostami et al., 2019). Our standards recommend TAN of <1ppm (pH dependent). 

Hydrogen sulphide: Hydrogen sulphide is toxic for shrimps (Torun et al., 2020, p.2). Chronic exposure can lead to damage of the gut structure and decrease the immunity of L. vannamei (Suo et al., 2017). It also inhibits nitrification, thereby increasing the accumulation of ammonia in the pond (Joye and Hollibaugh, 1995). Our standards recommend hydrogen sulphide of <0.1ppm (pH dependent). 

Alkalinity: Alkalinity levels above 120ppm promote nitrification and can prevent the build-up of toxic ammonia (Alune, 2020). However, Piérri et al. (2015) found no statistically significant impact of alkalinity on survival. Our standards recommend alkalinity of 120-200ppm. 

Water turbidity: Kathyayani et al. (2019) found that high turbidity increased stress, inhibited immunological activity, and reduced survival. Our standards recommend water turbidity (secchi disk visibility) of 40-45cm. 

Ozone levels: Adequate ozone levels (below 0.1-0.15 mg/L) help protect shrimps against soft-shell damage (Romano & Zeng, 2017, p.45). 

Conclusion

This report aims to identify which factors are most important for shrimp welfare (with a particular focus on L. vannamei), drawing on evidence from academic literature and conversations with farmers and aquaculturists. We are most confident in the importance of addressing eyestalk ablation, disease, stunning and slaughter, dissolved oxygen, and un-ionised ammonia to improve shrimp welfare. For these factors, we have very high confidence that small to medium improvements would reduce harm to shrimps. Of the remaining factors that we investigated, we have high confidence in the importance of addressing stocking density, environmental enrichment, transport and handling, food, pH, and high temperature, while we have medium confidence for salinity and low temperature. We would encourage further research into the welfare effects of these environmental factors and farming practices. We would also welcome further research on how best to measure shrimp welfare.

Appendix

Eyestalk Ablation - Impact on Aquaculture Outcomes

See above (section "Eyestalk Ablation") for the impact of eyestalk ablation on welfare.

Evidence is mixed on the effects of eyestalk ablation on aquaculture outcomes. Recent studies by Zacarias et al. (2019) and de Menezes et al. (2019) compared the reproductive performance of ablated and non-ablated L. vannamei. Zacarias et al. found that tanks with ablated shrimps produced ~80% more eggs and nauplii per day, whereas de Menezes et al. found the reverse effect, with ablated shrimps producing around 10-15% fewer eggs and nauplii per day. A likely explanation is that ablation had a stronger effect on mortality in the latter study (39% up from 29%), compared to the former (2% up from 1%). This suggests that ablation may be more harmful for reproductive performance when shrimps are susceptible to disease and mortality. 

The two studies are in accord regarding the detrimental impact of ablation on egg/nauplii production per spawned female (i.e. the number of eggs and nauplii produced per female who has mated and started spawning). This is likely because ablation requires shrimps to expend additional energy (Zacarias et al., 2019, p.464). 

Based on these studies, the sole benefit of ablation, in terms of reproductive success, is an improved mating rate per female. However, industry has observed a trend of improved mating success among non-ablated shrimps over several generations of closed-cycle breeding (Zacarias et al., 2019), although it is important to caution that selective breeding can be harmful for animal health and welfare (van Marle-Köster & Visser, 2021). 

As noted above (see section "Eyestalk Ablation"), ablation leads to reproductive exhaustion over time. Therefore, over the long-run, ablation may shorten the productive lifespan of females in addition to harming their welfare. 

Other factors may have as much or greater influence on reproductive performance. Albalat et al. (2022, p.3) note that stocking density of around 6-15 shrimps/m² is optimal for maturation, while hatching rates are affected by the spawning tank environment, variation in fertilisation rates, and the health of the female during breeding time (Zacarias et al., 2019. p.464). 

Disease - Preventative Tools

See above (section "Disease") for the impact of disease on welfare.

Probiotic use is an emerging preventative tool against disease, potentially protecting shrimps against pathogens while also improving water quality and feed efficiency (El-Saadony et al., 2022). A meta-analysis of 100 experiments found that probiotics improve survival, shrimp growth, and feed efficiency (Toledo et al., 2019).

Immunostimulants may also be effective as a disease prevention strategy when deployed alongside biosecurity protocols and procedures (Newman, 2019). These are naturally occuring substances that can be administered to shrimps either orally or by injection. Howell (2022) highlights the potential for immunostimulants to improve shrimps’ immune resistance against white spot disease. A major benefit of immunostimulants is that farmers are able to avoid the use of antibiotics, which can lead to antimicrobial resistance. Our standards require farms not to use antibiotics as a preventative measure against disease. 

Other emerging ways to control and prevent disease outbreaks include the use of “tolerines” (Flegel et al., 2008, p.365) and the rearing of specific pathogen-free (SPF) or specific pathogen-resistant (SPR) strains of shrimps (Alday‐Sanz et al., 2018). 

Stunning and Slaughter - Other Decapods

See above (section "Stunning and Slaughter") for details on the stunning and slaughter of shrimps.

Ice slurry has not been a successful stunning method for large decapods, failing to terminate neural activity (Fregin & Bickmeyer, 2016) or behavioural responsiveness (Roth & Øines, 2010) within an hour. 

Electrical stunning appears more promising for large decapods. As noted above (see section "Stunning and Slaughter"), Crustastun (an electrical stunning machine) is designed to stun and kill large decapods (Birch et al., 2021, p.71). Roth & Øines (2010) found that crabs could be rendered insensible within one second of high voltage electrical stunning, but a later study (Roth & Grimsbø, 2016) was less positive, as 7% of crabs discarded an appendage, which is a sign of stress (Conte et al., 2021, p.4). 

In a study of crabs and lobsters, electrical stunning did not increase stress levels (indicated by haemolymph lactate concentrations), aside from stress associated with handling (Neil & Thompson, 2012). However, in a study of fish, cortisol (a stress response) was more elevated after electrical stunning than after CO₂ stunning (Grans et al., 2016), which raises some doubt about whether electro-immobilised animals are in fact unconscious. CO₂ stunning is unlikely to be humane, as several studies of decapods and fish have indicated that it causes prolonged aversive behavioural reactions (Roth & Øines, 2010; Barkerud, 2021, p.6-8; Gardner, 1997, p.167). 

References 

Please find all the references here. 

Feedback

We value your feedback and suggestions. You can comment below or reach out to Lucas and Aaron at lucas@shrimpwelfareproject.org and aaron@shrimpwelfareproject.org. 

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. 

3. ^{^}

    All standards referenced in this report have been determined based on expert advice and research internal to SWP. 

4. ^{^}

   White spot disease causes shrimps to be lethargic, reducing feeding and preening. Yellow head virus targets multiple tissues including the gut, antennal gland, nervous tissue, and gonads, causing abnormal yellow colouring. Infectious myonecrosis leads to abdomen and tail muscle whitening and may cause lymphoid organs to be atrophied and enlarged (Albalat et al., 2022, pp.4-6). 

5. ^{^}

   However, note 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. 

6. ^{^}

   Antioxidants are “man-made or natural substances that may prevent or delay some types of cell damage.” (National Centre for Complementary and Integrative Health 2013). 

7. ^{^}

   See Kotiya and Vadher 2021; Krummenauer et al., 2010; Sookying et al., 2011; Kumar & Krishna, 2015; Galkanda-Arachchige et al., 2021; Durairaj et al., 2018; Samadan et al., 2018; Araneda et al., 2008; Krishna et al., 2015; Apún-Molina et al., 2017. However, see Mena-Herrera et al., 2006 for contradictory results. 

8. ^{^}

   These conclusions are based on SWP’s statistical analysis of the effects of reducing stocking density. 

9. ^{^}

   Abiotic factors are parts of an ecosystem that are not alive themselves but affect living organisms in the environment. 

10. ^{^}

    These optimal ranges are recommended based on expert advice and research internal to SWP. 

11. ^{^}

    Although we would typically only assign high confidence to the importance of welfare factors when backed by strong evidence in the academic literature, in this particular case we believe the wealth of knowledge from industry outweighs a single contradictory study.