ASC Position on Stocking Density and Fish Welfare

ASC does not impose a single, universal stocking density limit for farmed fish. Scientific consensus shows that stocking density cannot be used as a standalone indicator of fish welfare and must be assessed alongside other environmental and management factors.

Fish welfare outcomes depend on how density is managed together with water quality, oxygen availability, feeding practices, husbandry, and system design. When these factors are well controlled, higher densities can support good welfare outcomes for certain species and farming systems.

A fixed density limit is not appropriate because ASC standards apply to a wide range of species with very different biological and behavioural needs, from tropical species to salmonids and shrimp. Welfare requirements also vary by life stage, farming system, and local environmental conditions. A “one size fits all” approach risks being both ineffective and misleading.

Stocking density is often highlighted because it is easy to visualise. However, fish do not experience space in the same way humans do. While excessive densities can harm welfare, densities that are too low can also cause stress or aggression in some species. Others naturally school closely together. For example, farmed salmon are highly social and typically swim fin-to-fin; despite appearances, salmon pens consist on average of 96–98% water and only 2–4% fish.

How ASC addresses stocking density

The ASC Farm Standard takes a holistic, outcome-based approach to welfare. Stocking density is addressed within the farm’s Health and Welfare Management Plan (HWMP) and assessed through multiple Operational Welfare Indicators (OWIs), including:

• Water quality
• Behavioural observations
• Physical (morphological) condition
• Mortality trends

If negative trends are detected, farms must review and adjust their practices, which may include changes to stocking density alongside other management measures. This approach is more robust than imposing a fixed numerical threshold, particularly as density measurements vary widely between species, life stages, and production systems.

Alignment with science and expert opinion

Current scientific research consistently shows that there is no single “optimal” or “golden” stocking density that guarantees good welfare across systems. Studies demonstrate that welfare outcomes are strongly influenced by environmental management, such as oxygen levels, water flow, and feeding regimes, rather than density alone.

This perspective is echoed by the European Parliament, which recognises stocking density as a relevant welfare indicator but does not recommend universal density thresholds for farmed fish species.

ASC’s approach is also supported by fish welfare experts, including the FishEthoGroup, and has been recognised by the Aquatic Life Institute, which ranked the ASC Farm Standard as the leading aquaculture certification programme for animal welfare among major global schemes in its 2025 benchmark and first in both the 2023 and 2024 editions of this benchmark.

In summary

ASC does not rely on a single density number to define fish welfare. Instead, it applies a science-based, species-specific, and holistic approach that focuses on measurable welfare outcomes and continuous improvement across diverse aquaculture systems.

 

More detailed FAQs

What does the ASC Farm Standard say about stocking density?

Taking this more holistic approach, stocking densities are one of the many integral components to be considered within the Health and Welfare Management Plan (HWMP) in the new ASC Farm Standard.

In the Farm Standard, ASC requires the assessment of stocking density through various operational welfare Indicators (OWIs) (water quality, morphological scoring, behavioural scoring, and mortality) that can be used as proxies.

If a negative trend is observed on these indicators, then the farm should assess its farming operations, including stocking density, and modifying them accordingly.

This approach is more suitable than setting a metric limit, as accurate and reliable density figures are hard to obtain in aquaculture, and they vary between species, life stage and farming systems.

What does the latest science say about density and fish welfare?

ASC’s work always reflects the latest scientific thinking and our holistic approach using both environment- and animal-based indicators to assess welfare outcomes rather than using a specific ‘golden’ metric aligns with the research below.

Several studies, including those cited below, have shown that stocking density alone is not a definitive indicator of fish welfare in aquaculture. The effects of stocking density on welfare are heavily influenced by other factors, such as water quality, feeding rates, and species-specific behavioural needs.

For example, it has been demonstrated that the impact of stocking density on welfare is modulated by environmental management, such as oxygen levels, water flow, and the availability of food. Poor management of these factors can lead to stress and disease, regardless of stocking density.

• Saraiva, Joao & Rachinas-Lopes, Patrícia & Arechavala-Lopez, Pablo. (2022). Finding the “golden stocking density”: A balance between fish welfare and farmers’ perspectives. Frontiers in Veterinary Science. 9. 10.3389/fvets.2022.930221.

• Saraiva et al. (2022) indicates the complex relationship between fish welfare, stocking density and influencing factors make it challenging to define a specific optimal (“golden”) stocking density. It is difficult to set minimum and maximum stocking density levels that will protect welfare. However, there is little doubt that stocking densities that are too low or too high can have negative impacts on welfare and/or production if not managed properly.

• Jia, R.; Wang, L.; Hou, Y.; Feng, W.; Li, B.; Zhu, J. Effects of Stocking Density on the Growth Performance, Physiological Parameters, Redox Status and Lipid Metabolism of Micropterus salmoides in Integrated Rice–Fish Farming Systems. Antioxidants 2022, 11, 1215. https://doi.org/10.3390/antiox11071215

• Jia et al. (2022) also mention some systems, like integrated rice-fish farming, where higher stocking densities can be used without severe welfare issues as long as other parameters are tightly controlled. Fish in such systems can still thrive if water quality, oxygen levels, and feeding practices are optimised to prevent overcrowding effects.
• In addition, the same authors suggest that stocking density should be considered alongside other welfare indicators, rather than as a sole measure of fish welfare.

• The June 2023 “Animal Welfare of Farmed Fish” report by the European Parliament, emphasises the importance of considering stocking density alongside other welfare indicators to protect fish welfare. The report highlights the challenges associated with inappropriate stocking densities, particularly noting issues such as fin erosion, decreased growth, and increased aggression in juvenile fish. Additionally, it points out the risks of low oxygen saturation at high temperatures and densities. However, while the report suggests that fish density can be used as a welfare indicator post-smolt, it does not specify an exact recommended stocking density level (e.g. an XX kg/m³ threshold) for species like salmon.

• This approach aligns with the ASC methodology of integrating multiple operational welfare indicators (OWIs) into its standards, which are tailored to address the unique needs of various species and farming systems, rather than applying a blanket standard.

Do animal welfare groups support the ASC position on density?

FishEthoGroup (FEWG), is a non-profit association focused on improving fish welfare based on their behaviours and bridging the gaps between science and the aquaculture sector. FEWG has collaborated with ASC to ensure that our welfare standards reflect the latest scientific research and best practices, providing a solid scientific foundation for welfare improvements in aquaculture.

Aquatic Life Institute (ALI) has ranked the ASC Farm Standard as the leading organisation for animal welfare amongst the top three major aquaculture certification programmes globally. The tool analyses current welfare requirements within the primary farming standards of 8 global seafood certification schemes and 1 international ratings agency. The areas of assessment include water quality, stocking density and space requirements, environmental enrichment, feed composition, stunning and slaughter, neglected species prohibitions, and additional considerations. ASC also ranked first in the 2023 and 2024 editions of the ALI benchmark.

How is ASC driving progress on this topic?

ASC has convened a Technical Working Group (TWG) that includes some of the world’s leading scientists and stakeholders, including a leading animal welfare organisation, to explore and debate stocking density and other welfare issues. This multi-stakeholder approach ensures ASC standards remain rooted in the latest scientific thinking.

The group is currently focused on contributing to the development of an interpretation manual to support the implementation of the upcoming Farm Standard, including a recommendation table with considerations for stocking densities. As ASC operates on a continuous improvement model, following the launch of the Farm Standard, the group will meet quarterly to discuss new welfare topics or potential changes to the existing information on fish welfare, which may be considered for inclusion in future updates to the Farm Standard.

Does ASC impose a metric stocking density limit for fish welfare?

ASC does not impose a metric stocking density limit for fish. Stocking density is a complex topic in aquaculture welfare and the consensus in the scientific community is that it should not be used as a standalone welfare indicator. Several studies suggest that when stocking density is managed alongside other environmental and operational parameters (e.g. water quality, feeding, oxygen levels), high densities can still support good welfare practices depending on the species and system.

ASC has standards for a diverse range of species ranging from tropical barramundi to salmon and shrimp – each with unique welfare needs. This diversity and complexity make a ‘one size fits all’ approach unhelpful for addressing animal welfare. Our new Farm Standard takes a more holistic approach – you can read more details below.

Very often, the word ‘fish’ is used to describe a wide range of animals with hugely different characteristics and needs. But you can’t treat a salmon like a pangasius any more than you can treat a cow like a chicken – in fact a cow and a chicken are more closely related than a salmon and a pangasius!

Some animal welfare campaigners focus on stocking density and we understand it is easy to picture and emotive – for us humans there is nothing worse than being crammed on a rush hour train. But the truth is more complicated than that – of course, above certain densities fish can come to real harm, but for some species, a density which is too low can be detrimental to welfare and cause aggressive behaviour, while others will naturally school together and give them impression of having less space.

Farmed salmon (marine salmonids) are very sociable, so they often swim fin to fin and school together. This may give the appearance that they have small pens. However, in reality, on average, a salmon pen is made up of 2-4% salmon and 96-98% water. Keeping the stocking density of the salmon pens to this level ensures that the fish are kept healthy and limits their risk of illness.

So, we need to understand more and adopt a more holistic approach that takes into account what individual species really need and take together environmental conditions and specificities associated to a specific system or location.

What does the new ASC Farm Standard say about stocking density?

Taking this more holistic approach, stocking densities are one of the many integral components to be considered within the Health and Welfare Management Plan (HWMP) in the new ASC Farm Standard.

In the new Farm Standard, ASC requires the assessment of stocking density through various operational welfare Indicators (OWIs) (water quality, morphological scoring, behavioural scoring, and mortality) that can be used as proxies.

If a negative trend is observed on these indicators, then the farm should assess its farming operations, including stocking density, and modifying them accordingly.

This approach is more suitable than setting a metric limit, as accurate and reliable density figures are hard to obtain in aquaculture, and they vary between species, life stage and farming systems.

What does the latest science say about density and fish welfare?

ASC’s work always reflects the latest scientific thinking and our holistic approach using both environment- and animal-based indicators to assess welfare outcomes rather than using a specific ‘golden’ metric aligns with the research below.

Several studies, including those cited below, have shown that stocking density alone is not a definitive indicator of fish welfare in aquaculture. The effects of stocking density on welfare are heavily influenced by other factors, such as water quality, feeding rates, and species-specific behavioural needs.

For example, it has been demonstrated that the impact of stocking density on welfare is modulated by environmental management, such as oxygen levels, water flow, and the availability of food. Poor management of these factors can lead to stress and disease, regardless of stocking density.

• Saraiva, Joao & Rachinas-Lopes, Patrícia & Arechavala-Lopez, Pablo. (2022). Finding the “golden stocking density”: A balance between fish welfare and farmers’ perspectives. Frontiers in Veterinary Science. 9. 10.3389/fvets.2022.930221.
  • Saraiva et al. (2022) indicates the complex relationship between fish welfare, stocking density and influencing factors make it challenging to define a specific optimal (“golden”) stocking density. It is difficult to set minimum and maximum stocking density levels that will protect welfare. However, there is little doubt that stocking densities that are too low or too high can have negative impacts on welfare and/or production if not managed properly.

• Jia, R.; Wang, L.; Hou, Y.; Feng, W.; Li, B.; Zhu, J. Effects of Stocking Density on the Growth Performance, Physiological Parameters, Redox Status and Lipid Metabolism of Micropterus salmoides in Integrated Rice–Fish Farming Systems. Antioxidants 2022, 11, 1215. https://doi.org/10.3390/antiox11071215
  • Jia et al. (2022) also mention some systems, like integrated rice-fish farming, where higher stocking densities can be used without severe welfare issues as long as other parameters are tightly controlled. Fish in such systems can still thrive if water quality, oxygen levels, and feeding practices are optimised to prevent overcrowding effects.
  • In addition, the same authors suggest that stocking density should be considered alongside other welfare indicators, rather than as a sole measure of fish welfare.

• The June 2023 “Animal Welfare of Farmed Fish” report by the European Parliament, emphasises the importance of considering stocking density alongside other welfare indicators to protect fish welfare. The report highlights the challenges associated with inappropriate stocking densities, particularly noting issues such as fin erosion, decreased growth, and increased aggression in juvenile fish. Additionally, it points out the risks of low oxygen saturation at high temperatures and densities. However, while the report suggests that fish density can be used as a welfare indicator post-smolt, it does not specify an exact recommended stocking density level (e.g. an XX kg/m³ threshold) for species like salmon.
  • This approach aligns with the ASC methodology of integrating multiple operational welfare indicators (OWIs) into its standards, which are tailored to address the unique needs of various species and farming systems, rather than applying a blanket standard.

Do animal welfare groups support the ASC position on density?

• FishEthoGroup (FEWG), is a non-profit association focused on improving fish welfare based on their behaviours and bridging the gaps between science and the aquaculture sector. FEWG has collaborated with ASC to ensure that our welfare standards reflect the latest scientific research and best practices, providing a solid scientific foundation for welfare improvements in aquaculture.

• Aquatic Life Institute (ALI) has ranked the ASC Farm Standard top of its welfare-based aquaculture certification benchmark tool for 2024. The tool analyses current welfare requirements within the primary farming standards of 8 global seafood certification schemes and 1 international ratings agency. The areas of assessment include water quality, stocking density and space requirements, environmental enrichment, feed composition, stunning and slaughter, neglected species prohibitions, and additional considerations. ASC also ranked first in the 2023 edition of the ALI benchmark.

How is ASC driving progress on this topic?

ASC has convened a Technical Working Group that includes some of the world’s leading scientists and stakeholders, including a leading animal welfare organisation, to explore and debate stocking density and other welfare issues. This multi-stakeholder approach ensures ASC standards remain rooted in the latest scientific thinking.

The group is currently focused on contributing to the development of an interpretation manual to support the implementation of the upcoming Farm Standard, including a recommendation table with considerations for stocking densities. As ASC operates on a continuous improvement model, following the launch of the Farm Standard, the group will meet quarterly to discuss new welfare topics or potential changes to the existing information on fish welfare, which may be considered for inclusion in future updates to the Farm Standard.