A glance at fasting practices in Atlantic salmon aquaculture

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By Milthon Lujan

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Illustrations of cultured Atlantic salmon post-smolts with different conditions factors. These illustrations represent condition factors of approximately 1.3 (top), 1.15 (mid), and 1.0 (bottom). Source: Havas et al., (2024); Rev Aquac.

Atlantic salmon aquaculture, a thriving industry, faces a crucial challenge: balancing economic motivations for optimal growth with the ethical obligation to ensure fish welfare.

Concerns about animal welfare and environmental impact have led to increased scrutiny of aquaculture practices, including the use of fasting and food restriction.

In this vein, a team of scientists from the Institute of Marine Research, Nofima, and The Arctic University of Norway published a scientific review providing a synthesis of the various causes of fasting in Atlantic salmon aquaculture and evaluating their associated welfare implications to formulate guidelines for appropriate practices.

The article delves into specific welfare implications associated with different fasting scenarios, paving the way for responsible practice guidelines.

The issue of overfeeding

While aiming for maximum growth, fish farmers often overfeed their salmon, raising environmental and economic concerns including:

  • Nutrient pollution: Excess food contaminates surrounding water bodies with nitrogen and phosphorus, contributing to eutrophication.
  • Food waste: Unconsumed feed in open-sea cages attracts wild fish, impacting the ecosystem and increasing production costs.
  • Financial burden: Feeding is the highest expense in salmon farming, and overfeeding leads to resource waste.

Fasting in Atlantic salmon aquaculture

Atlantic salmon undergo fasting periods for various reasons, both voluntary and involuntary:

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  • Environmental factors: Stress from extreme temperatures, low oxygen levels, etc.
  • Diseases: Pancreas disease, gill amoebic disease, etc.


  • Feed withdrawal: Used before handling procedures such as transportation or delousing.
  • Harvest: The harvesting phase of salmon raised in recirculating systems includes a purging period during which the fish are not fed.

While involuntary fasting is generally short-lived (48 to 72 hours), voluntary fasting periods can last weeks or even months, significantly affecting production.

Concerns for animal welfare

Fasting raises ethical concerns regarding potential hunger and distress in fish, violating the principle of the absence of hunger and thirst. While fish welfare aligns with good economic practices, defining the exact threshold where welfare is compromised is challenging due to:

  • Perception differences: Fish experience the world differently from humans, making it difficult to assess their subjective experience of hunger.
  • Lack of scientific evidence: Current knowledge is insufficient to establish clear guidelines on responsible fasting durations based solely on welfare.

The need for context-dependent guidelines

The scientific review highlights the need for context-specific fasting guidelines, considering factors such as:

  • Reason for fasting: Voluntary fasting due to illness may not violate the absence of hunger, while sudden feed withdrawal may be stressful.
  • Fasting duration: Longer durations raise greater welfare concerns.

Understanding the response

Fortunately, Atlantic salmon exhibit a remarkable ability to adapt to fasting, particularly in later life stages:

  • Fry and young fish are highly sensitive to food deprivation and require careful handling.
  • Post-smolts (juvenile fish) and adults are more resilient and tolerate longer fasting periods without significant functional or health deterioration.
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Therefore, short-term involuntary fasting employed in standard farming practices, such as pre-transport withdrawal, should not pose significant welfare issues for post-smolts and adults.

Towards responsible practices

This review highlights the need for context-dependent fasting guidelines in Atlantic salmon aquaculture, based on:

  • Existing legislation on fish welfare.
  • Understanding of natural fasting periods in wild salmon.
  • Physiological responses and tolerance limits of farmed fish to fasting.
  • Implications of fasting in different farming systems (RAS, open-sea cages).
  • Understanding how salmon experience hunger and the potential benefits of strategic fasting.

Future research areas

The findings of the scientific review open doors to future research areas:

  • Understanding fish welfare during fasting.
  • Exploring the potential benefits of strategic fasting and compensatory growth.
  • Developing ethical and sustainable farming practices.


By understanding different fasting scenarios and their varied impacts on Atlantic salmon welfare, we can establish responsible practices that balance economic viability with ethical considerations. By prioritizing optimal environmental conditions, minimizing stress, and promptly addressing health issues, we can ensure the welfare of farmed fish while contributing to a sustainable industry.

Thus, balancing economic efficiency with animal welfare and environmental responsibility is crucial for the future of Atlantic salmon aquaculture. This review underscores the need for continued research and evidence-based guidelines for fasting practices that prioritize both fish welfare and sustainable production.

The study was funded by the Research Council of Norway through the FASTWELL project.

Malthe Hvas
Animal Welfare Research Group, Institute of Marine Research, Matre 5984,
Email: malthe.hvas@hi.no

Reference (open access)
Hvas M, Kolarevic J, Noble C, Oppedal F, Stien LH. Fasting and its implications for fish welfare in Atlantic salmon aquaculture. Rev Aquac. 2024; 1-25. doi:10.1111/raq.12898