The impact of crowding during harvest on the welfare and quality of rainbow trout

One of the critical procedures in the final stage of rainbow trout (Oncorhynchus mykiss) production is “crowding,” a technique that involves temporarily increasing fish density to facilitate harvesting. Although it is a common practice, its correct execution is fundamental to minimize stress and its negative consequences.

A recent study published in the Journal of the World Aquaculture Society by scientists from the Polytechnic University of Madrid and the Complutense University of Madrid delves into how different crowding methods and water temperature (seasonality) influence welfare indicators and fillet quality in rainbow trout. This article summarizes the key findings of this research, offering valuable information for professionals in the sector.

Why is it crucial to study crowding in rainbow trout?

Crowding, while reducing harvest time, can be a considerable source of stress for fish. This stress can trigger physiological responses that not only compromise animal welfare but can also deteriorate meat quality, affecting parameters such as color, texture, and muscle pH. Rainbow trout, a cold-water fish, is particularly sensitive to water quality and handling. Furthermore, global warming and the consequent increase in water temperature in farming systems add a layer of complexity to this management.

The study aimed to optimize crowding methods to minimize their negative impact, considering the additional influence of temperature fluctuations between winter and summer.

Understanding the study methodology: what was done and why?

Researchers evaluated two crowding methods in rainbow trout during two different seasons: winter (average temperature of 8.80°C) and summer (average temperature of 22.0°C). The crowding methods applied for 20 minutes were:

• Water level reduction (WL): The water column in the tanks was lowered to concentrate the fish.
• Lateral compression (LC): Mesh dividers were used to reduce the horizontal space available to the fish.

In both cases, the final fish density reached approximately 113.5 kg/m³.

To evaluate the impact of these treatments, various indicators were measured:

• Welfare indicators (physiological): Scientists analyzed plasma metabolites such as cortisol (primary stress indicator), creatine phosphokinase (K, related to muscle damage or metabolic demand), glucose, and lactate (indicators of energy mobilization and anaerobic metabolism).
• Product quality: Researchers evaluated the color of the skin, liver, and fillet (using the CIELab system to measure lightness L*, red-green component a*, yellow-blue component b*, chroma C*, and hue angle h*). Muscle pH and rigor mortis in the fillets were also measured at 0 and 24 hours post-mortem.

The objective was to determine which crowding method was less stressful and better preserved fillet quality, and how seasonal temperature influenced these results.

Which crowding method is better and how does temperature influence it?

The study revealed significant differences due to both the crowding method and the time of year.

Impact on plasma stress indicators

• Cortisol, K, and lactate: Plasma levels of cortisol, K, and lactate were consistently lower in fish subjected to the WL (water level reduction) method compared to the LC (lateral compression) method. This suggests a lower stress response with the WL method.
• Influence of temperature: Temperature was a crucial factor. In general, fish showed better results (less stress) in winter compared to summer, regardless of the crowding method. For example, K levels were lower in winter, while lactate levels were higher in winter, but glucose showed complex interactions. NEFA and LDH levels were lower in winter, and TGC levels were higher, indicating lower metabolic demand in colder temperatures.
• Complex interactions: Interactions between the method and the season were observed. For example, cortisol was lower with WL in winter, but low cortisol levels were also seen with LC in summer, which the authors suggest could indicate chronic thermal stress exacerbated by this crowding method, leading to an attenuated or exhausted cortisol response. Glucose was higher with LC in winter, coinciding with elevated cortisol in this group.

Effects on color (skin, liver, and fillet)

Color is an important quality attribute.

• Skin color: The WL method resulted in skin with a higher red component (a*) and lower values of yellow (b*), chroma (C*), and hue (h*), which, along with plasma data, suggests a lower stress response. In summer, fish showed higher b* and C* values, indicating a possible greater stress response due to temperature. LC crowding in summer showed an increase in C*, which could reflect cumulative stress.
• Liver color: The WL method was also associated with lower a* and C* values in the liver, indicating less stress. Liver color also varied with the season, with higher L* (lightness) and b* values in winter.
• Fillet color: The WL method in winter showed the lowest a* and C* values at 0 hours post-mortem. The hue (h*) of the fillet was lower (more desirable, towards red-orange tones) with the WL method. At 24 hours, summer fish showed higher a* values, possibly due to oxidative stress from high temperatures. The LC method in summer tended to show higher b* and C* values in the fillet at 24 hours, which could be a compensatory response rather than a real quality improvement. In general, the WL method and winter conditions favored a more stable and potentially better-perceived fillet color.

Muscle pH and rigor mortis

These parameters are crucial for fillet texture and shelf life.

• Muscle pH: Muscle pH at 0 hours post-mortem was higher (better) in the winter WL group compared to the winter LC group. In summer, initial pH values were generally lower, indicating greater stress. At 24 hours post-mortem, pH was lower in the WL group compared to LC. A less pronounced drop in pH between 0 and 24 hours, as observed in the summer groups, can be indicative of high pre-slaughter stress. The WL method showed a more significant pH drop between 0 and 24 hours post-mortem, which is a desirable characteristic.
• Rigor mortis: Rigor mortis at 0 hours was mainly influenced by the season, being lower in summer, which could imply a faster onset of rigor due to stress. No significant differences were found due to the crowding method in rigor mortis at 0 or 24 hours.

How does this help the trout farmer?

The findings of this study have direct practical implications for rainbow trout producers:

• Choice of crowding method: The water level reduction (WL) method consistently proved superior to lateral compression (LC) in of minimizing stress response and maintaining better fillet quality. This was reflected in lower levels of plasma stress metabolites, more favorable skin and liver color, and better fillet quality parameters such as pH and color. The authors suggest that WL allows for more natural fish movement, whereas LC might induce abnormal clustering near the surface to access more oxygen, increasing stress.
• Consideration of water temperature (Seasonality): Pre-slaughter management during the summer, with higher temperatures, results in greater physiological stress and a potential decrease in product quality, regardless of the crowding method used. Therefore, intense crowding processes during summer months or in warm water conditions are discouraged to avoid chronic stress situations.
• Animal welfare and product quality go hand in hand: The study reinforces the idea that practices improving animal welfare also tend to improve final product quality. Minimizing pre-slaughter stress is not just an ethical issue, but also an economic one.

Although the WL method is recommended, the authors acknowledge that its applicability may depend on the production system. However, when feasible, it is the preferable option. If lateral compression must be used, it is advisable to avoid it during periods of high temperatures.

Conclusion

The study underscores the critical importance of optimizing the crowding method during rainbow trout pre-harvest to minimize stress responses and maintain fillet quality. The research clearly demonstrates that the water level reduction (WL) method is more effective than lateral compression (LC) in achieving these goals. Furthermore, winter conditions (colder temperatures) resulted in less critical values compared to summer for all parameters evaluated.

For fish farmers, the recommendation is clear: whenever possible, crowding methods that reduce water depth (WL) should be chosen, and special caution should be exercised when performing these operations during periods of high temperatures to preserve both the welfare of the trout and the superior quality of the product that reaches the consumer.

Álvaro De la Llave-Propín
Department of Agricultural Production, Universidad Politécnica de Madrid
Complutense Avenue 3, 28040, Madrid, Spain.
Email: [email protected]

Reference (open access)
Villalba, A. M., Bermejo-Poza, R., Villarroel, M., Pérez, C., Cabezas, A., Garoz, R. G., Fernández-Muela Garrote, M., & Díaz, M. T. (2025). Effect of pre-slaughter crowding on rainbow trout welfare and product quality. Journal of the World Aquaculture Society, 56(3), e70025. https://doi.org/10.1111/jwas.70025