Induced swimming: an effective tool for reducing stress and enhancing immunity in rainbow trout

In aquaculture, optimizing fish welfare is not only an ethical matter but also a crucial factor for improving production performance and disease resistance. A recent study by researchers from the University of Porto, the University of Murcia, and The New Zealand Institute for Plant and Food Research Limited explores how an activity as natural as swimming, when induced in a controlled manner, can become a powerful tool to modulate stress, immunity, and the antioxidant state in juvenile rainbow trout (Oncorhynchus mykiss). The findings suggest that short-term “training” could better prepare fish to face the inevitable stressful handling in farming systems.

Key findings

• Induced swimming, whether at low, high, or oscillating speeds, significantly reduced plasma cortisol levels, indicating a decrease in stress compared to fish at rest.
• Exercise at constant speeds (low and high) improved skin mucosal immunity by increasing the activity of key enzymes like lysozyme and peroxidase, the fish’s first line of defense.
• Swimming at oscillating speeds led to a greater use of anaerobic metabolism, reflected in higher lactate levels, and increased oxidative stress in the liver.
• Exercise at constant speeds enhanced the antioxidant status in both red and white muscle, suggesting a greater capacity to combat cellular damage.

How were the effects of exercise assessed?

To understand the physiological response of rainbow trout to exercise, researchers designed a 6-hour experiment. They used swim tunnels where fish were individually subjected to four different conditions:

• Control (C): Fish kept in a minimal water flow, simulating resting conditions.
• Low Speed (L): Constant swimming at a speed of 0.8 body lengths per second (BLs⁻¹).
• High Speed (H): Constant swimming at a speed of 2.3 BLs⁻¹.
• Oscillating Speed (O): A variable swimming regime that fluctuated between low and high speeds (0.8 and 2.3 BLs⁻¹).

At the end of the exercise period, samples of plasma, skin mucus, muscle, and other organs were collected to analyze a wide range of indicators related to stress, metabolism, and the immune response.

Swimming to reduce stress: A notable drop in cortisol

One of the most compelling findings of the study was the response of cortisol, the primary stress hormone in fish. The results showed that all groups induced to swim (L, H, and O) had significantly lower plasma cortisol levels than the control group.

This discovery is particularly relevant. In farming conditions, confinement or suboptimal water flow can be stressful for active species like rainbow trout. The control group fish, being confined in the tunnels without the ability to swim actively, showed cortisol levels consistent with confinement stress reported in other studies. In contrast, swimming activity appears to attenuate this neuroendocrine stress response, reinforcing the idea that moderate exercise is beneficial for the species’ welfare.

A strengthened first line of defense: Mucosal immunity

The mucus covering a fish’s skin is its first line of defense against pathogens. The study revealed that induced swimming strengthened this protective barrier in a localized manner. Specifically:

• Lysozyme Activity: Increased in the mucus of fish from the low (L) and high (H) speed groups. Lysozyme is an enzyme with potent antibacterial activity.
• Peroxidase Activity: Was higher in the mucus of all swimming groups (L, H, and O) compared to the control group. Peroxidase is involved in oxidative responses against pathogens.

Interestingly, no significant changes were observed in the immune parameters measured in the plasma (the liquid part of the blood) or in the expression of pro-inflammatory genes in organs such as the head kidney, gills, or heart. This suggests that the exercise, under the applied conditions, generates a localized and controlled immunological activation on the fish’s surface without triggering a systemic inflammatory response that could be energetically costly.

Metabolism in action: How muscles utilize energy

Exercise, of course, demands energy. The study analyzed how the red and white muscles of the trout responded:

• Red Muscle (aerobic): In all swimming groups (L, H, and O), the activity of the enzyme citrate synthase (CS) was higher than in the control, indicating an increase in aerobic metabolism to sustain swimming. Furthermore, in the high-speed (H) and oscillating (O) groups, the enzyme cytochrome c oxidase (COX) also increased its activity, reflecting a greater mitochondrial oxidative capacity to meet high-energy demands.
• Anaerobic Metabolism: The oscillating swimming group (O) was the only one to show an increase in the enzyme lactate dehydrogenase (LDH) in red muscle and higher lactate levels in plasma. This indicates a greater reliance on anaerobic metabolism, likely due to the extra effort required for constant acceleration and deceleration.
• White Muscle (anaerobic): No significant changes were detected in the metabolic enzymes of this muscle, suggesting its involvement was limited under the sustained swimming conditions of the experiment.

The delicate balance of oxidative stress

The increase in metabolism during exercise can generate reactive oxygen species (ROS), causing oxidative stress. However, the body also activates antioxidant defenses to counteract this.

The study found that swimming at constant speeds (L and H) improved the antioxidant status in both red and white muscle. This was evidenced by an increase in the GSH/GSSG ratio, a key indicator of a healthy redox balance and a greater capacity to neutralize ROS.

However, the oscillating exercise (O) showed a different side. In this group, higher levels of lipid peroxidation (LPO) were observed in the liver, a sign of oxidative damage. This result suggests that while swimming is beneficial, regimes involving abrupt and frequent speed changes may impose a greater metabolic and oxidative challenge, at least at the hepatic level.

Practical implications for trout aquaculture

The results of this research demonstrate that short-term induced swimming, especially at constant speeds, can be a non-invasive and effective strategy to improve the welfare of rainbow trout.

Applying controlled periods of exercise before routine and stressful handling, such as transport, grading, or vaccinations, could “pre-condition” the fish. By reducing cortisol levels and strengthening the skin’s immune barrier, fish could be better prepared to face these challenges, reducing their susceptibility to diseases and improving their recovery capacity.

However, it is crucial to fine-tune the exercise conditions. While constant swimming appears to offer a positive balance, oscillating regimes could generate undesirable oxidative stress. Future research, applied to groups of fish and for longer periods, will be necessary to optimize these protocols and facilitate their implementation at a commercial scale.

Contact
Rodrigo O. A. Ozório
CIIMAR/CIMAR-LA, Interdisciplinary Centre of Marine and Environmental Research, University of Porto
Matosinhos, 4450-208, Portugal
Email: rodrigo.ozorio@ciimar.up.pt

Reference (open access)
Espírito-Santo, C., Alburquerque, C., Cavalheri, T. et al. Induced swimming reduced stress and modulated immune response and antioxidant status in juvenile rainbow trout (Oncorhynchus mykiss). Fish Physiol Biochem 51, 151 (2025). https://doi.org/10.1007/s10695-025-01569-w

Milthon Lujan

Editor at the digital magazine AquaHoy. He holds a degree in Aquaculture Biology from the National University of Santa (UNS) and a Master’s degree in Science and Innovation Management from the Polytechnic University of Valencia, with postgraduate diplomas in Business Innovation and Innovation Management. He possesses extensive experience in the aquaculture and fisheries sector, having led the Fisheries Innovation Unit of the National Program for Innovation in Fisheries and Aquaculture (PNIPA). He has served as a senior consultant in technology watch, an innovation project formulator and advisor, and a lecturer at UNS. He is a member of the Peruvian College of Biologists and was recognized by the World Aquaculture Society (WAS) in 2016 for his contribution to aquaculture.