Swimming behavior

Normal swimming behavior reflects good physiological condition and the ability of fish to cope with their environment. Disease, injury, or deformity can severely affect swimming behavior due to physiological changes the fish may be struggling to adapt to.

Level 0	Level 1	Level 2	Level 3
Normal, active swimming structure with appropriate cage use	Mostly normal swimming structure with minor variations	Clear disruption of normal swimming structure	Severe disruption of normal swimming structure

Protocol application

Use a consistent method for observing and assessing behavior to ensure data are standardized, including the location and time of observation. An example protocol could be: observe behavior for five minutes before feeding, five minutes during feeding, and then five minutes at another time in the day. For consistency, conduct observations at the same time each day.

Swimming behavior is assessed based on the visible portion of the population, acknowledging that using camera systems or surface observation will be unable to capture the behavior of all individuals in a given enclosure.

Using non-intrusive methods is preferred (e.g., underwater camera) as this avoids altering fish behavior through human presence.

When variations from normal swimming are observed, they should be documented in notes.

When in doubt, assess conservatively and use the more severe threshold level.

Scoring notes

When assessing swimming behavior, focus on organization and schooling rather than specific geometric patterns.
Activity level changes are more welfare-relevant than specific swimming patterns.
- Normal variation is expected with feeding and changing environmental conditions.
- Abnormal hyperactivity may indicate a chronic stressor, poor water quality, or disease.
- Lethargy may also indicate disease, severe stress, or poor nutrition.
Level 0: Normal active swimming structure includes organized schooling and responsiveness to stimuli.
Level 1: Minor variations from normal swimming structure include unusual avoidance of enclosure areas, some disruption to schooling, and occasional increased activity.
Level 2: Clear disruption to normal swimming structure includes fish clustering at the bottom or in the corners of the enclosure, disorganized movement, and episodes of hyperactivity or lethargy.
Level 3: Severe disruption to normal swimming structure includes fish avoiding open or feeding areas, constant bottom-dwelling, no schooling structure, extreme lethargy, and frantic swimming.
Good use of the enclosure space indicates comfort and environmental adequacy.
- Use of the whole water column indicates comfort throughout the cage.
- Bottom-dwelling may indicate low oxygen, poor water quality, or disease.
- Corner-clustering may indicate water flow problems, aggression, or fear response.
To facilitate consistency between observers, it is recommended to conduct periodic calibration sessions (scoring the same cage simultaneously and then discussing any points of disagreement).
Validation: The thresholds are largely based on expert opinion.

Key evidence

Source	Summary of source	Key result
Noble et al., 2020	Literature review on many rainbow trout welfare indicators, including swimming behavior.	Unstructured swimming at the bottom of the tank indicates acute stress, deviating from the normal circular schooling pattern typically seen in healthy fish.
Oppedal et al., 2011	Review article on the behaviors of Atlantic salmon.	Salmonids typically form a circular swimming pattern during the day and avoid the innermost part of the cage, the corners, and the bottom (though this structure breaks down during feeding). Fish that remain at the bottom of the tank may be experiencing environmental stress, such as low dissolved oxygen. It is normal for swimming speed to be reduced at night. A lack of reduced swimming speed at night may indicate disrupted circadian rhythms due to artificial lighting.

Supporting evidence

Arechavala-Lopez, P., J. C. Caballero-Froilán, M. Jiménez-García, X. Capó, S. Tejada, J. L. Saraiva, A. Sureda, and D. Moranta. “Enriched Environments Enhance Cognition, Exploratory Behaviour and Brain Physiological Functions of Sparus Aurata.” Scientific Reports 10, no. 1 (July 9, 2020). https://doi.org/10.1038/s41598-020-68306-6.
Arechavala-López, Pablo. “A Guide on Fish Welfare in Spanish Aquaculture - Volume 3: Welfare of Gilthead Sea Bream.” Spanish Aquaculture Business Association, 2024. https://drive.google.com/file/d/1Xp7Z-H53U_ZRmLf-cTFjCvs3dnHA-vOg/view.
García Palencia, H. “Monitorización Automatizada Del Comportamiento de Lubinas En Situaciones de Estrés Mediante Técnicas de Deep Learning.” Grado en Ingeniería Electrónica de Comunicaciones, Universidad Politécnica de Madrid, 2025. https://oa.upm.es/89484/1/TFG_GARCIA_PALENCIA_HECTOR.pdf.
Herbert, N. A., and J. F. Steffensen. “The Response of Atlantic Cod, Gadus Morhua, to Progressive Hypoxia: Fish Swimming Speed and Physiological Stress.” Marine Biology 147, no. 6 (April 2005): 1403–12. https://doi.org/10.1007/s00227-005-0003-8.
Hofer, R., and E. Gatumu. “Necrosis of Trout Retina (Oncorhynchus mykiss) after Sublethal Exposure to Nitrite.” Archives of Environmental Contamination and Toxicology 26, no. 1 (January 1994): 119–23. https://doi.org/10.1007/bf00212803.
Latremouille, David N. “Fin Erosion in Aquaculture and Natural Environments.” Reviews in Fisheries Science 11, no. 4 (October 1, 2003): 315–35. https://doi.org/10.1080/10641260390255745.
Martins, Catarina I. M., Leonor Galhardo, Chris Noble, Børge Damsgård, Maria T. Spedicato, Walter Zupa, Marilyn Beauchaud, et al. “Behavioural Indicators of Welfare in Farmed Fish.” Fish Physiology and Biochemistry 38, no. 1 (July 28, 2011): 17–41. https://doi.org/10.1007/s10695-011-9518-8.
Munday, B. L., C. K. Foster, F. R. Roubal, and R. J. G. Lester. “Paramoebic Gill Infection and Associated Pathology of Atlantic Salmon, Salmo salar and Rainbow Trout, Salmo Gairdneri in Tasmania.” In Pathology in Marine Science. Proceedings of the Third International Colloquium on Pathology in Marine Aquaculture, 1990.
Noble, C., K. Gismervik, M. H. Iversen, J. Kolarevic, J. Nilsson, L. H. Stien, and J. F. Turnbull. 2020. “Welfare Indicators for Farmed Rainbow Trout: Tools for Assessing Fish Welfare.” Nofima. https://nofima.no/wp-content/uploads/2020/05/Welfare-Indicators-for-farmed-rainbow-trout-Noble-et-al.-2020.pdf.
Oppedal, Frode, Tim Dempster, and Lars H. Stien. “Environmental Drivers of Atlantic Salmon Behaviour in Sea-Cages: A Review.” Aquaculture 311, no. 1-4 (February 2011): 1–18. https://doi.org/10.1016/j.aquaculture.2010.11.020.
Özcan, Filiz. “Prevalence and Causes of Skeletal Deformities in Cultured Juvenile Oncorhynchus mykiss. Skeletal Deformities in Cultured Juvenile Oncorhynchus mykiss.” Revista Colombiana de Ciencias Pecuarias 38 (2025). https://doi.org/10.17533/udea.rccp.e359311.
Papandroulakis, Nikos, Konstandia Lika, Tore S. Kristiansen, Frode Oppedal, Pascal Divanach, and Michael Pavlidis. “Behaviour of European Sea Bass, Dicentrarchus Labrax L., in Cages - Impact of Early Life Rearing Conditions and Management.” Aquaculture Research 45, no. 9 (January 2013): 1545–58. https://doi.org/10.1111/are.12103.
Pettersen, Jostein M., Marc B.M. Bracke, Paul J. Midtlyng, Ole Folkedal, Lars H. Stien, Håvard Steffenak, and Tore S. Kristiansen. “Salmon Welfare Index Model 2.0: An Extended Model for Overall Welfare Assessment of Caged Atlantic Salmon, Based on a Review of Selected Welfare Indicators and Intended for Fish Health Professionals.” Reviews in Aquaculture 6, no. 3 (June 30, 2013): 162–79. https://doi.org/10.1111/raq.12039.
Powell, M. D., M. A. Jones, and M. Lijalad. “Effects of Skeletal Deformities on Swimming Performance and Recovery from Exhaustive Exercise in Triploid Atlantic Salmon.” Diseases of Aquatic Organisms 85 (2009): 59–66. https://doi.org/10.3354/dao02056.
Remen, M., M. A. J. Nederlof, O. Folkedal, G. Thorsheim, A. Sitjà-Bobadilla, J. Pérez-Sánchez, F. Oppedal, and R. E. Olsen. “Effect of Temperature on the Metabolism, Behaviour and Oxygen Requirements of Sparus Aurata.” Aquaculture Environment Interactions 7, no. 2 (2015): 115–23. https://doi.org/10.3354/aei00141.
Wagner, Glenn N., Mark D. Fast, and Stewart C. Johnson. “Physiology and Immunology of Lepeophtheirus Salmonis Infections of Salmonids.” Trends in Parasitology 24, no. 4 (April 2008): 176–83. https://doi.org/10.1016/j.pt.2007.12.010.