Aberrant behavior

Aberrant behavior includes fish that are unusually lethargic or frantic, swimming deep or on their side, body rocking, surface gasping, burrowing, or clumping. This includes stereotypic behavior: repetitive behavior that lacks a clear purpose, often observed in captive animals with limited ability to engage in natural behaviors. It is generally accepted that these types of behaviors are a response to a poor environment. These behaviors may also indicate the influence of stressors, disease, or deformities.

Producers should understand normal swimming behaviors in their production to better identify abnormal behaviors.

Minor aberrant behavior

Level 0	Level 1	Level 2	Level 3
No minor aberrant behaviors	Low prevalence (< 2%) of minor aberrant behaviors	Noticeable prevalence (2–5%) of minor aberrant behaviors	High prevalence (> 5%) of minor aberrant behaviors

Major aberrant behavior

Level 0	Level 1	Level 2	Level 3
No major aberrant behaviors	Isolated individuals (< 0.1%) expressing major aberrant behaviors	Low prevalence (0.1–2%) of major aberrant behaviors	Noticeable prevalence (> 2%) of major aberrant behaviors

Overall: Overall score is determined by the most severe subindicator

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

Prevalence percentage estimates are 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.

Scoring notes

Fish can behave aberrantly in many ways, and not all aberrant behaviors indicate the same severity of welfare issues. For example:
- Minor aberrant behaviors can include:
  - Slight lethargy (reduced activity, but activity is still present).
  - Minor positioning changes (slightly deeper or higher than usual).
  - Reduced schooling participation.
  - Slightly increased ventilatory activity.
  - Some increased thigmotaxis.
- Major aberrant behaviors can include:
  - Surface gasping (indicating respiratory distress).
  - Body rocking or stereotypies.
  - Side swimming.
  - Severe lethargy (minimal movement, poor responsiveness).
  - Frantic swimming.
  - Loss of balance or equilibrium.
  - Erratic movements.
  - Significantly increased ventilatory activity.
  - Significantly increased thigmotaxis.
What behaviors are considered aberrant also depends on the baseline trends in the production system.
Acute and chronic aberrant behavior have different welfare implications. Acute behavior is linked to a specific stressor (e.g., a handling event) over a short period. Chronic behavior persists over weeks, indicating a systemic welfare problem.
- Where aberrant behaviors can be linked to an acute stressor, note this for future reference.
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 prevalence thresholds are derived from Pettersen et al. (2013) with a higher threshold applied to minor aberrant behaviors as the study did not distinguish between minor and major aberrant behaviors. The distinction between minor and major aberrant behaviors is based on expert opinion and should be adapted to the baseline behavior patterns of each production system.

Key evidence

Source	Summary of source	Key result
Davidson et al., 2011	Two experimental studies investigating the impacts of different freshwater RAS conditions on rainbow trout health and welfare. Experiment 1 compared very low water exchange and high water exchange in rainbow trout (151 g) over six months. Experiment 2 compared low water exchange to near-zero water exchange in rainbow trout (18 g).	In both studies, trout in lower-exchange systems swam significantly faster than those in higher-exchange systems. In study 1, high-exchange fish swam at approximately 0.7 BL/s, and low-exchange fish swam at approximately 1.4 BL/s. In study 2, the very low-exchange trout swam at approximately 2.1 BL/s, and the near-zero exchange trout swam at approximately 3.4 BL/s. The authors hypothesized that this was a physiological flight response caused by the chronically stressful water quality conditions. Approximately 4 times more side swimming was observed in the low-exchange system in study 1. The authors hypothesized this could be due to musculature imbalance, skeletal deformities, or swim bladder deviation. In study 2, the near-zero exchange group displayed more abnormal behaviors, including erratic swimming, swimming at oblique angles, surface swimming, and yawning/gulping. These behaviors became more severe over time. In the near-zero system, the prevalence of skeletal deformity reached up to 38%. The authors hypothesized that abnormal swimming may contribute to more deformities. Mortality was also higher in this group.
Sneddon, 2003	Experimental study investigating behavioral and physiological responses consistent with pain perception in 25 rainbow trout (~61 g) in freshwater laboratory conditions. There were 5 treatment groups: control (only handled), saline-injected, acid-injected, acid- and morphine-injected, and morphine-injected. The injection was in the trout’s lips.	Acid-injected fish rested on the substrate and rocked from side to side on their pectoral fins. The authors interpreted this as possible stereotypic or comfort-seeking behavior. Fish also rubbed their lips against gravel or tank walls, which the authors interpreted as an attempt to relieve pain. These behaviors were nearly absent in controls and were significantly reduced by morphine.
Pettersen et al., 2013	Review paper presenting a welfare index to assess the welfare of Atlantic salmon in sea cages. Behavior was included as an indicator.	The authors concluded that aberrant behavior was a sign of severe welfare issues. They suggested that 0% was ideal, < 0.1% acceptable, 0.1–2% poor, and > 2% critical for welfare. They categorized evidence of aggression as potentially serious and similar in severity to abnormal behavior. They suggested that surface gasping and lethargy could indicate severe gill disease.

Supporting evidence

Latremouille, David N. “Fin Erosion in Aquaculture and Natural Environments.” Reviews in Fisheries Science 11, no. 4 (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.
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 (March 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.
Sneddon, Lynne U. “The Evidence for Pain in Fish: The Use of Morphine as an Analgesic.” Applied Animal Behaviour Science 83, no. 2 (September 2003): 153–62. https://doi.org/10.1016/s0168-1591(03)00113-8.
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.