Aggressive behavior

Aggressive behavior has been defined as “behavior that actually or potentially causes harm to another animal,” excluding killing other animals for food or attacking a potential predator.²⁹ Aggressive behaviors, including nipping and increased chasing, can be evidenced by physical injuries such as bite marks. Aggression can lead to the formation of a hierarchy, in which subordinate fish are at higher risk of stress, e.g., due to an inability to compete for food. Aggressive behavior can also indicate insufficient resources or space.

Aggressive behavior during feeding time

Level 0	Level 1	Level 2	Level 3
No aggressive behavior OR Normal aggressive behavior with no harmful outcomes	Moderate aggressive behavior with minimal harm	Elevated aggressive behavior with visible harm	Severe aggressive behavior causing significant harm OR Fish are visibly excluded from the feeding area

Aggressive behavior outside feeding

Level 0	Level 1	Level 2	Level 3
No aggressive behavior OR Rare isolated aggressive interactions with no harmful outcomes only	< 5% of fish involved in aggressive interactions OR Any prevalence of aggressive interactions causing minor harm only (fin nipping, brief chasing)	5–20% of fish involved in aggressive interactions OR Any prevalence of aggressive interactions causing visible harm (bite marks, moderate fin damage)	> 20% of fish involved in aggressive interactions OR Any prevalence causing severe harm (open wounds, severe fin damage) OR Clear spatial exclusion or subordination tendencies

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. Feeding behavior should also be scored during this time. For consistency, conduct observations at the same time each day.

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.

Count and note aggressive acts during the observation period, then use a combination of prevalence and severity to determine the score.

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

Scoring notes

Normal aggressive behavior can involve brief chasing, establishment of a feeding hierarchy, threat postures, brief confrontations, or positioning disputes during feeding. These behaviors typically do not result in physical harm.
Moderate aggressive behavior can involve minor fin nipping, extensive chasing, or positioning disputes outside of feeding.
Elevated aggressive behavior can involve acts occurring more frequently and resulting in visible harm (such as bite marks).
Severe aggressive behavior can involve persistent harassment, causing severe harm such as open wounds or severe fin damage.
Prevalence outside feeding: Some aggressive behavior during feeding is normal (hierarchy establishment, competition for food). Aggressive behavior outside of feeding time is a stronger indicator of welfare problems, and the prevalence thresholds in the non-feeding subindicator reflect this: < 5% is likely individual personalities with some potential for emerging issues, 5–20% indicates systemic problems (environmental stressors, resource competition), and > 20% represents a population-level welfare problem requiring immediate intervention.
Visible harms resulting from aggressive behavior include wounds or tissue punctures from biting, fin damage as a result of nipping, and chasing wounds (e.g., scrapes or scale loss).
Aggressive behavior during feeding should be interpreted alongside the feeding behavior indicator — where aggression is elevated during feeding, check whether subordinate fish are being excluded from feeding areas and whether feed waste is elevated.
When aggressive behavior is observed, note the specific behaviors observed to provide more context.
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 based on expert opinion.

Key evidence

Source	Summary of source	Key result
Noble et al., 2007	Experimental study investigating the effects of different self-feeding regimes on growth, behavior, and fin damage in 100 juvenile rainbow trout (90 g) in freshwater laboratory conditions over 48 days.	Fish were provided with either a single 2-hour access period to self-feeders, three 2-hour access periods to self-feeders, or unlimited access to self-feeders. The authors found a significant increase in aggressive behavior in the “one access period” group.
Noble et al., 2020	Literature review on many rainbow trout welfare indicators, including aggressive behavior.	The review highlighted aggression during certain life stages or handling routines as a sign of social stress. Increased current speed can reduce aggression in salmonids, as fish expend more energy maintaining position.

Supporting evidence

Batzina, Alkisti, Christina Dalla, Zeta Papadopoulou-Daifoti, and Nafsika Karakatsouli. “Effects of Environmental Enrichment on Growth, Aggressive Behaviour and Brain Monoamines of Gilthead Seabream Sparus Aurata Reared under Different Social Conditions.” Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology 169 (March 2014): 25–32. https://doi.org/10.1016/j.cbpa.2013.12.001.
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.
Montero, D., G. Lalumera, M. S. Izquierdo, M. J. Caballero, M. Saroglia, and L. Tort. “Establishment of Dominance Relationships in Gilthead Sea Bream Sparus Aurata Juveniles during Feeding: Effects on Feeding Behaviour, Feed Utilization and Fish Health.” Journal of Fish Biology 74, no. 4 (March 9, 2009): 790–805. https://doi.org/10.1111/j.1095-8649.2008.02161.x.
Noble, C., K. Mizusawa, K. Suzuki, and M. Tabata. “The Effect of Differing Self-Feeding Regimes on the Growth, Behaviour and Fin Damage of Rainbow Trout Held in Groups.” Aquaculture 264, no. 1-4 (April 6, 2007): 214–22. https://doi.org/10.1016/j.aquaculture.2006.12.028.
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.
Peters, G., Mohamed Faisal, Tom Lang, and Iman Ahmed. “Stress Caused by Social Interaction and Its Effect on Susceptibility to Aeromonas Hydrophila Infection in Rainbow Trout Salmo Gairdneri.” Diseases of Aquatic Organisms 4 (May 31, 1988): 83–89. https://doi.org/10.3354/dao004083.
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.
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.

Damsgård, Børge, and Felicity Huntingford. “Fighting and Aggression.” In Aquaculture and Behavior, 248–85. Blackwell Publishing Ltd, 2012.