Feeding behavior

Active feeding with minimal waste can indicate a healthy population with appropriate feed management. Feed motivation can be reduced by stressors, which can lead to reduced feed intake and growth.30 Reduced feeding can also indicate disease or poor water quality.31 Fish often have a decreased appetite after experiencing handling procedures such as crowding, pumping, or transport. Uneaten feed can also contribute to deteriorating water quality. The amount of time taken to recover to normal feeding behaviors after such an event is also a useful indicator of welfare.32

Producers should understand normal feeding behaviors in their production to better identify and score feeding behavior.

Feeding behavior

Level 0 Level 1 Level 2 Level 3

Active anticipatory behavior before feeding
OR
Active behavior during feeding

Some anticipatory behavior before feeding
OR
Mostly active behavior during feeding

Reduced anticipatory behavior before feeding
OR
Slow or reluctant feeding

Absent anticipatory behavior before feeding
OR
Minimal activity during feeding

Feed waste

Level 0 Level 1 Level 2 Level 3

Feed is consumed efficiently, with < 2% feed waste within the normal feeding period

Most feed is consumed, with 2–5% of feed uneaten

5–10% of the feed remains uneaten

> 10% of the feed remains uneaten

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. Aggressive 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.

For consistency in measuring feed percentage, measure at the same time each day (e.g., 30 minutes after feeding). Calculate the waste percentage by dividing uneaten feed by total feed offered. Feed waste can be estimated using visual observation (e.g., with a camera or from the surface) or collected from waste feed collectors.

Two scoring systems are provided. It is recommended to score using both and then take the worst score. When in doubt, assess conservatively and use the more severe threshold level.

Scoring notes
  • Anticipatory behavior before feeding includes behaviors such as:
    • Increased swimming activity in the minutes before feeding.
    • Movement towards the feeding area when staff approach.
    • Schooling behavior near the expected feeding location.
  • Active anticipatory behavior means a clear increase in the above activities before feeding; some anticipatory behavior means some increase in activity; reduced anticipatory behavior means minimal behavioral change; and absent anticipatory behavior means no response.
  • A lack of anticipatory behavior should not necessarily be interpreted as low appetite if fish otherwise feed well.
  • Feed waste percentage is a more objective, evidence-based measure than anticipatory behavior. However, how applicable the feed waste score is depends on whether the amount of feed provided is appropriate.
    • Feed waste may not always be measurable, particularly in sea cages. In these cases, rely on the feeding behavior score.
  • Feeding behavior may be impacted by one-off events such as handling and weather events, which the assessor should also note. Daily appetite can also vary naturally, for example based on how much was eaten the previous day. In these cases, daily feeding behavior scores should be interpreted cautiously.
  • Persistent level 2 or level 3 feeding behavior may indicate a chronic welfare problem requiring investigation of disease, water quality, or chronic stressors.
  • Reduced feeding behavior may result from aggressive exclusion — see also the aggressive behavior indicator.
  • 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 behavioral thresholds are based on expert opinion, while the feed-waste percentage thresholds are based on Øverli et al. (2006) and informed by expert opinion.
Key evidence
Source Summary of source Key result
Øverli et al., 2006 Experimental study investigating how two selectively bred lines of 300 rainbow trout (bred to be either highly responsive or lowly responsive to stress) (456–493 g) relate to feed waste in laboratory conditions over 12 days.

Fish were hand-fed once daily over 20 minutes with graduated feeding regimes: 0.5% body mass/day (days 1–3), 1.0% body mass/day (days 4–6), and 2.0% body mass/day (days 7–12). Uneaten pellets were collected from PVC grid pellet traps 15 minutes after feeding ended.		 Fish from the highly stress-responsive line wasted approximately 11% of feed, while fish from the low-stress responsive line wasted approximately 4% of feed.

The difference in feed waste increased progressively as rations increased. The highly-responsive fish also demonstrated slower growth rates and poorer feed conversion efficiency. There was also a positive correlation between initial size variation within tanks and total feed waste.
McKay and Gjerde, 1985 Experimental study investigating the effects of different salinities on growth in 90 rainbow trout (50–150 g) over 12 weeks. Fish had been reared in freshwater and then transferred to salinities of 0, 10, 20, 24, 28, and 32 ppt. Appetite was significantly better at 0 ppt than at higher salinities. At 10 ppt, fish were leaving 39.5% of feed uneaten. At 20 ppt, fish were leaving 36.8% of feed uneaten, and at 32 ppt, fish were leaving nearly half of feed uneaten. Mortality increased with increasing salinity, from 0% at 0 and 10 ppt, but reaching 13% at 32 ppt.
Noble et al., 2020 Literature review on many rainbow trout welfare indicators, including feeding behavior. A poor or absent feed response can indicate disease or stress. While increased swimming speed before or at the start of feeding can indicate positive anticipation, persistence during meals or over several days may reflect competition or resource scarcity, signaling reduced welfare.
Supporting evidence
  • Andrew, J. E., J. Holm, S. Kadri, and F. A. Huntingford. “The Effect of Competition on the Feeding Efficiency and Feed Handling Behaviour in Gilthead Sea Bream (Sparus Aurata L.) Held in Tanks.” Aquaculture 232, no. 1-4 (April 5, 2004): 317–31. https://doi.org/10.1016/s0044-8486(03)00528-3.
  • 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.
  • Chandroo, Kris P., Steven J. Cooke, R. Scott McKinley, and Richard D. Moccia. “Use of Electromyogram Telemetry to Assess the Behavioural and Energetic Responses of Rainbow Trout, Oncorhynchus mykiss (Walbaum) to Transportation Stress.” Aquaculture Research 36, no. 12 (2005): 1226–38. https://doi.org/10.1111/j.1365-2109.2005.01347.x.
  • Gregory, T. Ryan, and Chris M. Wood. “The Effects of Chronic Plasma Cortisol Elevation on the Feeding Behaviour, Growth, Competitive Ability, and Swimming Performance of Juvenile Rainbow Trout.” Physiological and Biochemical Zoology 72, no. 3 (May 1999): 286–95. https://doi.org/10.1086/316673.
  • 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.
  • McKay, L. R., and B. Gjerde. “The Effect of Salinity on Growth of Rainbow Trout.” Aquaculture 49, no. 3-4 (November 1, 1985): 325–31. https://doi.org/10.1016/0044-8486(85)90089-4.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • Roque, Ana, M. Filipa Castanheira, Anna Toffan, Pablo Arechavala-Lopez, Edgar Brun, Morris Villarroel, Enric Gisbert, et al. “Report on Fish Welfare and List of Operational Welfare Indicators in Sea Bream. Deliverable 4.5 of the Horizon 2020 Project MedAID (GA Number 727315),” 2020. http://www.medaid-h2020.eu/index.php/deliverables/.
  • 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.

  1. 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.

  2. 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.

  3. 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.