Stocking density

Appropriate stocking density provides fish with sufficient space to swim, feed, and behave normally while maintaining good water quality. Stocking density, expressed as fish biomass per unit volume of water (kg/m³), is a complex indicator of welfare. The indicator involves multiple parameters that change over time as fish grow in a static volume. The search for a ‘golden’ stocking density has been a subject of much academic discussion, as it is not a good indicator of welfare per se. This is because the negative consequences of higher stocking densities can be mitigated through good management practices.33

However, across the literature, there is a clear trend: increasing stocking density results in poor welfare outcomes for rainbow trout (e.g., higher mortality and lower growth). Separating the direct impacts of high stocking density from associated issues, such as deteriorated water quality, can be very difficult. Further, some studies have found poor welfare outcomes at very low densities (e.g., at initial densities of 0.03 kg/m³ or 0.4 kg/m³).34 Producers should proceed with caution when considering very low density levels.

Level 0 Level 1 Level 2 Level 3
≤ 10 kg/m³
> 10 to ≤ 15 kg/m³
> 15 to ≤ 25 kg/m³
> 25 kg/m³
Protocol application

Calculate stocking density by dividing total fish weight (kg) by total water volume available to fish (m³). Recalculate density regularly (ideally weekly) using predicted fish growth data.

Scoring notes
  • At very low densities for commercial settings (e.g., < 5 kg/m³) and at level 2 and level 3, it is recommended to increase monitoring frequencies for other welfare indicators such as aggressive behavior, water quality impacts, and mortality.
  • When other important welfare indicators (such as dissolved oxygen, mortality, and aggressive behavior) score poorly, consider adjusting stocking density.
  • Validation: The prevalence thresholds are derived from considering the results in the studies listed below, as well as regulatory thresholds (the RSPCA Assured rainbow trout standard sets a limit of 15 kg/m³ over the site, and 17 kg/m³ in any one enclosure, and Norwegian regulation35 sets the maximum allowable stocking density for ongrowers in sea cages at 25 kg/m³). The level 0 threshold has been set at 10 kg/m³ as a precaution, however no papers cleanly test this exact density against higher densities.
Key evidence
Source Summary of source Key result
Zahedi et al., 2019 Experimental study investigating the effects of stocking density on health status in female ongrowing rainbow trout (405 g) in freshwater laboratory conditions over 60 days. Sampling was conducted every 20 days. Three different stocking densities were tested: 12, 24, and 44 kg/m³. One additional group was considered the ‘reduced density’ group, which moved from a density of 44 to 12 kg/m³ on day 40 for the final 20 days. To maintain the desired stocking density, the required number of fish was removed weekly. High density significantly reduced the final weight and condition factor. The reduced-density group did not have different growth parameters compared to the high-density group.

The low-density group had no mortality, whereas the medium- and high-density groups had mortality rates of 1.5% and 2.5%, respectively.

The gene expression of trout in the high-density group was indicative of chronic stress, with the low-density group exhibiting the lowest stress marker expression.
Liu et al., 2016 Experimental study investigating the effects of stocking density on health status in ongrowing rainbow trout (114 g) in laboratory conditions over 300 days.

Fish were initially housed in triplicate groups at three stocking densities: 4.6, 6.6, and 8.6 kg/m³. At the end of the experiment, densities reached 31.1, 40.6, and 49.3 kg/m³.

12 fish per density group were sampled at regular intervals.		 There were limited differences between the groups before 60 days, when all groups had densities below 16 kg/m³ (8.9, 12.7, and 15.9 kg/m³, respectively).

At day 120, the groups had densities of 12.0, 16.4, and 20.0 kg/m³, respectively. At this point, the highest-density group (20 kg/m³) showed a significantly higher cortisol response compared to the lowest-density group, though this difference was downregulated by day 300.

Low-density fish achieved significantly higher weights and lengths.

High-density groups also had significantly higher mortality (3.35% vs. 2.21% in low-density groups). High-density fish also showed elevated plasma levels, indicating chronic stress.
Aksakal et al., 2011 Experimental study investigating the effects of stocking density on stress and oxidative proteins on ongrowing rainbow trout (130 g) in laboratory conditions over 2 months.

Four different stocking densities were tested: 15, 20, 25, and 30 kg/m³. Consistent water quality parameters were maintained.		 The study demonstrated significantly elevated kidney G6PD activity and significantly depressed muscle G6PD activity at 20 kg/m³ and above, with HSP70 mRNA expression significantly increased at 25 kg/m³ and 30 kg/m³.

These changes indicate that increasing stocking densities beyond 15 kg/m³ induces additional stress in rainbow trout.
Supporting evidence
  • Aksakal, Ercüment, Deniz Ekinci, Orhan Erdoğan, Şükrü Beydemir, Zuhal Alım, and Saltuk Buğrahan Ceyhun. “Increasing Stocking Density Causes Inhibition of Metabolic–Antioxidant Enzymes and Elevates MRNA Levels of Heat Shock Protein 70 in Rainbow Trout.” Livestock Science 141, no. 1 (October 2011): 69–75. https://doi.org/10.1016/j.livsci.2011.07.006.
  • Alanärä, A., and E. Brännäs. “Dominance in Demand-Feeding Behaviour in Arctic Charr and Rainbow Trout: The Effect of Stocking Density.” Journal of Fish Biology 48, no. 2 (February 1996): 242–54. https://doi.org/10.1111/j.1095-8649.1996.tb01116.x.
  • Birolo, Marco, Francesco Bordignon, Angela Trocino, Luca Fasolato, Antón Pascual, Sergio Godoy, Carlo Nicoletto, Carmelo Maucieri, and Gerolamo Xiccato. “Effects of Stocking Density on the Growth and Flesh Quality of Rainbow Trout (Oncorhynchus mykiss) Reared in a Low-Tech Aquaponic System.” Aquaculture 529 (December 2020): 735653. https://doi.org/10.1016/j.aquaculture.2020.735653.
  • Cvetkovikj, Aleksandar, Miroslav Radeski, Dijana Blazhekovikj-Dimovska, Vasil Kostov, and Vangjel Stevanovski. “Factors Affecting Fin Damage of Farmed Rainbow Trout.” Macedonian Veterinary Review 38, no. 1 (November 24, 2014): 61–71. https://doi.org/10.14432/j.macvetrev.2014.11.032.
  • Ellis, T., B. North, A. P. Scott, N.R. Bromage, M. Porter, and D. Gadd. “The Relationships between Stocking Density and Welfare in Farmed Rainbow Trout.” Journal of Fish Biology 61, no. 3 (September 2002): 493–531. https://doi.org/10.1111/j.1095-8649.2002.tb00893.x.
  • 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.
  • Laursen, Danielle Caroline, Patricia I.M. Silva, Bodil K. Larsen, and Erik Höglund. “High Oxygen Consumption Rates and Scale Loss Indicate Elevated Aggressive Behaviour at Low Rearing Density, While Elevated Brain Serotonergic Activity Suggests Chronic Stress at High Rearing Densities in Farmed Rainbow Trout.” Physiology & Behavior 122 (October 2, 2013): 147–54. https://doi.org/10.1016/j.physbeh.2013.08.026.
  • Liu, Qun, Zhishuai Hou, Haishen Wen, Jifang Li, Feng He, Jinhuan Wang, Biao Guan, and Qinglong Wang. “Effect of Stocking Density on Water Quality and (Growth, Body Composition and Plasma Cortisol Content) Performance of Pen-Reared Rainbow Trout (Oncorhynchus mykiss).” Journal of Ocean University of China 15, no. 4 (May 23, 2016): 667–75. https://doi.org/10.1007/s11802-016-2956-2.
  • 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.
  • McKenzie, David J, Erik Höglund, A. Dupont-Prinet, Bodil Katrine Larsen, Peter Vilhelm Skov, Palle Pedersen, and Alfred Jokumsen. “Effects of Stocking Density and Sustained Aerobic Exercise on Growth, Energetics and Welfare of Rainbow Trout.” Aquaculture 338-341 (March 1, 2012): 216–22. https://doi.org/10.1016/j.aquaculture.2012.01.020.
  • Nahida, Rasheed, Manchi Rajesh, Prakash Sharma, Nityanand Pandey, Pramod Kumar Pandey, Arul Victor Suresh, Grace Angel, et al. “Stocking Density Affects Growth, Feed Utilisation, Metabolism, Welfare and Associated MRNA Transcripts in Liver and Muscle of Rainbow Trout More Pronouncedly than Dietary Fish Meal Inclusion Level.” Aquaculture 596 (February 15, 2025): 741717. https://doi.org/10.1016/j.aquaculture.2024.741717.
  • North, B. P., J. F. Turnbull, T. Ellis, M. J. Porter, H. Migaud, J. Bron, and N. R. Bromage. “The Impact of Stocking Density on the Welfare of Rainbow Trout (Oncorhynchus mykiss).” Aquaculture 255, no. 1-4 (May 2006): 466–79. https://doi.org/10.1016/j.aquaculture.2006.01.004.
  • Person-Le Ruyet, Jeannine, Laurent Labbé, Nicolas Le Bayon, Armelle Sévère, Annick Le Roux, Hervé Le Delliou, and Loïc Quéméner. “Combined Effects of Water Quality and Stocking Density on Welfare and Growth of Rainbow Trout (Oncorhynchus mykiss).” Aquatic Living Resources 21, no. 2 (April 1, 2008): 185–95. https://doi.org/10.1051/alr:2008024.
  • Pottinger, T. G., and A. D. Pickering. “The Influence of Social Interaction on the Acclimation of Rainbow Trout, Oncorhynchus mykiss (Walbaum) to Chronic Stress.” Journal of Fish Biology 41, no. 3 (September 1992): 435–47. https://doi.org/10.1111/j.1095-8649.1992.tb02672.x.
  • Raymo, Guglielmo, Fabiane Januario, Ali Ali, Ridwan O. Ahmed, Rafet Al-Tobasei, and Mohamed Salem. “Fecal Microbiome Analysis Uncovers Hidden Stress Effects of Low Stocking Density on Rainbow Trout.” Animal Microbiome 6, no. 1 (October 16, 2024). https://doi.org/10.1186/s42523-024-00344-1.
  • Roy, Jérôme, Frédéric Terrier, Michaël Marchand, Alexandre Herman, Cécile Heraud, Anne Surget, Anthony Lanuque, Franck Sandres, and Lucie Marandel. “Effects of Low Stocking Densities on Zootechnical Parameters and Physiological Responses of Rainbow Trout (Oncorhynchus mykiss) Juveniles.” Biology 10, no. 10 (October 13, 2021): 1040–40. https://doi.org/10.3390/biology10101040.
  • Sirakov, I., and E. Ivancheva. “Influence of Stocking Density on the Growth Performance of Rainbow Trout and Brown Trout Grown in Recirculation System.” Bulgarian Journal of Agricultural Science 14, no. 2 (2008): 150–54.
  • Suárez, M. D., C. E. Trenzado, M. García-Gallego, M. Furné, S. García-Mesa, A. Domezain, I. Alba, and A. Sanz. “Interaction of Dietary Energy Levels and Culture Density on Growth Performance and Metabolic and Oxidative Status of Rainbow Trout (Oncorhynchus mykiss).” Aquacultural Engineering 67 (July 2015): 59–66. https://doi.org/10.1016/j.aquaeng.2015.06.001.
  • Yarahmadi, Peyman, Hamed Kolangi Miandare, Seyed Hossein Hoseinifar, Nahid Gheysvandi, and Arash Akbarzadeh. “The Effects of Stocking Density on Hemato-Immunological and Serum Biochemical Parameters of Rainbow Trout (Oncorhynchus mykiss).” Aquaculture International 23, no. 1 (May 22, 2014): 55–63. https://doi.org/10.1007/s10499-014-9797-z.
  • Yarahmadi, Peyman, Hamed Kolangi Miandare, Sahel Fayaz, and Christopher Marlowe A. Caipang. “Increased Stocking Density Causes Changes in Expression of Selected Stress- and Immune-Related Genes, Humoral Innate Immune Parameters and Stress Responses of Rainbow Trout (Oncorhynchus mykiss).” Fish & Shellfish Immunology 48 (January 2016): 43–53. https://doi.org/10.1016/j.fsi.2015.11.007.
  • Zahedi, Saeed, Arash Akbarzadeh, Jalil Mehrzad, Ahmad Noori, and Mohammad Harsij. “Effect of Stocking Density on Growth Performance, Plasma Biochemistry and Muscle Gene Expression in Rainbow Trout (Oncorhynchus mykiss).” Aquaculture 498 (January 1, 2019): 271–78. https://doi.org/10.1016/j.aquaculture.2018.07.044.

  1. Saraiva, J. L., P. Rachinas-Lopes, and P. Arechavala-Lopez. “Finding the ‘Golden Stocking Density’: A Balance between Fish Welfare and Farmers’ Perspectives.” Frontiers in Veterinary Science 9 (July 25, 2022). https://doi.org/10.3389/fvets.2022.930221.

  2. Ellis, T., B. North, A. P. Scott, N.R. Bromage, M. Porter, and D. Gadd. “The Relationships between Stocking Density and Welfare in Farmed Rainbow Trout.” Journal of Fish Biology 61, no. 3 (September 2002): 493–531. https://doi.org/10.1111/j.1095-8649.2002.tb00893.x.

  3. Nærings- og fiskeridepartementet, Forskrift om drift av akvakulturanlegg (akvakulturdriftsforskriften) [Regulations on the operation of aquaculture facilities], FOR-2008-06-17-822 (2008), § 25, https://lovdata.no/dokument/SF/forskrift/2008-06-17-822.