Mortality

Mortality serves as a crude but important welfare indicator. While high mortality can reasonably be assumed to indicate poor welfare, fish in aquaculture systems can have poor welfare even with low mortality levels.

The Norwegian Food Safety Authority has stated that the average mortality rate in Norway is too high and unacceptable.26 In 2024, 2.4 million rainbow trout died during the marine phase in Norway, representing an estimated annual cumulative mortality rate of 15.0%.27 In 2021, approximately 5% of all Norwegian locations for Atlantic salmon and rainbow trout achieved mortality rates below 5% in the sea phase.28 Individual farmers, scientific institutions, and others have argued that this is a feasible mortality rate to achieve.

Beyond recording mortality rates, producers should also record causes of mortality to build an understanding of trends over time. This is crucial for enabling appropriate actions to avoid and prevent further mortalities.

Mortality per day

Level 0 Level 1 Level 2 Level 3
≤ 0.014 %/⁠day
> 0.014 to ≤ 0.029 %/⁠day
> 0.029 to ≤ 0.061 %/⁠day
> 0.061 %/⁠day

Mortality per annum

Level 0 Level 1 Level 2 Level 3
≤ 5 %/⁠year
> 5 to ≤ 10 %/⁠year
> 10 to ≤ 20 %/⁠year
> 20 %/⁠year

Overall: Overall score is determined by the most severe subindicator

Protocol application

The figures above refer to both annualized and daily mortality rates (with daily mortality rates extrapolated from annual rates, rounded for simplicity). Annual rates should be the primary metric for overall assessment, with daily rates necessary for operational tracking and management decisions.

Scoring notes
  • If daily mortality ≥ 0.029%, immediate investigation should be initiated.
  • Along with mortality rates, it is critical to track and record causes of mortality. Understanding why fish are dying is essential for identifying patterns, implementing preventive measures, and distinguishing normal background mortality from emerging problems. Where many mortalities (e.g., > 20% of recorded mortalities) cannot be explained, monitoring should increase, and management practices should be reassessed.
  • Raw mortality numbers are not the only important metric. Increasing mortality rates, even if absolute rates remain in the level 0 or level 1 categories, are still cause for concern.
  • If mortality rates increase or spike, this should be thoroughly investigated, and management practices should be reassessed.
  • Being aware of seasonal trends is also essential for making improved management decisions.
  • Validation: These thresholds are based on Norwegian industry aspirations, the Laksvel protocol, and expert opinion.
Supporting evidence
  • Davidson, John, Christopher Good, Carla Welsh, and Steven T. Summerfelt. “Abnormal Swimming Behavior and Increased Deformities in Rainbow Trout Oncorhynchus mykiss Cultured in Low Exchange Water Recirculating Aquaculture Systems.” Aquacultural Engineering 45, no. 3 (November 2011): 109–17. https://doi.org/10.1016/j.aquaeng.2011.08.005.
  • Fjelldal, Per Gunnar, Sankar Murugesan, Tone Vågseth, Audun Østby Pedersen, Angelico Madaro, Samantha Bui, Harald Kryvi, Lars Helge Stien, and Jonatan Nilsson. “Vertebral Deformities in Cultured Big Size Rainbow Trout: Radiological Analysis from Juvenile to Harvest Size.” Aquaculture 596 (2025): 741729. https://doi.org/10.1016/j.aquaculture.2024.741729.
  • Landbruks- og matdepartementet. “Melding Til Stortinget: Dyrevelferd. Meld. St. 8 (2024–2025).” Landbruks- og matdepartementet, 2024.
  • 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.
  • MacIntyre, Craig. “Water Quality and Welfare Assessment on United Kingdom Trout Farms.” PhD Thesis, 2008. https://dspace.stir.ac.uk/bitstream/1893/434/1/FINAL%20THESIS.pdf.
  • Moldal, T., J. Wiik-Nielsen, V. H. S. Oliveira, J. C. Svendsen, and I. Sommerset. “Norwegian Fish Health Report 2024.” Norwegian Veterinary Institute, 2025. https://www.vetinst.no/rapporter-og-publikasjoner/rapporter/2025/norwegian-fish-health-report-2024/_/attachment/inline/6b11b72c-ee8f-4529-921f-1a3d85dc419e:2d59843d7c1e34e9200669ae47f2974d8ee51b6a/Fish%20Health%20Report%202024.pdf.
  • Moutou, K. A., I. D. McCarthy, and D. F. Houlihan. “The Effect of Ration Level and Social Rank on the Development of Fin Damage in Juvenile Rainbow Trout.” Journal of Fish Biology 52, no. 4 (April 1, 1998): 756–70. https://doi.org/10.1111/j.1095-8649.1998.tb00818.x.
  • Nilsson, Jonatan, Kristine Gismervik, Kristoffer Vale Nielsen, Martin Haugsmo Iversen, Chris Noble, Jelena Kolarevic, Hilde Frotjold, et al. 2025. “Laksvel — a Standardised, Operational Welfare Monitoring Protocol for Atlantic Salmon Held in Sea Cages.” Institute of Marine Research. https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen-en-2025-40.
  • 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.
  • 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 31, 2006): 466–79. https://doi.org/10.1016/j.aquaculture.2006.01.004.
  • Park, Kunhong, Jinseo Choi, Younghun Lee, and Jeonghwan Park. “Evaluation of the Optimal Dissolved Oxygen Level for Rainbow Trout (Oncorhynchus mykiss) in the Recirculating Aquaculture System.” Journal of the Korean Society of Fisheries and Ocean Technology 59, no. 4 (November 30, 2023): 387–98. https://doi.org/10.3796/ksfot.2023.59.4.387.
  • 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 (2008): 185–95. https://doi.org/10.1051/alr:2008024.
  • Roque d’Orbcastel, Emmanuelle, Jeannine Person-Le Ruyet, Nicolas Le Bayon, and Jean-Paul Blancheton. “Comparative Growth and Welfare in Rainbow Trout Reared in Recirculating and Flow-through Rearing Systems.” Aquacultural Engineering 40, no. 2 (March 2009): 79–86. https://doi.org/10.1016/j.aquaeng.2008.11.005.
  • 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. Landbruks- og matdepartementet. “Melding Til Stortinget: Dyrevelferd. Meld. St. 8 (2024–2025).” Landbruks- og matdepartementet, 2024.

  2. Moldal, T., J. Wiik-Nielsen, V. H. S. Oliveira, J. C. Svendsen, and I. Sommerset. “Norwegian Fish Health Report 2024.” Norwegian Veterinary Institute, 2025. https://www.vetinst.no/rapporter-og-publikasjoner/rapporter/2025/norwegian-fish-health-report-2024/_/attachment/inline/6b11b72c-ee8f-4529-921f-1a3d85dc419e:2d59843d7c1e34e9200669ae47f2974d8ee51b6a/Fish%20Health%20Report%202024.pdf.

  3. Landbruks- og matdepartementet. “Melding Til Stortinget: Dyrevelferd. Meld. St. 8 (2024–2025).” Landbruks- og matdepartementet, 2024.