Farmed lumpfish (Cyclopterus lumpus) are the dominant cleaner fish used for biological sea-lice control in Norwegian salmon farming, yet mortality in the land-based hatchery phase is high and poorly attributed: most deaths are recorded by hatcheries as 'unspecified' or 'fin damage'. A 2024 compilation of production and fish-health data from four Norwegian hatcheries (12 fish groups, about 8.1 million fish) showed that mortality categorized as fin damage correlates with detection of Tenacibaculum sp. bacteria and Cyclopterus lumpus coronavirus (CLuCV), but could not determine whether environmental pathogens initiate the fin lesions or aggression-induced injuries act as portals for pathogens, and could not identify production- and tank-level risk factors because water chemistry, stocking density, sorting, feeding and vaccination records were too fragmented. This problem asks for causal attribution of hatchery lumpfish mortality: resolving the fin-lesion/infection direction with temporally resolved evidence, quantifying tank-level production risk factors from field data, and establishing the standardized underlying-cause mortality coding, culling-cause recording, individual welfare-indicator scoring and water-quality monitoring that make such attribution possible at commercial scale.

Background

Lumpfish (Cyclopterus lumpus) are the most widely farmed cleaner fish — fish stocked into salmon sea cages to graze parasitic salmon lice (Lepeophtheirus salmonis) off the farmed salmon — in Norwegian aquaculture, and essentially the whole national supply is hatchery-produced. Mortality during the hatchery (land-based juvenile) phase is high and, crucially, poorly attributed: hatcheries record a daily 'mortality cause' per tank, but the categories are non-standardized, dominated by 'unspecified', and mix risk factors, contributing causes, immediate causes and underlying causes. This last distinction follows the World Health Organization cause-of-death convention, in which the underlying cause is the disease or injury that initiated the train of events leading to death, as opposed to the final manifestation.

The motivating study (Boissonnot et al., 2024, Fishes 9(7):288) compiled production, fish-health and mortality data from 12 fish groups (8,112,848 individuals) in four commercial Norwegian hatcheries, production year classes 2020-2021. Where causes were specified, fin damage dominated, and mortality categorized as fin damage correlated with detection of infectious agents — above all the marine bacterium Tenacibaculum sp. (the agent of tenacibaculosis, an ulcerative skin and fin disease of marine fish) and Cyclopterus lumpus coronavirus (CLuCV), which co-occurred with Tenacibaculum sp. in six of seven detections. The data could not distinguish the two causal directions: environmental pathogens initiating fin lesions that lead to death, versus aggression-induced injuries (tail-fin biting, provoked for example by underfeeding or lack of resting substrate) acting as entry portals for pathogens. Risk-factor analysis was equally blocked: water chemistry was never sampled, stocking density could be reconstructed for only one hatchery, sorting and transfer dates went unregistered, vaccination dates were missing for 2 of 12 groups, and feed was logged only at group level. Culling — up to 63% of production in some groups — was recorded without reasons, and welfare-relevant conditions known to be common in hatcheries (cataracts, linked to nutrition in earlier epidemiological work; suction-cup deformities, which impair the adhesive disc lumpfish use to rest and can lead to exhaustion) were almost absent from health reports, even though validated lumpfish-specific welfare scoring instruments, such as the Operational Welfare Score Index, have existed since 2021.

Prior results frame what is known. Reynolds et al. (2022) investigated causes of mortality and loss of lumpfish in small- and large-scale research facilities and in deployed sea cages; for deployed (sea-phase) lumpfish, causes of loss are comparatively well described — handling/grading (21%) and mechanical delousing (20%) dominate, bacterial infections account for 17% — but no equivalent etiological picture exists for the commercial hatchery phase at scale, and lumpfish welfare reviews list mortality attribution among the central unresolved knowledge gaps (Garcia de Leaniz et al., 2022). A 2026 assessment of cleaner-fish use in Norway concludes that poor welfare and high mortality remain unresolved at commercial scale (Overton et al., 2026). The unresolved target is therefore the causal chain of events — the underlying causes — behind hatchery-phase lumpfish mortality, which the source study states requires, as a prerequisite, standardized monitoring protocols: harmonized mortality-cause coding focused on underlying causes, routine recording of culling reasons, individual-level operational welfare indicator (OWI) scoring (repeatable scored welfare measures such as fin, eye and deformity status), and systematic tank-level registration of water quality and production events.

Problem Statement

Determine the underlying causes — the chain of events leading to death — of lumpfish (Cyclopterus lumpus) mortality in the hatchery phase, at commercial-hatchery scale in Norway. Concretely, following the source study (Boissonnot et al., 2024), what it explicitly left unresolved and what it recommends as the prerequisite:

  1. Resolve the causal direction between fin lesions and infection: does infection with environmental pathogens (above all the marine bacterium Tenacibaculum sp., agent of the ulcerative fin and skin disease tenacibaculosis, and Cyclopterus lumpus coronavirus, CLuCV) initiate the fin damage that leads to death, or do aggression-induced injuries serve as portals for pathogens? Adjudicate the current evidence for each of the two causal directions and state what time-resolved evidence would discriminate them.

  2. Identify the production- and tank-level risk factors — such as water chemistry and its fluctuations, stocking density regimes, sorting and transfer schedules, feed strategy at tank resolution, and vaccination timing — that predispose lumpfish to aggression, lesions, and lethal infections, and set out how their contributions are to be quantified from tank-level records.

  3. Establish the standardized monitoring that the source names as the prerequisite for such causal and interventional studies: harmonized mortality-cause coding focused on underlying causes, routine recording of culling reasons, individual-level operational welfare indicator (OWI) scoring covering cataracts and suction-cup deformities, and systematic tank-level registration of water quality and production events.

These three components serve one objective — turning hatchery lumpfish mortality from a poorly categorized loss into a causally attributed and therefore mitigable outcome.

References

  1. Lauris Boissonnot, Camilla Karlsen, Thor Magne Jonassen, Silje Stensby-Skjærvik, Torolf Storsul, Albert Kjartan Dagbjartarson Imsland (2024). Understanding the Causes of Lumpfish (Cyclopterus lumpus) Mortality in Norwegian Hatcheries: Challenges and Opportunities. Fishes 9(7):288. DOI: 10.3390/fishes9070288. https://doi.org/10.3390/fishes9070288
  2. Albert Kjartan Dagbjartarson Imsland, Christiana Maple, Helena C. Reinardy, Lauri Kapari, Erik Höglund, Trond Ivarjord, Patrick Reynolds, Karin Pittman, Fredrik Staven, Lauris Boissonnot, Thor Magne Jonassen (2025). The effect of two rearing systems and water exchange rates on growth, welfare and robustness of juvenile lumpfish. Aquaculture Science and Management 2(1). DOI: 10.1186/s44365-025-00012-8. https://doi.org/10.1186/s44365-025-00012-8
  3. Kathy Overton, Luke T. Barrett, Frode Oppedal, Tina Oldham, Lena Geitung, Kristine Gismervik, Tim Dempster (2026). The rise and fall of cleaner fish use in Norwegian salmon farming. Aquaculture Environment Interactions 18:1-16. DOI: 10.3354/aei00520. https://doi.org/10.3354/aei00520
  4. Patrick Reynolds, Albert Kjartan Dagbjartarson Imsland, Lauris Boissonnot (2022). Causes of Mortality and Loss of Lumpfish Cyclopterus lumpus. Fishes 7(6):328. DOI: 10.3390/fishes7060328. https://doi.org/10.3390/fishes7060328
  5. Thor Magne Jonassen, M. Hamadi, S. C. Remø, Rune Waagbø (2017). An epidemiological study of cataracts in wild and farmed lumpfish (Cyclopterus lumpus L.) and the relation to nutrition. Journal of Fish Diseases 40(12):1903-1914. DOI: 10.1111/jfd.12664. https://doi.org/10.1111/jfd.12664
  6. Carolina Gutierrez Rabadan, C. Spreadbury, Sofia Consuegra, Carlos Garcia de Leaniz (2021). Development, validation and testing of an Operational Welfare Score Index for farmed lumpfish Cyclopterus lumpus L. Aquaculture 531:735777. DOI: 10.1016/j.aquaculture.2020.735777. https://doi.org/10.1016/j.aquaculture.2020.735777
  7. Carlos Garcia de Leaniz, Carolina Gutierrez Rabadan, Sara I. Barrento, Rebecca Stringwell, Paul N. Howes, Ben A. Whittaker, Jessica F. Minett, Robert G. Smith, Craig L. Pooley, Ben J. Overland, Leigh Biddiscombe, Richard Lloyd, Sofia Consuegra, Jake K. Maddocks, Paul T. J. Deacon, Ben T. Jennings, Sonia Rey Planellas, Amanda Deakin, Amber I. Moore, Daniel Phillips, Guillermo Bardera, Maria F. Castanheira, Maria Scolamacchia, Nancy Clarke, Ollie Parker, John Avizienius, Malcolm Johnstone, Michalis Pavlidis (2022). Addressing the welfare needs of farmed lumpfish: Knowledge gaps, challenges and solutions. Reviews in Aquaculture 14(1):139-155. DOI: 10.1111/raq.12589. https://doi.org/10.1111/raq.12589
  8. Fredrik R. Staven, Manuel Gesto, Marta C. Soares, Nina S. Iversen, Andreas R. Staven, Torvald B. Egeland, Silje Stensby-Skjærvik, Lauris Boissonnot, Thor Magne Jonassen, Sigurd Olav Stefansson, Erik Höglund, Torstein Kristensen, Albert Kjartan D. Imsland (2025). License to chill: Environmental enrichment for post-vaccination recovery and social training of lumpfish (Cyclopterus lumpus). Aquaculture 609:742861. DOI: 10.1016/j.aquaculture.2025.742861. https://doi.org/10.1016/j.aquaculture.2025.742861
  9. Karolina Yague-Barantchikova, Carlos Garcia de Leaniz (2024). An automated separator to exclude lumpfish with deformed suckers. BioShorts 1:6-9. DOI: 10.70145/bish0003. https://doi.org/10.70145/bish0003
  10. Albert K. D. Imsland, Patrick Reynolds, Lauris Boissonnot (2024). Effects of different feeding regimes on growth, cataract development, welfare, and histopathology of lumpfish (Cyclopterus lumpus L.). Aquaculture 578:740137. DOI: 10.1016/j.aquaculture.2023.740137. https://doi.org/10.1016/j.aquaculture.2023.740137