Introduction
Human activities and anthropogenic greenhouse gas emissions have driven climate change at unprecedented rates (IPCC 2014, 2019). Continued emissions will lead to further atmospheric and oceanic warming, triggering long-term shifts in the climate system that will exacerbate existing risks and introduce new challenges for all life on Earth.
Aquaculture is increasingly affected by climate-related stressors, including rising temperatures, more frequent extreme weather events, ocean acidification, and shifts in salinity and oxygen levels. The severity of these impacts will vary based on multiple factors, as will the sector’s capacity for adaptation (Falconer et al., 2022). In Norwegian aquaculture, high mortality rates stem from various causes, including pathogens, parasites, ulcers, gill diseases, and the strain of intensive handling procedures. As climate change accelerates, and its effects become more pronounced, environmental conditions will emerge as a significant risk factor - intensifying known threats as well as introducing new challenges to fish health. Notably, in 2024, injuries caused by jellyfish rose to rank among the top three causes of mortality in farmed salmon in Norway. Additionally, francisellosis, caused by Francisella noatunensis, reappeared in farmed cod after years without reported cases.
Temperature projections indicate that many regions in Norway may experience elevated temperatures over the next 10–15 years, including more frequent days exceeding optimal thresholds for currently farmed species. Suboptimal environmental conditions can induce micro-damages at the molecular and cellular levels in important organs, compromising fish health (Ytteborg et al., 2022). While ongoing technological advancements and new production methods continue to enhance fish health, the impacts of climate change may mask the benefits of these innovations.
Materials and methods
Using downscaled temperature projections based on the Intergovernmental Panel on Climate Change (IPCC) climate models (Shared Socioeconomic Pathways, SSPs), we analyzed potential future temperature conditions at existing and planned Atlantic cod farms in Norway. Regional climate projections were assessed using the Challenging Conditions Index (CCI), a method developed by Falconer et al. (2024) that evaluates environmental suitability based on a species’ thermal preferences.
To simulate near-future scenarios, a temperature trial was conducted, exposing farmed Atlantic cod ( Gadus morhua) to three temperature conditions: low (12°C), high (17°C), and fluctuating (12–17°C) temperatures . The robustness of cod reared at suboptimal temperatures was further tested by exposing fish to either jellyfish fragments or F. noatunensis. Skin and gill samples were collected from all temperature groups - including exposed and non-exposed individuals - and analyzed using histology and transcriptomics.
Results and discussion
The evaluation of climate projections for future cod farms indicates that several locations may experience temperatures exceeding those considered suitable for cod. The Challenging Conditions Index (CCI) further suggests that farming conditions over the next 10–15 years could become increasingly difficult.
Findings from the experimental trial revealed that temperature alone induce micro-damages in the epidermis of both skin and gills. Exposure to high and fluctuating temperatures led to increased breaches between individual epithelial cells, alongside heightened expression of stress-related genes such as sod , hsp70 , and catalase . Following exposure to jellyfish fragments or F. noatunensis, fish exhibited increased mucus production, elevated stress responses, and a higher frequency of morphological lesions - effects that were more severe under high and fluctuating temperature conditions.
Both skin and gills are highly sensitive to unfavourable environmental conditions, making their morphological analysis valuable for assessing the impact of environmental stressors and identifying potential challenges. The observed damage may reduce fish resilience, impairing their ability to withstand climate change-induced stressors, secondary environmental pressures, and general production-related stress. These findings underscore the importance of evaluating future environmental conditions and species-specific tolerances prior to site selection for future cod farming.
Funding
The work was supported by the Norwegian Troms and Finnmark county (TFFK2022-241 , ArctiCod) and the UK Research and Innovation Future Leaders Fellowship (MR/V021613/1).
References
IPCC. Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp; 2014.
IPCC. IPCC Special Report on the Ocean and Cryosphere in a Changing Climate. IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M. Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. 2019.
Falconer, L., Sparboe, LO., Dale, T., Hjøllo, SS., Stavrakidis-Zachou, O., Bergh, Ø., James, P., Papandroulakis, N., Puvanendran, V., Siikavuopio, SI., Hansen, ØJ., Ytteborg E. (2024) Diversification of marine aquaculture in Norway under climate change, Aquaculture 593.
Falconer, L., Telfer, T.C., Garrett, A., Hermansen, Ø., Mikkelsen, E., Hjøllo, S.S., McAdam, B.J., Ytteborg E. (2022) Insight into real-world complexities is required to enable effective response from the aquaculture sector to climate change. PLOS Climate 1(3): e0000017.