Policy – September 28, 2026

Salmon gills show climate stress before visible damage

Caption: Scientists tested how salmon gills respond to heat, low oxygen levels and jellyfish exposure. Credit: Terje Aamodt / Nofima

Farmed salmon can show biological signs of heat and environmental stress in their gills before visible damage appears, according to new research from Nofima.

The study examined how Atlantic salmon gills responded to a simulated marine heatwave, reduced oxygen levels and exposure to moon jellyfish (Aurelia aurita), conditions that can increasingly coincide in farming areas.

Researchers raised water temperatures from 12°C to 17°C over five days before holding them at the higher temperature for a further ten days. Some fish were also exposed to low oxygen levels or minced jellyfish.

The clearest effects came from higher temperatures.

Heat exposure altered the bacterial communities living on the gill surface and increased bacterial diversity. Researchers also recorded higher levels of bacteria from the Streptococcus and Staphylococcus genera.

While many species within those groups are harmless, some can become problematic when fish are stressed or in poor condition.

The fish showed no visible signs of disease during the experiment.

Gene-expression analysis nevertheless identified changes in more than 600 genes, with temperature producing the strongest response.

Genes associated with mucus production became more active, while parts of the innate immune system were suppressed. Genes associated with collagen production were downregulated, while keratin-related genes became more active.

Researchers said the pattern suggested that salmon were diverting biological resources away from growth and tissue maintenance towards an acute stress response and cellular protection.

Jellyfish exposure on its own produced relatively little change at gene level. However, fish exposed to jellyfish after experiencing elevated temperatures and low oxygen showed a stronger gill response, suggesting that combined environmental stressors may place greater pressure on the gill barrier.

“Gill health issue is a growing problem for salmon, and increasingly unpredictable environmental conditions are putting the gills under severe pressure,” said Nofima fish health scientist Carlo C. Lazado.

Senior scientist Elisabeth Ytteborg said the findings could help improve understanding of how climate-related environmental changes affect farmed salmon before damage becomes externally visible.

The study was carried out with Spain’s Instituto de Acuicultura Torre de la Sal as part of European research collaboration supported through the EU-funded EATFISH project.

The research has been published in Frontiers in Marine Science.