Salmon gills respond to climate stress before damage becomes visible
Marine heatwaves and more frequent jellyfish blooms pose a threat to farmed fish. But what actually happens inside the fish when its surroundings change?
A new study by Norwegian and Spanish researchers shows that salmon gills respond rapidly as the sea warms. Marine heatwaves leave biological traces before the fish show any signs of damage.
The gills are among the fish’s most important organs. Not only are they responsible for taking up oxygen and releasing carbon dioxide, they also act as an important barrier against pathogens and other environmental influences. When water conditions change rapidly, the gills must adapt to a range of new challenges simultaneously.
Marine heatwaves can lead to reduced oxygen levels and increased jellyfish blooms.
“Our study shows that, together, these stressors affect the bacterial flora on the gills and which genes the salmon switch on and off, long before anything can be seen on the gills. Gill health issue is a growing problem for salmon, and increasingly unpredictable environmental conditions are putting the gills under severe pressure,” says fish health scientist Carlo C. Lazado at Nofima.
A simulated heatwave
The experiment was not intended to recreate every condition salmon encounter in a sea cage, but to mimic key environmental changes already observed in farming areas. Marine heatwaves, periods of low oxygen and jellyfish blooms are becoming increasingly common, and can occur at the same time. By simulating these conditions in a controlled setting, the researchers were able to study how the gills respond biologically, while keeping the influence of other factors to a minimum.
The research examined how a simulated marine heatwave affected the bacterial flora and gene expression in the gills of farmed Atlantic salmon (ca 100 g). The heatwave consisted of a temperature rise from 12 to 17 °C over five days, after which the temperature was kept high for ten days. At the same time, some groups were exposed to reduced oxygen levels, while others were exposed to minced moon jellyfish (Aurelia aurita), a species that occurs regularly along the Norwegian coast.
The researchers then examined both the bacterial communities on the gill surface and the genes that were activated or suppressed in response to the environmental conditions.
Changes in gill bacteria
One of the clearest findings was that the heatwave altered the composition of the bacteria that naturally live on the gills. Fish exposed to high temperatures had a greater diversity of bacteria than control fish.
The bacterial analyses showed that the environmental treatments explained almost half of the variation in microorganisms on the gill surface. The heatwave led to a clear increase in bacterial diversity, as well as a substantial rise in the bacterial genera Streptococcus and Staphylococcus.
Many species within these bacterial groups are harmless, but some can cause problems when fish are stressed or in poor health. Although the fish in the experiment did not develop visible signs of disease, the results suggest that higher temperatures may create conditions that make it easier for certain bacteria to multiply and potentially contribute to health problems.
Gene expression reveals how salmon respond
In addition to measuring bacterial abundance, the researchers mapped which genes the salmon activated or downregulated in response to the stressors.
The analyses showed that more than 600 genes changed their expression, and that temperature had the greatest effect. Higher temperatures made genes linked to mucus production more active, while key components of the fish’s innate immune system became less active. Several genes linked to collagen production were downregulated, while keratin genes were upregulated. This pattern suggests a physiological reallocation, in which the salmon deprioritise growth and tissue maintenance in favour of an acute stress response and cell protection.
When the fish were exposed to moon jellyfish at stable temperatures, there was little effect at the gene level. However, when jellyfish exposure occurred after the fish had been through a heatwave and low oxygen levels, more changes occurred in the gills. This suggests that when several environmental changes occur at the same time, the protective barriers of the gills can come under strain.
According to senior scientist Elisabeth Ytteborg at Nofima, the findings provide a basis for understanding how combined climate stress challenges the gill health of farmed fish.
“Understanding how climate stress affects the gills of farmed salmon is important for the industry in order to deal efficiently with climate change. We have previously seen similar effects in fish skin, where higher temperatures weaken the skin’s natural protective barrier. These damages, which we call microdamages, are not visible on the fish, but they can make the fish more vulnerable when exposed to other stressors,” says Ytteborg, who was also involved in the experiment.
European collaboration
Lazado highlights the excellent collaboration with the Spanish institute IATS (Instituto de Acuicultura Torre de la Sal), which made the research possible. Among other contributions, PhD candidate Socorro Toxqui-Rodríguez from IATS spent a research stay in Tromsø and played a valuable part in the experiments. This was made possible through the EU-funded EATFISH project.
“This is an important collaboration that we hope to continue for many years to come. Climate change in the ocean affects all species, and the consequences are becoming increasingly clear. Through long-term collaboration, we can develop the knowledge needed to meet these challenges, both in aquaculture and in safeguarding wild marine stocks,” says Lazado.
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