Salmon ends up on plates around the world, but few stop to think about what happens inside, specifically in its gut, where a community of bacteria works silently. For years, science has studied how these microorganisms affect human health, but now a team of researchers has focused on the digestive tract of Atlantic salmon, analyzing samples from Norway, Scotland, and Ireland. The results, which add to growing concerns about climate change in European rivers, paint a complex picture for the species.
Meanwhile, in southern England, the River Itchen, famous for its crystal-clear, nutrient-rich waters, is suffering the consequences of an exceptionally hot summer. Water temperatures are exceeding 17 degrees Celsius, when the normal range is around 12 or 13 degrees. The salmon and trout that inhabit these waters are facing heat stress that threatens their survival. The combination of these factors has led scientists and conservationists to seek solutions, from restoring riverbeds to studying the microbiota as an indicator of health.
The gut microbiota: an invisible world that makes a difference
The study of the salmon microbiota, which analyzed 847 samples of stomach contents de peces A study comparing wild and farmed salmon reveals that bacterial diversity is significantly lower in caged specimens. While wild salmon exhibit a rich and varied microbial community, farmed salmon show more specialized bacteria, adapted to life in captivity. This difference, according to researchers, could have direct consequences for health: lower diversity is associated with a weaker immune system and a higher risk of developing diseases. vulnerability to diseases.
Furthermore, it has been observed that as salmon grow, bacterial diversity decreases, with a genus called Mycoplasma dominating the gut . This microorganism, which has co-evolved with the fish for millennia, appears to be taking over the space previously occupied by other bacteria. Although the exact impact of this change is still unknown, experts suspect it could affect both growth and resistance to pathogens. Diet, which accounts for about 23% of the variation in bacterial flora, is emerging as a key factor, since the standardized feed in fish farms does not compare to the variety of foods found in wild salmon.
Climate change is putting pressure on European rivers
In the River Itchen, in southern England, the situation is alarming. Successive heat waves have raised the water temperature to 21,7 degrees Celsius in July, well above the comfort threshold for salmon, which is between 10 and 18 degrees Celsius. This heat stress causes a decrease in dissolved oxygen , weakening the fish and making them more susceptible to predators such as seals and cormorants. In 2022, only 180 salmon were counted in the Itchen, and provisional figures for 2023 barely reach 250, far below the threshold needed to ensure the species' conservation.
Low water levels, exacerbated by the drought affecting three-quarters of England, are also concentrating pollutants in the rivers. Illegal discharges, such as those acknowledged by Southern Water in the River Test, are adding pressure to an already fragile ecosystem. Experts warn that, without action, the average temperature of waterways where trout live will exceed the tolerable limit in most areas by 2070, and for salmon, the critical threshold will be reached in half of their habitats by 2080.
River restoration: a hope for salmon
Faced with this situation, river restoration emerges as a viable solution. In the United States, the demolition of two dams on the Elwha River has allowed millions of tons of sediment to be displaced and salmon to return to their spawning grounds. This example, though distant, inspires European conservationists, who advocate planting trees to create shade, restoring rivers to their natural course, and reducing water withdrawals . On the Itchen River, some fishing sections have already been temporarily closed to limit stress on the fish, and work is underway to create refuges and improve river connectivity.
Science is also providing innovative tools. Monitoring the gut microbiome is being explored as a way to assess fish health before visible symptoms appear. Researchers have created a reference profile of the bacterial flora of wild salmon in different areas, which could be used to detect stress, disease, or environmental impacts early on. This technique, which could also be applied to other species, offers an early warning signal that would allow for rapid intervention.
The combination of these efforts, from habitat restoration to microbiota studies, points to a more promising future for Atlantic salmon. However, scientists emphasize that much research remains to be done. Understanding how gut bacteria function, their interaction with the immune system, and their role in the overall health of the fish is the next step. If the aquaculture industry harnesses this knowledge , it could generate positive ripple effects, not only for sustainable production but also for the conservation of wild populations.
Ultimately, the health of salmon depends on a delicate balance between genetics, diet, environment, and climate. The loss of bacterial diversity, rising temperatures, and river degradation are interconnected threats that require comprehensive solutions. The good news is that science and environmental management are moving in the right direction, and there are increasingly more tools to protect a species that is much more than a delicacy: it is an indicator of the health of our aquatic ecosystems.
