The Bidasoa River marks the southern limit of the European distribution of Atlantic salmon, acting as a canary in the mine in the face of climate change. And what the Orekan technical team (the public environmental management company of the Government of Navarra) has seen is that the Atlantic salmon is becoming accustomed to swimming upstream. more uphill than ever: It is neither fishing nor illness, more than half die because the river water is too hot. And the lucky ones who arrive are smaller.
The Bidasoa salmon is dying. News from Navarra echoes of Orekan’s research, which has been individually tracking Atlantic salmon by radio telemetry for seven years as they travel up the Bidasoa towards their spawning areas. In warmer years, mortality in summer exceeds 50%. Data collected between 2018 and 2025 show a direct and unequivocal correlation: the more days per year with water temperatures above 20°C, the higher the percentage of fish that do not make it to autumn.
From 20 °C onwards, Atlantic salmon enter physiological stress. Science documents it solidly.: The optimal growth of juveniles occurs between 16 and 20 °C and stops when approaching 23 °C, which explains the decrease in size. Furthermore, the lethal limit of the species is estimated at 27.8 °C, with 33 °C as the absolute mortality temperature. At these temperatures, adults that are in the migration phase (which do not feed and spend energy traveling up rivers) are especially vulnerable.
He Orekan report It is a full-fledged warning: with three exceptions, the species has been below the Critical Conservation Limit in Bidasoa of one million eggs per season for more than 26 years. The population is kept alive artificially thanks to annual repopulation from the Oronoz-Mugaire fish farm.
Why is it important. Because Atlantic salmon is a umbrella species: its conservation guarantees the population of the entire Bidasoa river ecosystem and other species, such as the European mink, the otter or the common trout. As a biological indicator, its decline is an alarm signal about the health of the peninsula’s rivers and the loss of biodiversity.
But also, the fact that it is the southernmost population of the species in the European Eastern Atlantic makes it especially unique in that they concentrate the greatest adaptive genetic diversity (they live on the edge), so that they function as sentinels of environmental change. In short: it is a matter of a few decades before what we see today in the Bidasoa reaches rivers further north. Losing the Bidasoa salmon is not only saying goodbye to the presence of a species in one of the most southern and important places, but also losing the entire genetic archive of adaptations to temperatures and drought.
Context. The decline of Atlantic salmon is not exclusive to Bidasoa or Navarra. The International Council for the Exploration of the Sea public last year an alarming report: 2023 and 2024 were years in which the return of salmon marked historic lows in most North Atlantic countries. And the ICES is clear: not even fishing restrictions are stopping the decline of the species because it is a marine issue.
In the Iberian Peninsula the problem is more serious: a study of 65 years in the Asturian Sella River demonstrated that the factor that best predicts the collapse of catches is not fishing pressure but temperature, both local and oceanic. Thus, Iberian populations make the longest oceanic migrations towards subpolar feeding areas, which implies more energy expenditure and greater exposure to predators.
In detail. Warming is strangling the Bidasoa salmon twice: in the Navarrese river and in the sea. The temperature series surface of the North Atlantic processed by the Climate Change Institute of the University of Maine through ClimateReanalyzer.org confirm it: the average daily water temperature in the salmon feeding area has increased compared to the last years of the 20th century. This displaces their prey, forcing a longer migration. The result? Their biggest threats are climate change and predation.
In the river the consequences go beyond mortality. Orekan records the reduction in size in young individuals and delays in the migration and reproduction calendar. As pointed out this global reviewthe timing of migration and the bioenergetics of the adult are the stages of the life cycle most sensitive to changes in temperature and flow.
Yes, but. The Orekan data is robust at tracking individual fish, but has an important limitation: seven years of measurement is a short time series to isolate the climate signal from interannual noise. Without going any further, the studio on the river Sella needed 65 years of data to build statistically robust models.
On the other hand, it is important to highlight that the Bidasoa population is artificially supported by repopulation, which masks the real magnitude of the natural collapse. Without Oronoz-Mugaire’s annual releases of fry, the wild population would probably have collapsed by now. Or what is the same: that the observed abundance data are more optimistic than the real ecological situation and that any interruption in the breeding program could precipitate extinction in a few years. And it leaves another question on the table: if the sport fishing for salmon in the Bidasoa It is still ethically sustainable when the population has been below its critical conservation limit for 26 years. This 2026 is prohibited.
In Xataka | The great salmon hoax: its true color is gray but the industry has spent years spending millions to hide it
In Xataka | We are drugging the salmon with cocaine and anxiolytics. And that’s causing them to behave strangely.
Cover | Annual and Hector Berganza
GIPHY App Key not set. Please check settings