The Seine nearly hit 28°C this summer, a record that spells trouble for trout and pike
Fish under threat, record-breaking temperatures and a river that has never suffered so much from the heat: the situation in the Seine this summer of 2026 is causing serious concern among specialists. Readings taken by the MeSeine observatory paint an unprecedented picture for the Parisian river.
Temperature records never before seen in the Seine
On 26 June, sensors operated by the MeSeine observatory recorded 28.1°C in the river. A few weeks later, on 17 July, the thermometer was still reading 27°C. Both measurements represent absolute records for the Parisian river, according to data from the organisation, which is the environmental monitoring network created by the public wastewater management authority for the Île-de-France region, the SIAAP.
These figures did not come out of nowhere. Between 1870 and 2019, the river’s overall temperature rose by 2.5°C. By 2100, current projections anticipate a further increase of 2 to 3°C, driven by heatwaves of which this summer appears to be a foretaste. Climate change is a long-term reality, and the river is absorbing its effects directly.
Aquatic wildlife under threat on multiple fronts
For the species that inhabit the river, this rise in temperatures represents a concrete and immediate danger. Higher-than-usual mortality is already being observed, even if it remains difficult to quantify precisely. The heat is acting on several fronts at once.
First, the initial heatwave coincided exactly with the breeding season. Young fish need cool water to develop properly, and this unfortunate timing has compromised population renewal at a particularly vulnerable moment.
Second, warmer water holds less dissolved oxygen. This reduction in available oxygen poses a direct threat to species such as trout, bullhead, dace and pike, all of which are present in the Parisian stretch of the river and all of which are sensitive to this kind of thermal stress.
Invasive species are thriving as the water warms
The rise in temperatures is not affecting everyone equally. Species of tropical origin, such as the black-spotted goby and the pumpkinseed sunfish, are finding the warmer waters increasingly hospitable. These invasive species are establishing themselves at the expense of native ones, which they are helping to push out.
This is all the more worrying given that it follows an encouraging period. Water quality had improved in the wake of the Olympic Games, allowing certain historically present species to return to the Seine. That revival of biodiversity, the result of significant effort, is now being threatened by the ongoing thermal shift.
- 28.1°C recorded on 26 June, an absolute record for the Parisian river
- 27°C recorded on 17 July of the same year
- +2.5°C rise in temperature between 1870 and 2019
- A further +2 to 3°C projected by 2100
- Trout, bullhead, dace and pike directly threatened by oxygen depletion
A step backwards after recent progress
The current situation stands in stark contrast to the advances made in recent years. The return of historically present species to the river had been a positive signal, a sign that efforts to improve water quality were paying off. Now, that encouraging picture is being called into question by a factor that improved sanitation alone cannot counteract.
Heat acts independently of water cleanliness. Even a well-oxygenated, low-pollution river suffers the effects of a prolonged temperature rise. As a result, those working in environmental monitoring are hoping that appropriate solutions will be put forward soon to address this new climatic reality.
