Home - Mission - Stimulate Research and Development



Collective science-advice study on fish mortality in the Loue river made public

A general drop in river quality over several decades is thought to be the cause of the fish mortalities in the Loue river in 2010 and 2011. That is the conclusion of the collective science-advice study commissioned by the prefect of the Doubs department, managed by Onema and made public on 7 March 2012. The group of experts made a series of recommendations on monitoring, research and future work.

The upper Loue river, in the Doubs department, is in good condition according to the criteria of the Water framework directive and is famous for the quality of its recreational fishing. But since January 2010, there have been two events of exceptionally high fish mortality, primarily trout and grayling, as well as small benthic species such a bullhead fish. At the same time, proliferation of toxic cyanobacteria was observed.

Scientists brought in from various fields
What is the reason for the death of the fish? Is there a link with the bacteria? To answer these questions, Onema set up, at the request of the prefect, a group of experts in February 2011. For a full year, eleven biologists, hydrologists and chemists analysed the data and existing publications, and discussed the issue with local experts and stakeholders. The initial conclusion is that there is no connection between the death of the fish and the development of the cyanobacteria. However, the two events taken together signal poor overall condition of the river. A number of causes may be involved, i.e. excessive nutrients in the water, extensive development work on the river affecting the physico-chemical and biological quality, the probable presence of pollutants and a level of vulnerability specific to the river due to the karstic soil, etc.

A general drop in river quality
The general drop in river quality over the past decades is thought to have caused the poor condition of the river populations. The diversity and occasionally the quantity of algae, macro-invertebrates and fish in the Loue is currently low. Certain species of macro-invertebrates that are sensitive to the quality of their environment have disappeared and been replaced by species less sensitive to degraded environments.
This degradation would appear to be the consequence of a modification and an increase in certain human activities, though it was not possible to determine the most important factors among household pollution, agricultural practices, the wood industry, fishing, urbanisation, etc.

Understanding the river before taking action
Following the science-advice study, the working group recommended setting up monitoring systems for physico-chemical and biological aspects as well as on human activities and the resulting pressures. To improve river status, the group listed a number of priority projects including limiting the flow of nutrients to the river and the elimination of certain weirs and dams to accelerate the flow rate. Finally, to better understand river operation, a number of key fields for research were listed, namely the impact of toxic substances on fish, the impact of nitrogen pollution on aquatic organisms, the impact of fishing practices, the past history of degradation in the Loue river and identification of the environmental conditions leading to massive development of toxic cyanobacteria.


Back to Newsletter