The trout, a sentinel organism for water quality of rivers

The behaviour and survival of trout in rivers are indicators of the quality of the water. Researchers have demonstrated through a study of this sentinel organism that the trout is an indicator of the quality of aquatic ecosystems and the functioning of catchment basins.

The trout is a bio-indicator of the quality of aquatic ecosystems
A sentinel organism or bio-indicator is a species whose behaviour, survival or other reactions sound the alarm bell when faced with disruption of the environment. The common trout (Salmo trutta), a well known fish in French rivers, is a very good example of a bio-indicator especially the juvenile stages which are the most delicate and the most sedentary.
Tests have shown that the mortality of eggs and young trout larvae which live in gravel increases with levels of particulate matter and ammonia in rivers. When particulate matter is deposited it seals the gravel in rivers where incubation of eggs takes place and where very young trout alevins live. This results in a reduction in dissolved oxygen and an increase in ammonia in the water between the gravel. In addition to an increase in mortality of alevins this type of pollution results in retarded growth in survivors and a reduction in the food production capacity of the river for these young trout. By a series of chain reactions the numbers in the trout population of the whole river can drop.

Revealing the quality of water
The classic method of testing the quality of water consists in carrying out physico-chemical analyses. Another approach is bio-indication which consists in analysing the consequences of physico-chemical disruption on organisms which live in aquatic environments. In this way widespread but chronic pollution such as some types of agricultural pollution can be revealed. Different methods exist based on observation of the biodiversity of certain living organisms (for example insect larvae), on measuring the bio-accumulation of pollutants (heavy metals in mosses), or on the response of certain sentinel species such as the trout in this case.

Where does particulate matter come from ?
Particulate matter is solid mineral (clay, alluvium or sand) or organic matter. This matter comes from erosion of the ground in catchment basins and is transported to rivers by surface run-off during rainfall. This can involve large quantities where cultivated land, bare in winter, is situated on slopes or near rivers and where there are no hedges or grass strips to retain the particles. Particulate matter can also come from erosion of river banks during flooding or excessive trampling by animals.

Within the framework of INRA-CEMAGREF cooperation a project concerning the effects of catchment basin management on the transfer of particulate and dissolved matter and on the biological quality of surface water in livestock rearing zones, has been conducted in association by nine teams of researchers including specialists in the fields of landscape ecology, agronomy, earth and hydrology sciences and aquatic ecology. This project has revealed not only the role of land use planning and landscape structure of catchment basins on the flux of particulate matter but also that of agricultural practices near river banks in particular. A difference in particulate matter flux between two small catchment basins in Lower Normandy (Basse Normandie) has been revealed by certain bio-indicators including in particular the survival of trout during juvenile stages.