Situation and restoration of amphyhalin migratory fishes in the Loire basin.

The migratory context of the Loire basin is caracterized by its hydro-ecolo gical diversity and its geographic extension. This hydrographic system stem ming from ancient massifs has a large potential for migratory fish, as show n by historical data of the ligerian (Loire) fisheries production. Species diversity has mainly been preserved but, from the quantitative point of vie w, the state of amphihaline populations has seriously declined. The limitin g factors of these stocks result from the cumulative effect of impacts that occur along the migration circuits and which increase according to the siz e of the basin. While the main Ligerian rivers have been relatively spared by major hydraulic developments, they are nevertheless seriously affected b y a large number of obstacles that delay migration and can compromise the a ccess to the spawning areas. The same is true of the eutrophication which w orsens as more pollutants are added, culminating with a maximum of turbidit y in the estuary (bouchon vaseux). Under these conditions, the ligerian sto cks are strongly dependent on the hydrology. Interannual flow variations ca n temporary reduce the influence of the limiting factors or, on the contrar y, lead to crisis situations that threaten the preservation of the species. In addition to the restoration actions undertaken since the beginning of t he 1980fi, major operations such as the removal of the Maisons Rouges dam ( Vienne) have been carried within the frame of the Plan Loire Grandeur Natur e. These operations should help to reach the minimum necessary limits for s tocks reconstitution. Tangible signs of stock recovery are appearing in the Vienne basin. The first results of stocks reconstitution have been quantif ied on the Allier River at Vichy dam, which is equipped with a control stat ions. However, achieving the objectives and evaluating the results for the whole basin remains governed by equipment of one major obstacle on each of the other three main migration axes of the basin (Loire, Vienne, Creuse), w ith the installation of efficient migration and monitoring systems.