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. In the opposite to the situation one hundred years ago, when the Rhine’s Allis shad population collapsed, due to rigorous overfishing, the implementation hydraulic engineering measures and the progressing deterioration of water quality, and the Allis shad finally became extinct from the Rhine system within a few decades and without suitable countermeasures could had been initiated, today the opportunity is given to save the largest remaining Allis shad population in the Gironde watersheds from suffering a similar fate.
The Allis shad fisheries landing figures in the Dutch stretch of the Delta Rhine testify the shads populations decline at the end of the 19th century. Actually the rigorous fisheries were amongst the main factors for the collapse of the Rhine's Allis shad population. Data after de Groot 1990
Yield (blue columns), effort (yellow-white columns) and catch per unit effort (red line) of Allis shad fisheries on the Garonne by professional fishermen between 1979 and 2007 (data source CEMAGREF). Even the landing figures in the Gironde watersheds have collapsed abruptly. Since 2007 a fishing ban is in force.
Numbers of adult shads counted at the monitoring facilities on the lower Garonne and the Dordogne dams from 1993 to 2011 (data source MIGADO). In spite of the fishing moratorium being in force since 2007 the Allis shad population hasn't sustainably recovered yet.
Already the most downstream situated transversal barriers (Golfech/Garonne, Bergerac/Dordogne) obstruct the Allis shad from reaching spawning areas in the Gironde watersheds middle and upper courses. The present fish ways presumably do only an insufficient share of the adults seeking to migrate upstream facilitate to pass-over the transversal barriers (Map: Migado; Pictures: A. Scharbert).
As at that time in the Rhine area, in the Gironde watersheds the catch of Allis shad during their spawning migration has not only a long tradition and economic importance for the numerous professional fishermen along the Garonne and the Dordogne, but is a culinary attraction for human population and the tourism in the region.
In analogy to the process occurring in the Rhine area in the early 20th century, the sudden drop of the fisheries catch yields from 2005 onwards, indicates the onset of a severe shad population decrease. Although a moratorium which prohibits the catch and sale of Allis shad caught in the Gironde watersheds was set into force in 2008, neither the number of shad at the monitoring facilities on the Garonne and the Dordogne nor acoustic surveys on the number of shads spawning in the river sections downstream of the dams point to a sustainable recovery of the Allis shad population.
Since the fishing ban has not led to a recovery, there are most probably further reasons behind the Allis shad populations collapse in the Gironde watersheds. Nevertheless and in the opposite to situation once on the Rhine, the rivers in the southwest of France have not suffered far-reaching modifications (hydraulic engineering, deterioration of water quality) in the last years or even decades, which might explain the decline of the Allis shad stock. However, during a long-term monitoring surveys on the of YOY shads in the Gironde estuary carried out monthly, a strong decrease of the YOY was already observed years before the number of adult returners showed a similar tendency. This might indicate to an impaired recruitment, which is most probably insufficient to maintain the population.
This could on one hand be due to the unsuitability of existing fish ways for migrating shad, which are thus forced to spawn in stretches below the dams where habitats for spawning are less suitable and the reproduction less successful. However, still little is known about the Allis shads’ complex life-cycle and particularly the young-of-the-year (YOY) requirements towards the ambient environment during the freshwater phase, i.e. from hatching until the emigration into the estuary in late summer are quite unknown. In general the first weeks of life are regarded as the most critical in the life-cycle of fish and accompanied by high mortality rates even when the requirements are rather met, whereas the recruitment may be drastically reduced under unsuitable habitat conditions, insufficient food availability, changes in water chemistry or an increased predatory pressure.
By means of comprehensive studies on these topics, the influence of the several factors on the recruitment of Allis shad, shall be assessed and shall finally lead to denomination of countermeasures in order to protect the population in the Gironde watersheds.
In order to make the re-introduction of the Allis shad to the Rhine system a success, it is required to continue the stocking measures initiated in the previous LIFE project for a long term to found a sustainable basis for a stock to return for spawning each 3 to 5 years later. These measures are integrated into the LIFE+ Allis shad project and thus ensured for the period from 2011 until 2015. Accompanying investigations shall parallel to the studies conducted in France help to close the gap of knowledge on Allis shad and to assess and to further improve actions for re-establishing shad populations.
