In spring and this summer, the “Migrateurs Rhône-Méditerranée” (MRM) association was present for several days in the Rhone Gard. On the Cèze, upstream of the Chusclan, surveys were carried out to determine whether the sectors were favorable for the reproduction of the shad. Currently, this migratory fish cannot travel upriver that far due to the Chusclan Threshold. He will soon be able to do so thanks to the next works which include the construction of a fishpond. The data is still being studied but Pierre Campton, technical director of MRM, provides the first results.
Objectif Gard: What can you tell us about shad fish?
Peter Campton: It is an endemic species of the Rhône-Mediterranean basin. It is therefore a species that is not found elsewhere in France or in the world. There is still a major conservation issue. It is a species that reproduces on rivers and will grow in the sea.(…) With global warming, the water needs of agriculture are increasing and there is less and less water in our rivers. The shad is a fish very sensitive to water level, depth and temperature. This will then affect the success of the reproduction. The management of access to water resources has a direct impact on the resilience possibilities of the species.
What was the purpose of your study conducted on the Cèze?
The objective was to identify potentially favorable habitats for the reproduction of the shad, between the threshold of the Chusclan and the Sautadet waterfalls. This is part of a much more global view that aims to identify which sectors are at stake in terms of accessibility for migratory fish. To then adjust our follow-ups once the main axes are reopened. This is the case of the Cèze where there is a project to restore ecological continuity on the threshold of the Chusclan.
Are the shads today unable to cross the threshold of the Chusclan to reproduce?
No, they do not. Today we are identifying which sectors of streams upstream of the sill would be the most favorable to its reproduction. We look at the heights of the water, the speed of the water, the ideal granulometry (size of the pebbles on the bottom of the river, Editor’s note). Based on this knowledge, when the windowsill is reopened, we can verify whether the shads reach it and reproduce effectively.
What are the first results of the study?
We have many potentially favorable habitats. One of the remarkable aspects of the Cèze is that there is no damming structure upstream of the sector and there is no hydroelectric power station. Once the shads have reproduced, they return to the sea and the presence of a micro power station can have an impact and cause mortality on the spawners, but also on the fry that hatch from the eggs and also return to the sea. .
What other criteria are important?
When shads go to spawn in a spawning area, if there are stones clogged with organic matter or algae, survival will be limited. This aspect is more difficult to grasp. We are in an experimental phase, we are trying to implement protocols to define if a reproductive area (reproduction area, ed) is heavily clogged. The overall management of the environments by the territories is therefore also a problem.
Have you already studied this sector between Chusclan and the Sautadet waterfalls?
Yes, we had already been there a few years ago. The idea was also to see how the environment had evolved. Because in the meantime there have been heavy floods. Floods that are called “morphogens”. We wondered if there hadn’t been movement of gravel, of substrates that would have changed the habitat of the shad. It was more of an upgrade job. But overall, we stay on the same quantities. Whether there are any changes to the breeding grounds remains to be seen.
We just have to wait for the work.
We are eagerly waiting for them. You should know that in recent years the Cèze has been one of the tributaries of the Rhône most colonized by shads. We have had excellent results as we are monitoring spawning downstream of the Chusclan and angling. There is great interest in giving this species access to high quality breeding areas so that the maximum number of fry can survive.
When is the Chusclan Doorway construction due?
Currently, the Chusclan sill prevents the shad from moving upriver. A fish passage must be created to allow these migratory fish to go up the Cèze to the Sautadet waterfalls, 14km further on. The yard also plans to lower the threshold by 50 cm. It would be around 750,000 euros excluding VAT (80% financed by the Water Agency). The works, brought by the Agglomeration, should have started this summer, but due to the drought and the high risk of fire, the construction machinery could not function. A priori, the site is rescheduled for June 2023 because we have to wait for a period when the water level will be quite low. The project
Can major droughts like this summer have an impact?
Shads breed at a time of year when we don’t have this drought problem. The fry however, which remain on site until September, may be disturbed. We talk about the consequences of drought but we also have to think about how to manage these times of crisis. We must ensure that the environment is resilient to global warming: how to ensure that there is a minimum of water in our rivers during these times of crisis? We must not think only in terms of continuity for migratory fish but of the environment.
What is the “Migrateurs Rhône-Méditerranée” association?
The “Migrateurs Rhône-Méditerranée” association was created in 1993, at the initiative of the Ministry of the Environment and the fishing federations, in response to the observed decline in migratory fish populations in the Rhône-Mediterranean basin. Since then, he has ensured the conservation and restoration of their living environment. Their action covers 21 departments, including the Gard, and 11,000 rivers. The association is based in Arles and employs eight permanent staff. The 21 departmental fishing federations are also part of it.