Background

The pearl mussel (Margaritifera margaritifera) was once common in Europe, but more than 95% of the total number have disappeared over the past hundred years. In Belgium, there are only a few remaining populations, found in very clean watercourses in the Rulles, Sûre, Vierre and Our basins. Most of these are small, with only one population containing more than 1 000 individuals.

Decline of water quality, unintentional crushing and localy deliberate killing of mussels in the hope of finding pearls have been some of the main reasons for the species' decline. The life cycle of the pearl mussel is very complex, as the larvae have to spend around ten months in the gills of a trout (Salmo trutta forma fario) in order to change into a small mussel. After this, they bury themselves in a particular substratum of the riverbed and remain there for around five years before continuing their development to sexual maturity on the riverbed.

This developmental process means that the species is very sensitive to the deterioration of its natural habitat. Livestock and forestry machinery in the river can kill mussels, as can initiatives to stabilise riverbanks and riverbed reprofiling. The larvae and young mussels are strongly affected by pollution which modifies nitrate, phosphate and oxygen concentrations and increases the turbidity of the water and the siltation of the riverbed.
Objectives

This LIFE Nature project aimed to restore populations of the pearl mussel through the long-term conservation of their habitats in the four catchment areas in Belgium where they still occur.

The project planned to increase understanding of the current mussel populations, their habitat and ecological requirements through studies, increased monitoring and a mapping exercise.

Appropriate actions to improve these habitats and ecological conditions would then be undertaken, including restoring riverbanks, removing conifers from river valleys and planting deciduous, riverine woodlands.

Efforts would be made to reconcile conflicts between the needs of the mussel and those of other stakeholders including the farming and forestry sectors. This might necessitate land purchase along relevant sections of the riverbank to implement conservation areas or the implementation of land-use agreements.  

The project sought to draw on the experience and learning acquired in other countries and other projects - including LIFE projects - in the pursuit of these objectives.
Results

The project successfully provided more up-to-date and detailed understanding of the pearl mussel and its habitat and implemented actions to foster the development of the populations in Belgium.

The team intensively prospected 289 km of river, monitoring mussel numbers, brown trout populations, habitat types and valley bottoms. Water quality was monitored at 200 stations. The University of Liege carried out two detailed studies of the quality of the sediment.

These efforts highlighted 600 problem areas, including 180 newly discovered concerns. Based on these findings, the project then implemented a series of management initiatives to tackle problems in key target areas, managing to solve around 20% of all identified issues.

A key element was the erection of 76 km of fencing, construction of ten wooden bridges and installation of 119 drinking troughs to prevent damage to riverbanks, disruption of the riverbed and trampling of the mussels by livestock. This work was carried out with 43 participating farmers in the project’s target area.

The project convinced more than 80 landowners to remove around 70 ha of coniferous trees, which had gradually replaced original grasslands in the river basin. This opened the valley bottom and restored a network of humid meadows, which are much more effective at holding riverbanks together, provide organic particles to feed the mussels and allow more light to reach aquatic fauna in the river.

Whilst opening the river in general to promote plantlife and the oxygen content of the water, deciduous riverine forests were restored across 16 ha of riverbank. These were planted to provide shade to specific parts of the river and ensure the cool temperatures needed by the mussels in the summer.

Following the work of the project, the Government of Wallonia approved an investment plan for water-treatment stations taking into account the needs of the pearl mussel. This marked the first time in Belgium that nature conservation objectives had been a determining factor at such a level of water-treatment policy making.

Where management measures were found to be incompatible with farming or forestry requirements, land was purchased where possible to prioritise the environmental objectives. A total of 132 ha were purchased under the LIFE project, along with an additional 43 ha from other funding sources - from more than 100 owners.

Conservation status was applied to more than 230 ha of the river basin, restricting land use and certain activities such as fishing. Key aspects behind the overall success of this project were that all the actions were carried out within one overall strategy and that all the different sectors of activity - local authorities, water authorities, anglers, famers and forestry workers - were involved.

Further information on the project can be found in the project's layman report and After-LIFE Conservation Plan (see "Read more" section).