Development of release methods for critically endangered freshwater pearl mussels (Margaritifera margaritifera)

Biodiversity loss is a global problem with freshwater bivalves considered amongst the most<br/>endangered biota. The freshwater pearl mussel, Margaritifera margaritifera, is declining<br/>throughout its range owing to habitat degradation and overexploitation. In most of its range,<br/>populations are regarded as reproductively non-functional which has led to the development<br/>of captive breeding programmes. A novel method of releasing M. margaritifera was trialled,<br/>with captive-bred juveniles being released into the rivers caged in ‘mussels silos’ (protective<br/>concrete domes with ventilation creating upwelling to ensure water through flow). We<br/>released 240 juvenile mussels and survival and growth rates were monitored for 18 months<br/>post-release for three size classes: A (13.01-20.00mm); B (10.01-13.00mm); and C (4.01-<br/>10.00mm). We explicitly tested two experimental treatments; one where sediment was added<br/>to each silo (allowing mussels to orientate and burrow) and one without sediment. Survival<br/>by the end of the experiment at month 18 was significantly higher for the largest size class at<br/>97% (though growth was lowest in this cohort), and lowest for the smallest size class at 61%<br/>(though growth was highest in this cohort). Survival and growth were unaffected by the<br/>experimental treatment suggesting that adding sediment offered no advantage. Growth was<br/>positively correlated with both water temperature and the particle size of suspended solids<br/>(both of which were collinear, peaking in summer). There are a large number of ex situ<br/>breeding programmes for freshwater pearl mussels throughout Europe and our finding<br/>suggest that the use of ‘mussel silos’ could be a useful tool to protecting juvenile mussels<br/>allowing them to be released at a relatively early stage of development, minimising the risk of<br/>domestication.