Status assessment and recovery potential of Denmark´s only freshwater pearl mussel (Margaritifera margaritifera) population in the Varde River system

The freshwater pearl mussel ( Margaritifera margaritifera ) is a strongly declining target species for conservation. In Denmark, its conservation status is unclear, as the last living specimen was found in the Varde River in 1995. This study assessed the current status and the recovery potential of M. margaritifera in the Varde River system by: (i) dive monitoring searching for living mussels, (ii) assessing habitat quality in the Varde River and its tributaries, (iii) assessing the host fish population status and trends, and (iv) identifying potential donor populations based on genetic analyses of DNA from dead shells. Dive monitoring detected only empty shells, confirming the historic presence of the species. Habitat assessment revealed that most sites were unsuitable for juvenile pearl mussels due to mobile sand, low interstitial oxygen levels, or a lack of medium grain sizes, but few suitable spots were identified within a restored stretch of two tributaries (Holme stream and Linding stream). Host fish population sizes have declined in recent years, despite removal of migration barriers. The historic populations of M. margaritifera in Varde River were closely related to their conspecifics from Estonia and Southern Sweden, and not to the nearest population from Northern Germany. Knowledge on potential donor populations and increasing stream restoration activities provide recovery potential for M. margaritifera in the Varde River system, but existing habitat quality needs substantial improvement to sufficiently support the return of viable populations of this flagship species.