The culture of juvenile freshwater pearl mussels Margaritifera margaritifera L. in cages: a contribution to conservation programmes and the knowledge of habitat requirements

Published source details Buddensiek V. (1995) The culture of juvenile freshwater pearl mussels Margaritifera margaritifera L. in cages: a contribution to conservation programmes and the knowledge of habitat requirements. Biological Conservation, 74, 33-40.
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Summary

The freshwater pearl mussel Margaritifera margaritifera has become extinct over much of its European range. Many extant populations have declined by 90% or more. In suitable rivers in the Lüneburg Heide (Luneburg Heathlands) area of northern Germany, culture of juvenile mussels in cages was undertaken to assess if this would be useful in raising early post-parasitic stages (i.e. after glochidia growth on gills of fish hosts) for conservation purposes.

Commencing 1989, large numbers of post-parasitic mussels were obtained by infecting brown trout Salmo trutta in aquaria. Young mussels were collected daily and transferred to a ‘culture cage’. These comprised three plastic sheets (125 x 85 mm) with 96 holes (6 mm diameter) drilled in each. Five juvenile mussels were enclosed within 92 holes (‘cells’) of a 9-mm thick sheet covered by fine plastic gauze. These were placed in four rivers (A1 - supporting the last known northern German mussel population; A2 - mussels until the early 1980s (the most polluted study river); B - mussels extinct (extirpation date unknown); and C - probably never supporting mussels).

Survival and growth were determined at intervals of a sample of at least 30 individuals. Juveniles kept in river B were transferred to A1 (at 13 months old) to assess the possibility of poorer-quality rivers supporting mussels initially reared in more suitable (less polluted) sites; survival and growth were compared. The influence of 12 water chemistry variables on growth and survival was analysed. Experiments in river C were abandoned due to high sedimentation rates.

At the end of the period reported, some musselshad been maintained in cages for up to 52 months. Mortality was high in the initial months (June-December) of the post-parasitic stage in all rivers, but declined subsequently. Growth was restricted to warmer months (April-September) and was similar between rivers (although several individuals in A2 grew much faster). There was a negative relationship between growth and eutrophication. Compared to wild Scottish juveniles, average growth of young in cages was retarded.

Survival over their first winter was largely influenced by size; all <700 µm (22.8% of the total population) died. Large individuals > 900 µm had a 50% chance of reaching their second growing season. Sediment accumulation also influenced survival. Cells containing moderate amounts had higher survival than those with little or none; survival was lowest in cells more than two-thirds full.

Survival and growth of juveniles transferred from river B to Al, was similar those retained in Al.

Overall, survival was equal to free-living juveniles and cage culture appears useful in raising juvenile mussels. Several areas for culture improvement were identified.

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