Population density and age structure of the freshwater pearl mussel, Margaritifera margaritifera, in two Iberian rivers ADOLFO OUTEIRO*, PAZ ONDINA*, CARLOS FERNA ´ NDEZ † , RAFAELA AMARO † AND EDUARDO SAN MIGUEL † *Departamento de Bioloxı ´a Animal, Facultade de Veterinaria, Universidade de Santiago de Compostela, Lugo, Spain † Departamento de Xene ´tica, Facultade de Veterinaria, Universidade de Santiago de Compostela, Lugo, Spain SUMMARY 1. The European freshwater pearl mussel, Margaritifera margaritifera (Bivalvia: Unionoida), is one of the most threatened mussels. The Iberian populations of this species are considered peripheral because their distinct characters such as growth rate and longevity, and require development of effective conservation strategies. 2. We assessed population density and age structure of pearl mussels in two Galician rivers (Eo and Masma in north-west Spain). Four sampling sites were selected in each river to cover stretches of 100 m. The mean density of mussels in each of these sampling transects was estimated using the adaptive sampling technique, given that mussels occur at low densities and are highly aggregated in these rivers. 3. Age structure was inferred for each population using length–age keys. The empty shells encountered during sampling were used to determine the length of the specimens at different ages (years), together with length-at-age data from shells previously analysed for computing growth rates from the same rivers. Water samples from both rivers were analysed for typical physicochemical parameters. 4. Mean densities were very variable, even within the same river (from 0.27 to 6.55 m )2 in the River Eo and from 0.98 to 2.38 m )2 in the River Masma). Individuals in the 0- to 5-year age class were scarce in both rivers. 5. Margaritifera margaritifera showed a preference for the strip of river bed within 1.5 m from the river bank and avoided sites at greater distances. The species also showed a preference for sites with more than 80% tree cover and avoided sites with <50% cover. 6. Iberian populations exhibit the highest growth rate, together with the lowest maximum age and maximum length known for M. margaritifera. Detailed knowledge about these peripheral Iberian populations will contribute to developing strategies for conservation and management of this endangered species. Keywords: adaptive sampling, endangered species, length–age key, Margaritifera margaritifera, peripheral populations Introduction In the last century populations of the freshwater mussel (Margaritifera margaritifera L.) decreased dras- tically throughout the entire geographical range of the species, on the Atlantic basins of Europe and North America at latitudes above 40°N. Many Central European populations have disappeared and others are incapable of reproducing successfully (Bauer, 1983, 1986, 1988; Valovirta, 1998; Ziuganov et al., 1998; Young, Cosgrove & Hastie, 2001a). The decline in populations has been attributed to different types Correspondence: Adolfo Outeiro, Departamento de Bioloxı ´a Animal, Facultade de Veterinaria, Universidade de Santiago de Compostela, 27002, Lugo, Spain. E-mail: outeiro@lugo.usc.es Freshwater Biology (2007) doi:10.1111/j.1365-2427.2007.01913.x Ó 2007 The Authors, Journal compilation Ó 2007 Blackwell Publishing Ltd 1