Implications for conservation
Threats to Indochinese pearl shell mussel populations in the Nam Ou Drainage include three main issues. First, M. laosensis is a traditional food harvested by rural villagers in the Phongsali Province of Laos. Villagers also harvest and sell thiarid gastropods and corbiculid bivalves. Villagers harvest Margaritifera incidentally when foraging for other freshwater species, typically during the dry season (Fig. 2C). Second, river regulation and dam construction threaten Indochinese pearl mussels. Simple dams made of stones, trunks or bamboo stakes are frequently constructed by villagers to increase water levels by 0.3–1.0 m. The upper Nam Pe River has three such primitive dams and the Nam Long River has two dams. Impounded reaches upstream from these dams contain thick layers of decomposing plant litter, and M. laosensis were not found in the dammed areas of rivers. Finally, deforestation and agriculture are leading to increased water pollution in this region. Deforestation is widespread in the Nam Long and Nam Pe River Basins, but is primarily associated with slash-and-burn agriculture. The most widespread types of land cover include evergreen shrubs and fragmented or regenerating forests (~80 % of total area in our study watershed). Steep, devegetated mountain slopes and river valleys lacking plant coverexperience intense erosion during monsoon rains. As a result, many streams are highly impacted by clay and other fine sediments. Interestingly, N and P concentrations remain very low in rivers because fertilizer use is uncommon and most villages and cattle farms in the region are small or not located close to our study rivers (Table 1).