Pearl mussel density is very low at all sites surveyed (Appendix 1). We located a total of 252 individuals from both streams, but only 78 (31.0 %) were alive (Fig. 2B); 79 individuals (31.3 %) had likely died after harvesting by local people (Fig. 2C) and 95 shells (37.7 %) probably died by other causes (e.g., post-flood stranding, mammal and bird predation, disease). The largest mussel population was found in the River Nam Long (66 individuals, 84.6%). The majority of individuals encountered were observed near the riverbank in the shadow of overhanging trees and bushes (67 individuals; 85.9 %). The remaining 11 mussels (14.1 %) were found in mid-channel habitats. Pearl mussels primarily occurred in coarse sand and fine gravel substrates, frequently in micro–grottos downstream from cobble and boulder substrates (58 individuals; 74.4 %). Twenty individuals (25.6 %) occupied sandy and fine gravel substrates between stones. Many of the mussels encountered (~50%) were almost completely burrowed beneath the substrate with only t