Changes in the Asian Clam Population in a Tropical Mesotrophic Reservoir during Severe Drought

Abstract

This study examined changes in the Asian clam (Corbicula spp.) population of a productive mesotrophic reservoir during severe drought. Two important clam habitats of the Ubolratana reservoir in northeastern Thailand were surveyed: Tha Lat and Nong Muad Ae. Sampling was performed five times during periods of low storage volume (22–25 % of maximum capacity) in April, June, July, August, and November 2019. Both clam areas had low population densities, and density tended to decrease over the study period. The adult Corbicula clams ranged in size from 16 to 25 mm (shell length). Total clam density had negative relationships (p<0.05) with water temperature and sedimentary organic content.  The clam habitat of Nong Muad Ae tended to be more suitable than Tha Lat because it had a sandy sediment with comparatively lower water content (17.79–33.03 %) and organic matter content (1.60–5.09 %). During all of the sampling periods, the level of sedimentary sulfides remained at a safe level for benthic organisms. However, the inflows during 2019 were also dramatically decreased compared to 2018, and are expected to restrict population development. No new-born Corbicula clams were found except in the last sampling in November 2019. The density of small (0–10 mm) clams had positive relationships (p<0.05) with the prior five months’ rainfall, inflow, relative change of inflow, and relative change of water storage. The overall view of this study implies that the Corbicula population in this reservoir may reach a deteriorated status during periods of ongoing drought. Future management approaches should therefore be conservative and carefully considered.