Damming in the Mekong River and Its Impacts on Fish Migration

Although scientists agree that a significant portion of fishes in the Mekong River is migratory throughout their lives, little quantitative investigations have been conducted to study migration ecology of the species. It is, however, imperative that the major migration routes of at least commercially and biologically important fish species and their crucial habitats (e.g. spawning habitats) along the routes be identified in order to predict and avoid the adverse effects of dams that are currently planned in the basin.

New technology is now available to scientists to analyze an elemental signature of a small calcified structure called otolith and to ultimately elucidate migration history of individual fish based on the signature. Otolith, or ear bone, continually grows by incorporating calcium carbonate from ambient water as fish grows together with other minor or trace elements such as strontium, barium, manganese, magnesium, etc. Since the otolith is an acellular and metabolically inert tissue, any chemicals accreted on to the growing surface are permanently retained, serving as a “data logger” as to chemical environments to which the fish has been exposed.

This technology has been applied to Siamese mud carp (Henicorhynchus siamensis), the most abundant in the Mekong and yet little understood species about their ecology. The otoliths of approximately 150 individuals collected at > 15 locations throughout Laos, Thailand and Cambodia were analyzed using a laser ablation inductively coupled plasma mass spectrometry. The concentrations of elements such as strontium and barium showed significantly positive correlations between otolith “surface” and river water where the fish was collected, indicating these elements can be used as a marker when reconstructing the migration history. Elemental signatures along the core-to-edge axis of otolith sections were 1) considerably similar between individuals collected at the same locations, 2) different between locations, especially those in different tributaries, and 3) were generally symmetrical or U shaped. These observations suggest that individuals of a given population migrate in schools through the same migration routes, that different populations have different migration routes, and that the species has a homing instinct to return to native spawning grounds.