Downstream Migration of Juvenile Fish Associated with the Drift of Water Hyacinth ( Eichhornia crassipes )

Passive downstream migration of juvenile fish associated with the invasive water hyacinth ( Eichhornia crassipes ) was investigated in the delta of the Mekong River. The behavioral program of juveniles aimed at leaving the coastal attached vegetation and entering the transit flow for migration was the same in the case with drifting water hyacinth. In general, the passive downstream migration has three components: the drifting of juveniles with thickets of water hyacinth; the entry of juveniles into the transit flow from coastal biotopes during the twilight-nocturnal period; the entry of juveniles into the transit flow from thickets of drifting hyacinth during the twilight-nocturnal period. It should be noted that downstream migrations of juvenile fish associated with drifting water hyacinth in the Mekong Delta may end by their mass death in salt water after reaching the sea with floating hyacinth.