The Migration Problem a Dam Creates
Mahseer in the Cauvery and hilsa in the Ganga do not choose to migrate. Their reproductive biology compels it. Hilsa, Tenualosa ilisha, travel up to 1,200 kilometres from the Bay of Bengal into freshwater to spawn, navigating by olfactory cues, the chemical signature of the river reach where they hatched. A concrete barrage does not redirect that instinct. It terminates it. Upstream populations are cut off from downstream gene flow, and downstream populations lose access to spawning gravels. Within one or two generations, local stocks collapse into reproductively isolated remnants.
