The fish that reads the river like a map
The Hilsa, Tenualosa ilisha, can detect a change in water salinity of less than 0.1 parts per thousand. That sensitivity is not incidental. It is the opening move in one of the most precise navigational performances in freshwater biology. Every monsoon, adult Hilsa leave the Bay of Bengal, push into the Ganga, the Brahmaputra, and the Hooghly, and travel up to 1,200 kilometres upstream to spawn. They do this without visual landmarks. They do it against the current. And they arrive, year after year, at the same stretches of river where they were born.
