Why hilsa fishes come to the rivers from the sea to lay egg in bangladesh??

Question

Why hilsa fishes come to the rivers from the sea to lay egg in bangladesh??

Answer 1

Its not about migration.

Eggs of Hilsa are  very sensible.

In sea/ocean due to continuous waves, they can not lay eggs.

So they come inside rivers where the water is much more calm than the sea.

Also only after coming inside the river they becomes more tasty than when caught from the sea. Thats why in the sea they always need to fight with the sea waves but after coming to river its like taking rest for them. So they accumulate fat in their bodies which in turn makes them tastier.

Answer 2

They're probably migrating from place to place.