There are always few comercial folks that do netting at the mouth of the backwater (for Mullets) where it opens in the sea. A guy from Sri Lanka said that it was illegal to net at the river mouth in Sri Lanka as it affects the ecosystem largely and also told me that it is possibly banned in India too. I heard from Pro guys here that neting at the river mouth in Kovalam makes it very hard to catch other species like Barracuda, Barramundi and Giant Trevallys by sport fishermen in that area as the neting activity disturbs the entire area heavily.
I am not sure if it is legal to net at the river mouth area in India. Also any input on this topic is very much welcome.