There are many things that are not allowed by the Constitution of India, especially with regard to protecting wildlife and habitat. Unfortunately, lots of these things are conveniently overlooked by the very people who are tasked with protecting them.
Historically, in many places around the World, anglers set up bodies to protect the environment in which they pursue their sport. They have the utmost regard for the fish they try to catch and wish to preserve the river or wetland habitat.
In so many places, if it were not for anglers, the authorities would get away with breaching their duties.
I am not arguing about the merits of the practise of angling at this point, merely stating: without anglers, the rivers will die!

10 years ago, the upper reaches of the Kaveri were being systematically poisoned and netted for anything that swam. Thanks to a local group, who have angling as a primary consideration with regard to the river, it is now a haven for all kinds of wildlife. Still under extreme pressure, but safe for now.
The Forestry Dept long since gave up any pretence that they were concerned or could protect this area. In the last two years they have extended effective control of the entire catchment to this group thanks to the work they do.
Without angling, this group would be far less a body and may even struggle to survive.