To protect depleting fish stocks, Assam enforces regulations on:

The correct answer is: d) All of the above

Assam enforces regulations on mesh size of nets, closed fishing seasons, and minimum catch sizes to protect depleting fish stocks.

• Mesh size of nets is regulated to prevent the capture of small fish that have not had a chance to reproduce.
• Closed fishing seasons are established to allow fish populations to recover during times when they are most vulnerable to fishing.
• Minimum catch sizes are set to ensure that only fish that are large enough to have reproduced are caught.

These regulations are designed to ensure the sustainability of fish stocks in Assam.