Fishing is prohibited in March.

I advise you not to eat March crucian carp, there are thousands of fish in your stomach.

This sentence means to advise you not to eat crucian carp in March, which is the breeding season of crucian carp, and thousands of young fish in Qian Qian are in their bellies.

The breeding season of many fish begins around March, so it is recommended not to go fishing during this time.