Illegal fishermen were caught and their fish confiscated by residents concerned they were fishing unlawfully in the River Blackwater.
Simon Wakefield, 57, company director at Yellow Admiral, a marine recruitment firm, spotted the fishermen near the Stone Inn.
The fishermen had a seal trapped in their nets and were catching undersized sea bass illegally.
Legally sea bass can only be caught with a rod and line when they are 36cm or over.
Mr Wakefield and his neighbours confiscated the fish, which had died, and contacted the coast guard, police and Maldon District Council.
A council spokesman said: “That part of the river is a public fishery so anybody can set their own nets, but everyone must adhere to the strict size limits and no undersize fish can be taken for any purpose."