HMS Mersey, one of the Royal Navy's three fishery protection vessels, was on routine duties when she spotted the FV Elizabeth sailing 40 nautical miles (74km) off the Norfolk coast.
The Commanding Officer decided to send over a boarding team, led by Marine Enforcement Officer Midshipman Phill Fordham, to carry out a full inspection.
On the vessel, the team found that the two cod-ends (the section of the net that catches and holds the fish) were significantly undersized, meaning that young fish and vulnerable species were unable to escape as the nets were dragged through the water.
On reporting the infringement to the Marine Management Organisation, HMS Mersey issued the trawler's captain with an £8,000 financial administration penalty.
The Commanding Officer of HMS Mersey, Lieutenant Commander Mark Anderson, said:
"This is a great result for my ship's company and another example of the continued effectiveness of Royal Navy fishery protection vessels working to enforce important fisheries legislation in and around UK waters."
HMS Mersey is one of three ships that form the Royal Navy's Fishery Protection Squadron, the oldest of the Royal Navy's squadrons. The ships patrol UK and EU waters inspecting fishing vessels at sea and enforcing fisheries regulations on behalf of the Marine Management Organisation, a government agency.