English: River-Class patrol vessel HMS Mersey of the Royal Navy's Fishery Protection Squadron at sea near Portsmouth.

Mersey is the youngest of the original batch of River-Class vessels built for the Fishery Protection Squadron, and is the oldest unit in the Royal Navy. She patrols the waters of the UK and up to 200 miles in the Atlantic ensuring that fishing boats and trawlers adhere to internationally-agreed quotas.

Mersey was the last ship to be built at Vosper Thornycroft yard in Woolston, Southampton.

Mersey and her sister ships are not owned by the RN, but are leased by BAe who retain responsibility for major maintenance and upkeep.

She is a regular visitor to UK ports and often hosts organised groups to show what the Royal Navy is about. She operates a crew rotation system which means that a member of the ship’s company would normally spend four weeks aboard and two weeks ashore on leave or on courses, allowing the ship herself to be on almost constant patrol.