English: River Class Patrol Vessel HMS Mersey of the Fishery Protection Squadron is pictured at sea near Portsmouth. HMS Mersey is the youngest of the original batch of River-class patrol vessels built for the Fishery Protection Squadron – 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 stick to internationally-agreed quotas. Mersey was the last ship to be built at Vosper Thornycroft yard in Woolston, Southampton. Mersey like her sister ships is not owned by the RN, she is in fact leased by BAE who retain the responsibility for any major maintenance and upkeep. The ship 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 onboard and two weeks ashore catching up on leave or on courses, allowing the ship herself to be on almost constant patrol.