Fisheries Sea Days - For their sea day, the Fishery Protection Patrol Vessels MERSEY and SEVERN were able to demonstrate both their navigational skills with a series of co-ordinated complex manoeuvres at high speed; and showed the Force Protection capabilities with a live firing exercise using their general purpose machine gun and 20mm cannon.     Present was Second Sea Lord Vice Admiral (David Steel), who spent the day on board,    Each of the three Fishery Protection Patrol Vessels (the TYNE is the other one) each spend over 300 days each at sea every year.     Working with the Marine Management Organisation, the patrol duties of the fishery limits surrounding England and Wales are shared between the three ships.