Minefield Decay

Minfefields decay at a specified rate. Those in deep water decay by 33% per day, those in shallow water decay by 5% per day, and those in friendly bases with a port size 1 or higher decay by 1% per day. [Manual, section, p. 134]

Minefield Maintenance at Bases

Minefield Tenders (ship type ACM) can maintain the mines in a friendly base. Each ship can maintain 150 mines per day, and no action is required of the player. To fill this role, an ACM must be in port and not part of a task force. [Manual, section, pp. 134-5]

There are some basic requirements for minefield maintenance to take place. First, the port must have fuel available. If the ACMs can't be fueled, they can't maintain mines. Second, the ACMs must be relatively free of damage. Their limit is about 10% (the exact number is either flexible or not published). [Forum post by Don Bowen, 29 Aug 2010]

ACMs do not use up the supply of mines in their maintenance role. It is assumed that they are repairing mines, not replacing them. [Forum post by Don Bowen, 19 Apr 2011]