Page 25 of the rulebook states:
Taking Control of Enemy Ports - If the land area connected to a Port is successfully attacked and taken control of by another player, that player may immediately replace any enemy Ship units in the Port with an equal number of his own available Ship units (or as many Ship units as he has available, he desires, or is allowed by supply limits). Excess enemy ships are simply removed from the board.
What does available mean in this context? Available as in a player has ships left that (s)he has not yet mustered or available as in ships that have already been mustered?
Where do the own ships come from? Are they existing ships on the board that simply move into the port now or are new ships mustered instead?