In its current sea zone a fleet can patrol, attempting to intercept all hostile ships, by using the telescope cursor. A fleet can blockade a designated port, using a ship with a red X cursor, by clicking near the port when that cursor appears. Both of these commands are used against enemy forces. A blockade has greater chance of success against ships that enter or leave the selected port. A blockade does not seek other ships in the sea zone, but hostile patrolling fleets can intercept your blockaders.
Both blockades and patrols can result in battle against hostile warships or in interception of merchant ships. When merchants are intercepted the intercepting forces can sink or capture the merchant ships. Additionally, it is possible to capture the cargo being carried. These details are summarized in the battle report at the end of the turn.
