What immediate action should you take when you hear the fog signal of another boat you cannot see?

When you hear the fog signal of another boat that you cannot see, you should immediately reduce your speed to a safe level and maintain a proper lookout. Use your radar and other navigational aids to assess your surroundings. Be prepared to take evasive action if necessary, and ensure that your sound signals (such as your horn) are ready to communicate your presence. Finally, consider altering your course to increase safety while avoiding potential collisions.