What immediate action should be taken when operating a boat if you hear the fog signal of another boat you cannot see?

If you hear the fog signal of another boat that you cannot see, you should immediately take precautionary measures to ensure safety. Reduce your speed to a safe and manageable level, and maintain a proper lookout for other vessels. Use your sound signals to communicate your presence, and be prepared to take evasive action if necessary. Additionally, consider navigating to a safe area or anchoring if the visibility is extremely low.