What immediate action should you take when operating a vessel and you hear the fog signal of another boat you cant see?

When you hear the fog signal of another vessel that you cannot see, you should immediately reduce your speed and maintain a safe course. It’s crucial to keep a lookout for other vessels and use radar or sound signals to assess your surroundings. Additionally, be prepared to take evasive action if necessary to avoid a collision. Always ensure your navigation lights are on and stay alert for further signals.