What is the recommended signal for a vessel engaged in fishing?

The recommended signal for a vessel engaged in fishing is indeed a prolonged blast. This signal serves as a way to indicate the vessel's activity to other nearby vessels, thereby promoting safety in navigational waters. When a fishing vessel uses a prolonged blast, it communicates its presence and helps prevent misunderstandings or potential collisions with other vessels that may be navigating in the area.

In maritime communication, the acknowledgement of various signals helps crew members and vessel operators understand the nature of nearby vessels' activities. A prolonged blast specifically denotes that the vessel is engaged in fishing, making it an essential part of maintaining situational awareness on the water.

Other choices do not correlate with the specific signal prescribed for fishing vessels, which is why they do not serve as the correct answer in this context. Instead, they represent different signals used in other navigational circumstances, which are distinct from the identification of a fishing vessel.