A vessel showing a green light over a white light in a vertical line above the level of the sidelights is __________.

A vessel that displays a green light over a white light in a vertical line above the level of the sidelights is indeed engaged in trawling. This combination of lights is specifically mandated by the International Regulations for Preventing Collisions at Sea (COLREGs) to indicate that the vessel is fishing by trawling, which involves dragging a net through the water behind the boat.

The green light indicates the starboard side of the vessel, while the white light typically indicates that the vessel is engaged in a specific activity, which in this case is trawling. This configuration is crucial for other vessels on the water to recognize the activity the trawling vessel is engaged in, helping to prevent collisions and ensure safe navigation.

Understanding the significance of these lights is important for mariners, as it allows them to assess the situation and take appropriate action based on the activities of the vessels around them. Other types of fishing vessels or those operating under different conditions have different light configurations, which is why correctly identifying this specific arrangement is vital for safe navigation.