Which signal may be exhibited by a vessel trawling at some time?

The signal indicating that a vessel is engaged in trawling consists of a white light over a red light arranged in a vertical line. This is a specific identification signal used to alert other vessels to the nature of the trawler's activity, indicating that it is actively fishing and has gear deployed.

This signal is essential for ensuring the safety of navigation, as it informs nearby vessels to exercise caution when approaching or passing the trawling vessel, particularly because fishing gear may extend from the vessel and could pose a hazard. The visibility of these lights helps in maintaining safe distances and avoiding entanglement with fishing lines or nets.

The other signals do not represent a trawling activity. Two white lights in a vertical line typically indicate that a vessel is engaged in fishing but not specifically trawling. Two red lights in a vertical line signify a vessel not under command, and one green light does not represent any specific activity relevant to trawling. Hence, the signal of a white light over a red light is the correct and recognized signal for a vessel that is trawling.