What is considered a mandatory distress signal when at anchor?

In the context of vessels at anchor, a mandatory distress signal is not explicitly outlined for that scenario under the COLREGs (International Regulations for Preventing Collisions at Sea) or other relevant maritime statutes. When vessels are at anchor, they are not typically considered to be in immediate distress, as the act of anchoring itself is a stable state of operation.

While flares, sound signals, and radio signals are used in various circumstances to communicate distress, none of these are categorized as mandatory signals for a vessel that is simply at anchor. Flares can signal distress situations but need to be used when there is an actual emergency. Sound signals, such as horns or bells, could indicate an emergency situation but are not specifically mandated for vessels at anchor. Radio signals can be used to communicate distress, but again, this is dependent on the circumstances surrounding the situation rather than a mandatory requirement.

In summary, when a vessel is at anchor, there is no defined mandatory distress signal that must be employed under those conditions, leading to the conclusion that "neither" is considered a mandatory distress signal when at anchor.