50 C.F.R. § 697.9
Facilitation of enforcement
(a) General. See § 600.730 of this chapter.
(b) Radio hails. Permit holders, while underway, must be alert for communication conveying enforcement instructions and immediately answer via VHF-FM radio, channel 16, when hailed by an enforcement officer. Vessels not required to have VHF-FM radios by the Coast Guard are exempt from this requirement.