50 CFR 300.173: Vessel identification.

Each U.S. vessel fishing under the Treaty must be marked for identification purposes, as follows:

(a) A vessel used to fish on the high seas within the Convention Area as defined in § 300.211 must be marked in accordance with the requirements at §§ 300.14 and 300.217.

(b) A vessel not used to fish on the high seas within the Convention Area as defined in § 300.211 must be marked in accordance with either:

(1) Sections 300.14 and 300.217, or

(2) The vessel's name and U.S. Coast Guard Documentation number (or if not documented, the state registration number) followed by the letter U must be prominently displayed where they are clearly visible both from the air and from a surface vessel. Numerals and the letter U must meet the size requirements of § 660.704 of this title. Markings must be legible and of a color that contrasts with the background.