46 USC 7312 - Scale of employment

(a) Individuals qualified as able seamenunlimited under section 7307 of this title may constitute all of the able seamen required on a vessel.
(b) Individuals qualified as able seamenlimited under section 7308 of this title may constitute all of the able seamen required on a vessel of less than 1,600 gross tons as measured under section 14502 of this title, or an alternate tonnage measured under section 14302 of this title as prescribed by the Secretary under section 14104 of this title or on a vessel operating on the Great Lakes and the Saint Lawrence River as far east as Sept Iles. Individuals qualified as able seamenlimited may constitute not more than 50 percent of the number of able seamen required on board other vessels.
(c) Individuals qualified as able seamenspecial under section 7309 of this title may constitute
(1) all of the able seamen required on a vessel of not more than 500 gross tons as measured under section 14502 of this title, or an alternate tonnage measured under section 14302 of this title as prescribed by the Secretary under section 14104 of this title or on a seagoing barge or towing vessel; and
(2) not more than 50 percent of the number of able seamen required on board other vessels.
(d) Individuals qualified as able seamenoffshore supply vessels under section 7310 of this title may constitute all of the able seamen required on board a vessel of less than 500 gross tons as measured under section 14502 of this title, or an alternate tonnage measured under section 14302 of this title as prescribed by the Secretary under section 14104 of this title engaged in support of exploration, exploitation, or production of offshore mineral or energy resources.
(e) When the service of able seamenlimited or able seamenspecial is authorized for only a part of the required number of able seamen on board a vessel, the combined percentage of those individuals so qualified may not be greater than 50 percent of the required number.
(f) Individuals qualified as able seamenfishing industry under section 7311a of this title may constitute
(1) all of the able seamen required on a fish processing vessel entered into service before January 1, 1988, and of more than 1,600 gross tons but not more than 5,000 gross tons as measured under section 14502 of this title, or an alternate tonnage measured under section 14302 of this title as prescribed by the Secretary under section 14104 of this title; and
(2) all of the able seamen required on a fish processing vessel entered into service after December 31, 1987, and having more than 16 individuals on board primarily employed in the preparation of fish or fish products but of not more than 5,000 gross tons as measured under section 14502 of this title, or an alternate tonnage measured under section 14302 of this title as prescribed by the Secretary under section 14104 of this title.