50 CFR 25.31: General provisions.

Whenever a particular public access, use or recreational activity of any type whatsoever, not otherwise expressly permitted under this subchapter, is permitted on a national wildlife refuge or where public access, use, or recreational or other activities previously permitted are curtailed, the public may be notified by any of the following methods, all of which supplement this subchapter C:

(a) Official signs posted conspicuously at appropriate intervals and locations;

(b) Special regulations issued under the provisions of § 26.33 of this subchapter C.

(c) Maps available in the office of the refuge manager, regional director, or area director, or

(d) Other appropriate methods which will give the public actual or constructive notice of the permitted or curtailed public access, use, or recreational activity.