(a) Acreage limitation and exceptions. (1) On the mainland such leases may be for an area not exceeding 640 acres. A lease may cover an entire island, provided the area thereof does not exceed 30 square miles, and provided the need for such entire island is clearly established. Islands so close together that animals can cross from one to the other and whose combined area does not exceed 30 square miles, will be treated as one island. Islands having an area of more than 30 square miles will be treated as mainland.

(2) Where a lease is granted for an area in excess of 640 acres on an island, the manager may, after notice to the lessee, reduce the area to an amount not less than 640 acres, if he determines that the lessee cannot reasonably use all of the area for which the lease was granted.

(b) Lands subject to lease. (1) Vacant, unreserved, and unappropriated public lands are subject to lease.

(2) Except for lands under the jurisdiction of the Fish and Wildlife Service and the National Park Service, public lands withdrawn or reserved for any purpose are subject to lease, if the department or agency having jurisdiction thereof consents to the issuance of the lease.