At some time in history, the Chikama clan became a private retainer of the Hōjō Tokusō family, which was the de facto ruler of the shogunate.
Apart from the Manose River, he also owned Bōnotsu, a trading port that was connected to Hakata Bay, China and the Ryukyu Islands.
Another source probably making reference to this is a map of Japan stored at the Kanazawa Bunko, a library of the Hōjō clan.
It depicts a land mass outside the boundary of Japan where the caption reads "龍及國宇嶋身人頭鳥雨見嶋私領郡" (*U-shima, State/Province of Ryūkyū [where people have] a human body but a bird head; Amami Island(s), a privately owned district).
The latter half suggests that (the Hōjō clan considered that) the Amami Islands were not part of Japan but anyway owned by Japanese.