Reading The Making of Modern Japan, I read that the Daimyo of Tsushima, through whom all trade with Korea went, had a domain of 5,000 koku, most of which were from the Korea trade. Since daimyos were strictly defined as being those samurai with estates rating over 10,000 koku, how was the ruler of Tsushima a Daimyo?
This might be a mistake on the part of the author. While the island of Tsushima was valued at about 6000 koku the domain itself had a couple of external fiefs in Hizen province (modern Saga prefecture) that push it fairly comfortably over 10,000 koku.
Though due to the vital role played by the domain in diplomacy and trade with Korea, it was given treatment far above its agricultural production.