Even before the end of hostilities, the Ainu and the peoples of the Amur region had been secretly trading valuable furs with the connivance of the Mongol officials in Nurgan. Soon after the subjugation of the Ainu, Ainu elders made tributary visits to Yuan posts located at Wuliehe (兀列河; in the Tym basin), Nanghar (囊哈兒; near present-day Langry), and Boluohe (波羅河; the Poronay), and received gifts in return, making presenting tribute a form of trade. The center of the northern trade gradually shifted to Chinese posts in the Amur Estuary and on Sakhalin following the Mongol conquests, and Sakhalin itself became a conduit of trade between the Mongol Empire in mainland Eurasia and the Japanese archipelago. The Mongols retreated from Sakhalin after 1320, and tributary trade in Sakhalin and the Amur basin ceased in the mid 14th century as the Yuan dynasty fell into decline. The Chinese under the Ming dynasty re-established a presence in the area in 1409 and collected tribute from the Nivkh and the Ainu until late 15th century. Regardless of the political situation, a network of trade linking Manchuria, Japan, and Kamchatka with Sakhalin at the center persisted until the 18th century. The Mongol peace put a stop to the conflicts between the Nivkh and the Ainu, with the Ainu keeping to southern Sakhalin, leaving the north to the Nivkh. Hostility gave way to a relationship characterized by intermarriage, commercial trade, and cultural exchanges between the two peoples. A number of important cultural elements that became the hallmarks of modern Ainu culture, including the iomante bear ceremony, were introduced to the Ainu from the Okhotsk culture carried by Nivkh as a result. For the Nivkh, the increasing focus on trade led to the rise of the ethnographic Nivkh culture at the expense of the original Okhotsk culture: while rituals involving orcas and bears remained, other cultural characteristics like pottery disappeared. The peace between the Nivkh and the Sakhalin Ainu as well as the Mongol presence in Sakhalin also meant that the Hokkaido Ainu could not freely migrate to Sakhalin across the La Pérouse Strait as they had done over the centuries. The population pressure that pushed the Ainu north now encountered resistance, and the resulting reaction in the southern direction brought the Ainu into increasing conflict with the Japanese. Around the time of the Mongol invasions of Sakhalin, the Ainu of Tsugaru rose up against the