In a hostile 1642 encounter between Ngāti Tūmatakōkiri and Dutch explorer Abel Tasman's crew, four of Tasman's crew members were killed, and at least one Māori was hit by canister shot. Europeans did not revisit New Zealand until 1769, when British explorer James Cook mapped almost the entire coastline. Following Cook, New Zealand was visited by numerous European and North American whaling, sealing, and trading ships. They traded European food, metal tools, weapons, and other goods for timber, Māori food, artefacts, and water. The introduction of the potato and the musket transformed Māori agriculture and warfare. Potatoes provided a reliable food surplus, which enabled longer and more sustained military campaigns. The resulting intertribal Musket Wars encompassed over 600 battles between 1801 and 1840, killing 30,000–40,000 Māori. From the early 19th century, Christian missionaries began to settle New Zealand, eventually converting most of the Māori population. The Māori population declined to around 40% of its pre-contact level during the 19th century; introduced diseases were the major factor.