The first contact between the Māori and Europeans was in 1642, when Dutch explorer Abel Tasman arrived and was fought off, and again in 1769 when the English navigator Captain James Cook claimed New Zealand for Britain at the Mercury Islands. Nevertheless, the British government showed little interest in following up this claim for over half a century. The first mention of New Zealand in British statutes is in the Murders Abroad Act of 1817, which clarified that New Zealand was not a British colony (despite being claimed by Captain Cook) and "not within His Majesty's dominions". Between 1795 and 1830 a steady flow of sealing and then whaling ships visited New Zealand, mainly stopping at the Bay of Islands for food supplies and recreation. Many of the ships came from Sydney. Trade between Sydney and New Zealand increased as traders sought kauri timber and flax and missionaries purchased large areas of land in the Bay of Islands. This trade was seen as mutually advantageous, and Māori tribes competed for access to the services of Europeans that had chosen to live on the islands because they brought goods and knowledge that were essential to the local "iwi" (the Māori word for the social unit often called "tribe" or "people"). At the same time, Europeans living in New Zealand needed the protection that Māori chiefs could provide. As a result of trade, Māori society changed drastically up to the 1840s. They changed their society from one of subsistence farming and gathering to cultivating useful trade crops.