As far as it is possible to tell, slavery seems to have increased in the early-19th century with increased numbers of prisoners being taken by Māori military leaders (such as Hongi Hika and Te Rauparaha) to satisfy the need for labor in the Musket Wars, to supply whalers and traders with food, flax and timber in return for western goods. The intertribal Musket Wars lasted from 1807 to 1843; northern tribes who had acquired muskets captured large numbers of slaves. About 20,000 Maori died in the wars, which were took place mainly in the North Island. An unknown number of slaves were captured. Northern tribes used slaves (called "mokai") to grow large areas of potatoes for trade with visiting ships. Chiefs started an extensive sex trade in the Bay of Islands in the 1830s, using mainly slave girls. By 1835 about 70 to 80 ships per year called into the port. One French captain described the impossibility of getting rid of the girls who swarmed over his ship, outnumbering his crew of 70 by 3 to 1. All payments to the girls were stolen by the chief. By 1833 Christianity had become established in the north of New Zealand, and large numbers of slaves were freed.