Hostilities increased further with the arrival of 600 colonists from Norfolk Island between 1807 and 1813. They established farms along the River Derwent and east and west of Launceston, occupying 10 percent of Van Diemen's Land. By 1824 the colonial population had swelled to 12,600, while the island's sheep population had reached 200,000. The rapid colonisation transformed traditional kangaroo hunting grounds into farms with grazing livestock as well as fences, hedges and stone walls, while police and military patrols were increased to control the convict farm labourers. Violence began to spiral rapidly from the mid-1820s in what became known as the "Black War". While black inhabitants were driven to desperation by dwindling food supplies as well as anger at the prevalence of abductions of women and girls, whites carried out attacks as a means of exacting revenge and suppressing the native threat. Van Diemen's Land had an enormous gender imbalance, with male colonists outnumbering females six to one in 1822—and 16 to one among the convict population. Historian Nicholas Clements has suggested the "voracious appetite" for native women was the most important trigger for the explosion of violence from the late 1820s. From 1825 to 1828 the number of native attacks more than doubled each year, raising panic among settlers. Over the summer of 1826–7 clans from the Big River, Oyster Bay and North Midlands nations speared stock-keepers on farms and made it clear that they wanted the settlers and their sheep and cattle to move from their kangaroo hunting grounds. Settlers responded vigorously, resulting in many mass-killings. In November 1826 Governor George Arthur issued a government notice declaring that colonists were free to kill Aborigines when they attacked settlers or their property and in the following eight months more than 200 Aborigines were killed in the Settled Districts in reprisal for the deaths of 15 colonists. After another eight months the death toll had risen to 43 colonists and probably 350 Aboriginals. Almost 300 British troops were sent into the Settled Districts, and in November 1828 Arthur declared martial law, giving soldiers the right to shoot on sight any Aboriginal in the Settled Districts. Martial law would remain in force for more than three years, the longest period of martial law in Australian history. In November 1830 Arthur organised the so-called "Black Line", ordering every able-bodied male colonist to assemble at one of seven designated places in the Settled Districts to join a massive drive to sweep Aboriginals out of the region and on to the Tasman Peninsula.