In order to neutralise this objection and further the anti-slavery campaign, in 1816 Lord Exmouth was sent to secure new concessions from Tripoli, Tunis, and Algiers, including a pledge to treat Christian captives in any future conflict as prisoners of war rather than slaves and the imposition of peace between Algiers and the kingdoms of Sardinia and Sicily. On his first visit he negotiated satisfactory treaties and sailed for home. While he was negotiating, a number of Sardinian fishermen who had settled at Bona on the Tunisian coast were brutally treated without his knowledge. As Sardinians they were technically under British protection and the government sent Exmouth back to secure reparation. On August 17, in combination with a Dutch squadron under Admiral Van de Capellen, he bombarded Algiers. Both Algiers and Tunis made fresh concessions as a result. However, securing uniform compliance with a total prohibition of slave-raiding, which was traditionally of central importance to the North African economy, presented difficulties beyond those faced in ending attacks on ships of individual nations, which had left slavers able to continue their accustomed way of life by preying on less well-protected peoples. Algiers subsequently renewed its slave-raiding, though on a smaller scale. Measures to be taken against the city's government were discussed at the Congress of Aix-la-Chapelle in 1818. In 1820 another British fleet under Admiral Sir Harry Neal again bombarded Algiers. Corsair activity based in Algiers did not entirely cease until its conquest by France in 1830. In thalassocratic Austronesian cultures in Island Southeast Asia, maritime raids for slaves and resources against rival polities have ancient origins. It was associated with prestige and prowess and often recorded in tattoos. Reciprocal raiding traditions were recorded by early European cultures as being prevalent throughout Island Southeast Asia. With the advent of the Islam and the colonial era, slaves became a valuable resource for trading with European, Arab, and Chinese slavers, and the volume of piracy and slave raids increased significantly. Numerous native peoples engaged in sea raiding, they include the Iranun and Balanguingui slavers of Sulu, the Iban headhunters of Borneo, the Bugis sailors of South Sulawesi, and the Malays of western Southeast Asia. Piracy was also practiced by foreign seafarers on a smaller scale, including Chinese, Japanese, and European traders, renegades, and outlaws. The volume of piracy and raids were often dependent on the ebb and flow of trade and monsoons, with pirate season (known colloquially as the "Pirate Wind") starting from August to September.