However, the economic warfare was intensified in a way that amounted to a veritable siege of the Republic as a whole. In the first place, the naval war intensified. The Spanish navy harassed Dutch shipping, which had to sail through the Strait of Gibraltar to Italy and the Levant, thereby forcing the Dutch to sail in convoys with naval escorts. The cost of this was borne by the merchants in the form of a special tax, used to finance the Dutch navy, but this increased the shipping rates the Dutch had to charge, and their maritime insurance premiums also were higher, thus making Dutch shipping less competitive. Spain also increased the presence of its navy in Dutch home waters, in the form of the "armada" of Flanders, and the great number of privateers, the Dunkirkers, both based in the Southern Netherlands. Though these Spanish naval forces were not strong enough to contest Dutch naval supremacy, Spain waged a very successful "Guerre de Course", especially against the Dutch herring fisheries, despite attempts by the Dutch to blockade the Flemish coast.