The herring trade, an important pillar of the Dutch economy, was hurt much by the other Spanish forms of economic warfare, the embargo on salt for preserving herring, and the blockade of the inland waterways to the Dutch hinterland, which were an important transportation route for Dutch transit trade. The Dutch were used to procuring their salt from Portugal and the Caribbean islands. Alternative salt supplies were available from France, but the French salt had a high magnesium content, which made it less suitable for herring preservation. When the supplies in the Spanish sphere of influence were cut off, the Dutch economy was therefore dealt a heavy blow. The salt embargo was just a part of the more general embargo on Dutch shipping and trade that Spain instituted after 1621. The bite of this embargo grew only gradually, because the Dutch at first tried to evade it by putting their trade in neutral bottoms, like the ships of the Hanseatic League and England. Spanish merchants tried to evade it, as the embargo also did great harm to Spanish economic interests, even to the extent that for a time a famine threatened in Spanish Naples when the Dutch-carried grain trade was cut off. Realizing that the local authorities often sabotaged the embargo, the Spanish crown built up an elaborate enforcement apparatus, the "Almirantazgo de los países septentrionales" (Admiralty of the northern countries) in 1624 to make it more effective. Part of the new system was a network of inspectors in neutral ports who inspected neutral shipping for goods with a Dutch connection and supplied certificates that protected neutral shippers against confiscation in Spanish ports. The English and Hanseatics were only too happy to comply, and so contributed to the effectiveness of the embargo.