The marshes have always known cycles of prosperity and decline, due to series of bad or excellent harvests, disastrous storms or demand problems. As there were limited means of storage, surpluses were stored in a simple layer of earth until the middle of the 20th century.
The true historical centre of salt "from Guérande" has always been Batz and its villages, Kervalet, Trégaté and Kermoisan, where it has been almost the unique activity for a thousand years. Trading activities were also predominant, thanks to the presence of the sea for transport via the ports of Croisic and Pouliguen, before the creation of the railway (initially used for transporting salt), then road transport.
From the beginning of the basin's development, Guérande pushed its salt workers out of its walls to the villages of Saillé, Clis and Queniquen, but the town didn't forget to reap the colossal profits. In the 9th century, the term "Varrande" (Guérande) was well known to the Scandinavians; two centuries later we find "Garrande". It was salt which made the riches of the town, well before its fortification.
Up to the 17th century, the salt trade and transport was assured by the Bretons, who used the sea and rivers to ensure its distribution, whilst enabling the local lords to levy a tax on boat traffic. In the 17th century, trade became more international, with the Dutch, then the Swedes in the 18th century.
At the end of the 18th century, the Guérande people complained of unfair competition due to taxes and from Russian and Iberian products, and despite the better quality local product (there were guarantees that the salt was not mixed with that of Noirmoutier), exports declined.
The 19th century was marked by the disastrous consequences of Napoleon's European policies. Guérande salt was affected by new taxes replacing the "gabelle" (an unpopular salt tax), the continental blockade and an 1840 law which reduced import taxes and opened western markets to rock salt.
At the beginning of the 20th century, the French salt market was still regional, with each region using its own local production method. Guérande salt supplied the west of France through numerous traders working directly with the producers.
From the middle of the 1960s, improvements in road transport and the industrialisation of production in the south and east of France hurt the small regional producers who were also now experiencing European competition. It is from this date that the "Compagnie des Salins du Midi et des Salines de l'Est" (Company of salt works in the South and East) was created, with the aim of organising the monopoly in French salt production.
During the 1970s, a "salt war" took place in Europe. The trend for refined products and the reduction in cooking salt consumption as a conservation product led to a reduction in demand for salt "de terroir" and the failure of numerous traders in the Guérande basin. The most important were bought out by the "Compagnie des Salins du Midi et des salines de l'Est", which opted for the disappearance of the western salt works, which were considered to be outdated and not profitable enough.
This situation led to a reduction in prices, the closure of numerous production sites and the disappearance of many companies. Guérande salt fell from favour, sales slumped and the salt marshes declined.