Acumulações de conchas mortas de moluscos na ilha da Ínsua (Caminha, Portugal)

Abstract

Following information gathered in the historical Franciscan chronicles by Fr. Manoel da Esperança (1666) and Fr. Pedro de Jesus Maria José (1760), we found in Braga District Archive a transcription (probably dated from the 18th century) of a document, composed of two parts (dated 1441 and 1467), about the shells of Ínsua island. According to the first part of the document, Caminha Town Council had been levying taxes on seashells that Franciscans collected in Ínsua island. Franciscans protested and managed to have these taxes cut off. However, some years later, Caminha Town Council raised again this kind of tax, since in the second part of the document, the House of Vila Real, which owned the landlord of Caminha since 1464, informs the Town Council that could no longer tax the shells, since they then possess the rents of Caminha. Such an acute contest about this natural resource could result from the fact that seashells were indeed very important and abundant in the island. The importance was probably due to their use in lime manufacture (mentioned by Manoel da Esperança), at that time an indispensable ingredient to prepare mortars used in buildings, and probably also as an agricultural fertilizer. In order to check seashells abundance at Ínsua and understand their dynamic accumulation, we carried out field work in the summer 2010. Big accumulations of dead mollusc shells were indeed found, mainly in the west and south sides of the island, with a total mass exceeding 10 tons. Shells of Mytilus galloprovincialis, Nucella lapillus, Patella spp. and Gibbula umbilicalis, dominated the deposits, with some specimens of big size. Estuarine and fluvial molluscs were residual. It was concluded that these accumulations probably result from molluscs that live in the rocks surrounding the island and are released from the substrate after dead or strong tempests. The abundance and prosperity of the molluscs of Ínsua island are probably related with high levels of nutrients of currents that run along the island, dragging waters from river Minho. In the whole, these deposits form the biggest natural accumulation of dead seashells in the Portuguese continental coast. This is a rare (or unique) case of the use of seashells for lime production in medieval Portugal. Results of the biological research program support the authenticity of the document analyzed and of the Franciscan chronicles.