Finding the right plaice at the right time:Multi-molecular analysis of flatfish reveals historical catch habitats

Flatfish are ecologically diverse species that commonly occur in marine environments, but also in estuarine and riverine habitats. This complicates the examination of the potential role of flatfish in the ‘marine fish event horizon’, an economic shift in human exploitation from freshwater to marine fish species during the 10–11th centuries CE around the southern North Sea. This study represents the first multi-disciplinary investigation of flatfish remains to make species-specific interpretations of flatfish exploitation. Peptide mass fingerprinting and multi-isotope analysis of carbon (δ13C), nitrogen (δ15N) and sulphur (δ34S) was performed on collagen from 356 archaeological flatfish and 120 comparative archaeological marine or freshwater species to explore the catch habitat of individual flatfish species between 600 and 1600 CE from the North Sea area. European flounder show signals reflecting both freshwater and marine environments, while other flatfish show only those of marine habitats. A subtle shift towards more marine exploitation towards the end of the period is identified, corresponding to the observed transition in targeted species from flounder to plaice throughout the medieval period. Sites show slight differences in δ13C and δ34S within the same species, related to the local environments. Remarkable is the high abundance of marine plaice and flounder during the early medieval period, which shows clear marine or coastal exploitation of flatfish early on, well before the previously accepted onset of the marine fish event horizon. This indicates a gradual shift from coastal to open marine fish exploitation over the medieval period.