Abstract: Human populations have relied on fish for food for thousands of years, as evidenced in numerous Palaeolithic archaeological sites across Europe. In this paper, we investigate past populations of brown trout (Salmo trutta L.) along the Bay of Biscay coast before, during, and after the Last Glacial Maximum and compare their phenotypic diversity with modern populations. A total of 270 salmonid vertebrae from 11 archaeological sites in Spain and France were analysed, which span c. 30 to 9 kyr cal B.P. Species identification, combining ZooMS analyses with vertebral body size, confirmed 219 specimens as brown trout, and we present a model for salmonid species identification based on vertebral size. Stable isotope analyses of carbon and nitrogen reveal a continuum of habitat use patterns. After testing multiple published regression models for body length estimation in brown trout, we develop a novel linear regression model alongside a protocol for age estimation. Overall, the analysis of body size, age and migratory ecotype demonstrates that the phenotypic diversity of brown trout populations during the Pleistocene-Holocene transition is comparable to that of modern populations, reinforcing the role of the Iberian glacial refugium in maintaining diversity during the Last Glacial Maximum. The finding of large freshwater trout alongside sea trout in human-occupied archaeological sites further suggests the exploitation of both riverine and marine fish resources by human populations.

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