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Same old Salmo? Changes in life history and demographic trends of North Iberian salmonids since the Upper Palaeolithic as revealed by archaeological remains and beast analyses

Life history traits determine fitness and hence the ability of populations and species to survive through adverse conditions. Therefore, identifying temporal changes in life history traits over large timescales is necessary to understand and predict the effect of current global change on wild populations. In this study, we compare life history traits between Palaeolithic and present‐day vertebrates, analysing the number of winters spent in the river and at sea by North Iberian salmonids (Salmo salar and Salmo trutta) from two separate time frames: the Upper Palaeolithic, based on archaeological remains, and the present day, based on sport catches. The river stage did not change significantly, but the marine period has been shortened in modern anadromous specimens, accordingly shortening mean generation time. Population growth rates have been estimated through Bayesian analyses of the mitochondrial DNA control region of modern specimens for the two Salmo species using two different mutation rates (1% and 3.6%). Coincidence of coalescent Ne estimates with independent Ne calculated from catches suggests that the 3.6% mutation rate fits better the evolution of the studied populations. Population growth rate declines would have occurred in the last millennium and could be explained by a combination of climatic events and anthropogenic activities.

CHRONOLOGICAL CHANGES IN UPPER PALEOLITHIC FISHERIES REVEALED BY MUSEUM ARCHIVAL MATERIAL

ABSTRACT Salmonid vertebrae in mixed faunal remains from North Iberian archaeological sites of the upper Paleolithic were analyzed to determine specimen age, migratory status, and the seasonality of catch, based on annual growth marks. Fish size was back calculated from vertebra size using published equations. Although sample size is very small, significant changes in the average size and migratory status of fished specimens, and in the seasonality of fishing were detected. Salmonids (Atlantic salmon Salmo salar and brown trout Salmo trutta) were present as a resource in human diet in different climatic conditions. However, preferential winter harvest was prevalent starting after the Last Glacial Maximum, although this may be an artifact of the data related to sea-level rise. The main potential impact of prehistoric fishing habits on salmonid populations was the removal of the larger breeders from the rivers, thus indirectly promoting the reproduction of smaller fish (i.e., selection for small size, although most likely unintended). The methodology described in this study, if applied to larger collections and/or samples, can provide information on how salmonids reacted to past changes in harvest or climate, and could help to predict the consequences of current environmental and climatic changes.

Shrinking fish: comparisons of prehistoric and contemporary salmonids indicate decreasing size at age across millennia

A comparison of Upper Palaeolithic and contemporary salmonid vertebrae from the Iberian Peninsula indicates that there has been a significant decrease in the mean body size for a given age among Atlantic salmon and brown trout inhabiting the southernmost range of their endemic distribution. Mean size at age was greater in prehistoric specimens for all age classes during the freshwater phase of their life histories. Fisheries-induced evolution (selection for smaller sizes) is an obvious explanation for the observed reduction in fish body size, but recent changes in the aquatic habitat affecting density-dependent growth cannot be ruled out.