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.