Scale microchemistry as a tool to investigate the origin of wild and farmed Salmo salar

Atlantic salmon Salmo salar are extensively farmed throughout their natural range, and unintentional interactions between farmed fish and wild populations have been implicated in the decline of wild salmon. The trace element composition of salmon scales distinguishes wild from farmed fish, and potentially provides a rapid and cheap method to assess the extent of escaped farmed fish within a wild population. Scale samples from wild and farmed fish from sites throughout Scotland were analysed for a large number of trace elements using inductively coupled plasma mass spectrometry (ICP-MS). Discriminant function analysis of the resulting data classified wild and farmed fish with 98% accuracy. Mn is identified as the element contributing most to the dissimilarity between wild and farmed fish, with scales from farmed fish yielding significantly higher concentrations of Mn. Scale chemistry also differed between farms. 87% of samples taken from six farm sites around the west coast of Scotland were correctly classified to their farm of origin. Scale chemistry provides a powerful tool to determine the origin of S. salar, despite the potential for post-depositional change in elemental concentrations of scale bioapatite.