Geochemical signatures in scales record stream of origin in westslope cutthroat trout

We used laser ablation inductively coupled plasma mass spectrometry (ICP-MS) to quantify the elemental composition of Mg:Ca, Mn:Ca, Sr:Ca, Ba:Ca, and Pb:Ca ratios in scales from juvenile westslope cutthroat trout Oncorhynchus clarkii lewisi collected from 13 streams in three major drainages (North Fork, South Fork, and Middle Fork) of the upper Flathead River system in Montana during summer 2001 and 2002. We also determined element: Ca levels within natal streams in summer 2001 and 2002. The concentrations of Sr:Ca and Ba:Ca in westslope cutthroat trout scales were highly correlated with those in the water. The multivariate elemental signatures of the scales differed significantly among streams. A forward stepwise discriminant function analysis was used to classify individual fish, first to the drainage of origin and then to the natal stream while considering all 13 streams simultaneously. At the drainage level, cross-validated classification accuracy was 91% in the Middle Fork, 81% in the North Fork, and 78% in the South Fork; overall accuracy was 82%. Of the fish that were correctly classified at the drainage level, 88% were correctly classified to their natal stream at accuracy levels of 100% in the Middle Fork, 88% in the North Fork, and 80% in the South Fork. Finally, the Mn:Ca, Sr:Ca, and Ba:Ca ratios in westslope cutthroat trout scales were significantly correlated with values in the otoliths of individual fish, suggesting that scales may provide a nonlethal alternative to otoliths as natural markers. Our data indicate that elemental signatures in scales may be used as natural tags to identify the natal stream origin of westslope cutthroat trout in the upper Flathead River system and in other similar freshwater environments. However, future work needs to determine whether elemental signatures are sufficiently stable over time to allow for accurate classification of adult fish after emigration from natal streams.