|Muhlfeld, C. C., B. Marotz, S. R. Thorrold, and J. L. FitzGerald, Geochemical signatures in scales records stream of origin in westslope cutthroat trout, Trans. Am. Fish. Soc.134: 945-959, 2005|
We used laser ablation inductively coupled plasma mass spectrometry (ICP-MS) to quantify Mg:Ca, Mn:Ca, Sr:Ca, Ba:Ca, and Pb:Ca in scales from juvenile westslope cutthroat trout Oncorhynchus clarki lewisi collected from 13 streams in the upper Flathead River system in Montana during summer 2001 and 2002. We also determined Mg:Ca, Sr:Ca, and Ba:Ca levels within natal streams in summer 2001 and 2002. The chemical composition of trout scales were highly correlated with Sr:Ca and Ba:Ca ratios in the water. Multivariate elemental signatures of the scales differed significantly among streams, and canonical discriminant analyses revealed that streams were clearly separated in discriminant space. Forward stepwise discriminant function analyses were used to classify individual fish back to their natal stream using a multi-spatial scale approach. At the drainage level, classification accuracy was 91% (range, 86-100%) for the five streams in the North Fork, 98% (range, 96-100%) for the four streams in the Middle Fork, and 78% (range, 72-86%) for the four streams in the South Fork. At the basin scale, classification accuracy was 73% (range, 29-96%) for the 13 study streams when the three drainages were combined. Finally, Mn:Ca, Sr:Ca, and Ba:Ca ratios in the trout scales were significantly correlated with values in the otoliths of individual fish, suggesting that scales may provide a non-lethal alternative to otoliths in determining life-history characteristics of freshwater fish populations. Our data indicate that elemental signatures in scales may be used as natural tags to identify natal stream origin of cutthroat trout in the upper Flathead River system and in similar freshwater environments. However, future work needs to determine if elemental signatures are sufficiently stable over time to allow for accurate classification of adult fish after emigration from natal streams.