Relationships between Water, Otolith, and Scale Chemistries of Westslope Cutthroat Trout from the Coeur d'Alene River, Idaho: The Potential Application of Hard-Part Chemistry to Describe Movements in Freshwater

Abstract

We quantified Mg:Ca, Mn:Ca, Sr:Ca, and Ba:Ca molar ratios from an area representing the summer 2000 growth season on otoliths and scales from 1-year-old westslope cutthroat trout Oncorhyncus clarki lewisi collected from three streams in the Coeur d'Alene River, Idaho, system. We also quantified Mg:Ca, Sr:Ca, and Ba:Ca molar ratios in the water during summer 2000 and used regressions to model the assimilation of the various ratios into the otoliths and scales. Otolith and scale chemistries were linearly related to the Sr:Ca and Ba:Ca ratios in the water. The partition coefficients for the Sr:Ca ratios in otoliths and scales are higher in this freshwater system than in experimental results from a saline environment; we attribute these differences to differences in the biology of saltwater and freshwater fish. In contrast, the Ba:Ca partition coefficients are similar between the two environments, suggesting that our estimates are representative of those for a wide range of concentrations, temperatures, salinities, and at least two families of fish. The Mg:Ca, Sr:Ca, and Ba:Ca ratios varied significantly in otoliths from the three streams and could be used to reclassify individual fish to the streams from which they were collected with 100% accuracy. The Mn:Ca, Sr:Ca, and Ba:Ca ratios varied significantly in scales from the three streams and could be used to classify individuals with 82% accuracy. Given the heterogeneity of basin geology, the stability of water chemistry, and the degree of discrimination noted for the three streams we sampled, we believe that examination of the elemental composition of fish otoliths and scales could be used to describe the movements of fish in this and similar freshwater systems. Further, the high correlation between the ratios in scales and those in otoliths suggests scales may offer a nonlethal sampling alternative.