Predicting strontium isotope variation and fish location with bedrock geology: Understanding the effects of geologic heterogeneity

• Strontium ratio of streams accurately modeled for a large watershed using the bedrock geology of upstream watersheds.
• Strontium ratios modeled from bedrock maps were used to successfully determine fish location using otoliths.
• Geology alone can be used to determine if watersheds are isotopically distinct.
• Geologic heterogeneity and scale effect the ability to predict 87Sr/86Sr from geologic maps.
• Heterogeneity metrics could constrain the spatial limits of prediction in future isoscape models.

Recent advances in using naturally occurring isotopes to reconstruct movement patterns have revolutionized the study of migration and spatial patterns across taxa. Isoscape approaches utilize isotopic variation in the underlying geology to quantify migration pathways. Spatial patterns in the geology can be used to predict isotopic variation, such as 87Sr/86Sr; however, previous attempts to create predictive models have had mixed results. Our primary objective was to investigate the relationship between bedrock lithology and 87Sr/86Sr ratio as a tool to extend the spatial resolution of animal migration studies. Secondly, we investigated the ability to use geologic prediction as an a priori tool for determining chemically distinct watersheds. We first developed a regression model to relate known stream water 87Sr/86Sr to rock information from geologic maps, then used model outputs to classify adult fall Chinook salmon to their juvenile rearing location from 87Sr/86Sr signatures recorded in their otoliths (ear bones). We discuss the effect of scale and geologic heterogeneity on our ability to determine 87Sr/86Sr ratios within the study area. Our results indicate that the relationship between 87Sr/86Sr values and bedrock lithology can be used to accurately determine the rearing location of fish using otolith 87Sr/86Sr signatures. The scale at which geology can be used as a predictor of 87Sr/86Sr values is constrained by geologic heterogeneity and inherent variability in 87Sr/86Sr ratios within major rock categories. Further, our results indicate that geological data alone can be used to quantitatively investigate which watersheds are likely to be distinguishable using this method within a basin. Geologic prediction also has the potential to improve the scale and resolution of isotopic studies and the development of isoscapes. By applying measures of spatial heterogeneity we will be better able to quantitatively place limits on the accuracy of geologic predictions of 87Sr/86Sr ratios.