The Limits of Prediction: Applying Strontium Isoscape Predictions to Otolith Studies Across Spatial Scale and Geologic Heterogeneity

The use of strontium isotopes in otoliths has been striking in the breadth of spatial and temporal scales at which fish movement can be determined. Isoscapes, maps of isotopic variation across the landscape, could potentially further improve the application of strontium isotope tracers in otoliths. The tight relationship of strontium isotopes to underlying geology has been used to predict ⁸⁷Sr/⁸⁶Sr ratio isoscapes in water, with varying degrees of success. Recently, several publications have shown promise in predicting ⁸⁷Sr/⁸⁶Sr from geologic maps using different methods. These predictive approaches have the potential to increase the scale and resolution of otolith studies; identify unknown population sources; and provide a priori assessment of potential study sites and fish locations. Our results indicate that geologic prediction can effectively determine fish location from otoliths, subject to basin-specific limits of scale and geologic heterogeneity. We present results from three otolith-based studies of fish movement; the Snake River and Henry’s Fork watersheds of Idaho, and the Amazon basin in South America. These study sites vary widely in spatial scale, geologic makeup and the ways in which isotope prediction were applied. Taken together they illustrate both the utility and limits of strontium isotopic prediction in otolith microchemical studies.