Dual-isotope isoscapes for predicting the scale of fish movements in lowland rivers.

Abstract

Assessments of patterns of animal movements are important for understanding their spatial ecology. Geostatistical models of stable isotope (SI) landscapes (‘isoscapes’) provide a complementary tool to telemetry for assessing and predicting animal movements, but are rarely applied to riverine species. Often single isotope gradients in freshwater environments are insufficiently variable to provide high isoscape resolution at relatively fine spatial scales. This is potentially overcome using dual-isotope assignment procedures and thus the aim here was to apply single (δ13C) and dual (δ13C and δ15N) isoscapes to assigning riverine fish to origin and predicting their movements. Using the River Bure, England, as the study system, the foraging locations of a small-bodied lowland river fish (roach Rutilus rutilus) of low vagility were predicted using their SI data and those of a common prey item (amphipods). These foraging locations were then compared to their capture locations, with the distance between these being their ‘predicted displacement distance’. The results indicated significant enrichment of δ13C and δ15N with distance downstream in roach fin tissue and amphipods; roach bivariate isotopic niches were spatially variable, with no niche overlap between upstream and downstream river reaches. Furthermore, the dual-isoscape assignment procedure resulted in the lowest predicted displacement distances for roach, therefore enhancing model performance. The dual isoscape approach was then applied to determining the predicted displacement distance of individual common bream Abramis brama, a larger, more vagile species, with these data then compared against the subsequent spatial extent of their movements recorded by acoustic telemetry. When using a high probability density threshold for isotope assignment, the predicted displacement distance of common bream was a significant predictor of the spatial extent of their subsequent movements recorded by acoustic telemetry, although it was less able to predict the direction of displacement. This first probabilistic assignment to origin for riverine species using a dual-isotope isoscape technique demonstrated that where the required spatial resolution of animal movements in freshwater is moderately broad (5 – 10 km), dual-isotope isoscapes can provide a reliable alternative or complementary method to telemetry.