Eye lens stable isotope analysis reveals potential discharge‐related changes in habitat use by Blue Sucker ( Cycleptus elongatus )

Stable isotope analysis (SIA) of eye lenses can provide a retrospective view of resource use over a fish's life. This tool can be valuable for studying long‐term isotopic changes in non‐game species that typically receive limited research emphasis. We assessed shifts in isotope composition of Blue Sucker ( Cycleptus elongatus ) from eye lenses of fish ( n = 39) collected from the James River, South Dakota, that ranged in total length from 315 to 821 mm and age from 1 to 41 years. We assessed the chronology of δ 13 C and δ 15 N values from individual fish lens layers and modelled population‐level trends across the observed lens diameter range using Bayesian general additive models. We assessed changes in δ 13 C and δ 15 N isotope values of juvenile (<2 years) Blue Sucker lens layers over time and across the two‐year, grand mean (April–October) discharge of the James River using Bayesian linear models. Lens diameter was linearly related to fish length and exponentially related to age. The mean population‐level trend in δ 13 C values increased by ~3‰ as eye lens diameter increased above 2 mm, whereas δ 15 N values were consistent across the range of lens diameters (within ~1‰). Juvenile mean posterior estimated δ 13 C values generally decreased over time. As river discharge increased, the juvenile mean posterior estimated δ 13 C values decreased by 5‰ across the range of observed discharge values (8.0–224.0 m 3 /s), while changes in δ 15 N values were notably low at ~1‰. Lower δ 13 C values for younger fish (e.g., smaller lens diameters) during high river discharge likely indicate increased residence time of juveniles in floodplain habitats. These results demonstrate that eye lens SIA can provide insight into changing habitat use by long‐lived, vulnerable fish.