Adfluvial smallmouth bass in a tributary of Lake Huron

Fish exploit opportunities within aquatic ecosystems to increase their likelihood of survival, growth, and reproduction, ultimately to maximize fitness over the entire life cycle. Fitness is increased by moving to find abiotic and biotic conditions suitable for various life history stages. Smallmouth bass (Micropterus dolomieu) are known to spawn in lakes and flowing waters displaying high nest site fidelity in both environments. Some populations of smallmouth bass are adfluvial, living as adults in lakes and ascending rivers to spawn. However, this life history variant is poorly understood, particularly in the Great Lakes. The objective of our study was to determine if smallmouth bass migrate 13 km upstream to a barrier in the Maitland River from Lake Huron. Stable isotopes were used to make inferences about the migratory status of Maitland River smallmouth bass. We found significant separation in the isotopic space for adult bass captured downstream of a migration barrier (mean δ¹³C = −18.93 ‰; mean δ¹⁵N = 11.02 ‰) compared to bass found upstream (mean δ¹³C = −26.58 ‰; mean δ¹⁵N = 15.29 ‰) river segments. Stable isotope values for juvenile bass, forage fishes, and invertebrates all grouped in similar δ¹³C isotopic space across the lower and upper river segments. Adfluvial smallmouth bass are underappreciated, yet an important life history variant supporting population biodiversity in the Great Lakes.