Forensic geochemistry identifies the illegal introduction of Walleye into Lake Cascade, Idaho

Abstract

Objective: The illegal introduction of fish species can disrupt ecosystems, collapse food webs, and undermine recreational fishing opportunities. Determining whether introduced fish are locally reproducing is important for resource managers. Here, we used the geochemical analysis of otoliths to investigate the potential illegal introduction of a Walleye Sander vitreus caught in 2022, in Lake Cascade, Idaho. Lake Cascade is known for its recreational Yellow Perch Perca flavescens fishery and has no documented resident population of Walleye. Methods: To determine if the Walleye was spawned in Lake Cascade or introduced from elsewhere, we analyzed otoliths for strontium isotopes (87Sr/86Sr) and compared them to local water samples and three locally caught Yellow Perch. Result: The Walleye otolith revealed a shift from higher 87Sr/86Sr values (0.70878) in its early life to a period of intermediate value (0.70842) equal to Payette Lake water, then finally to a lower value (0.70807) comparable to the Lake Cascade water and resident Yellow Perch otoliths. Conclusion: These results suggest that the Walleye was initially transplanted to the Payette Lake area from a currently unknown source in 2020, 2 years before its capture. It resided there briefly before migrating south into Lake Cascade. This study further highlights the benefit of geochemical analyses to identify the illegal introduction of fish and to provide resource managers with a powerful tool for early detection and prevention of the establishment of illegally introduced fish species.