Identification of Freshwater and Anadromous Inconnu in the Mackenzie River System by Analysis of Otolith Strontium

Abstract

Because trace elements from the environment are incorporated into the otoliths of fish as they grow, otolith microchemistry can be used to reconstruct key aspects of their environmental history, such as migratory patterns. We used scanning proton microprobe analysis on otoliths of inconnu Stenodus leucichthys from the Mackenzie River system (Northwest Territories, Canada) to determine whether patterns of strontium (Sr) distribution supported the hypothesis that some populations are anadromous while others remain in a low-Sr freshwater environment. The otoliths of inconnu from the Slave River had low, flat Sr profiles, indicating that these fish spent their entire lives within a freshwater environment. In contrast, while the otoliths of inconnu from the Arctic Red River varied substantially, they generally had low, flat profiles near the core followed by oscillating patterns of high and low Sr through to the outer edge of the otolith. These latter profiles indicate that these fish spent their first 1–2 years in a freshwater environment and then moved annually into a higher-Sr environment, such as the Mackenzie estuary or Beaufort Sea. A subset of fish from the Arctic Red River, primarily males, revealed smaller annular maxima or ranges of Sr concentrations and were considered partially anadromous. Because the salinity of the environment consistently correlates strongly with Sr levels in otoliths, otolith Sr can provide a reliable tool for examining the life history and migratory patterns of diadromous species that are otherwise difficult to study directly.