Evaluation of IUCN spatial distribution metrics for a migratory species, Fraser River Sockeye salmon

The International Union for Conservation of Nature (IUCN) has developed criteria to assess extinction risk; the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) is an independent agency that assesses the vulnerability of Canadian species using IUCN criteria. Criterion B uses extent of occurrence, area of occupancy, and number of locations to evaluate risks associated with restricted spatial distribution in conjunction with several subcriteria. Concerns have been expressed about the use of these distribution metrics for migratory aquatic species as they often have naturally restricted distributions that change at different life stages. The conservation status of Sockeye salmon (Oncorhynchus nerka) of the Fraser River, British Columbia, Canada is currently being assessed and distribution metrics have been estimated. We compared metrics for Sockeye salmon to similar values for 57 assessments of vertebrate, invertebrate and plant species conducted by COSEWIC. Relationships among the metrics were generally similar between Fraser River Sockeye salmon and COSEWIC assessments, suggesting that despite concerns about the applicability of occurrence and abundance metrics and corresponding methodologies to highly migratory and aquatic taxa, Fraser River Sockeye salmon do not demonstrate large departures compared to other species assessed in Canada. The majority of Fraser River Sockeye salmon distribution metrics fell below thresholds for endangered or threatened statuses. However, we also observed that reported values for species assessed by COSEWIC had little relationship to the status that was ultimately assigned. Thus, based on results of this study, the ultimate role of the distribution metrics in COSEWIC status assessment is uncertain.