Evaluation of Fraser River Sockeye salmon (Oncorhynchus nerka) spawning distribution following COSEWIC and IUCN Redlist guidelines

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Sockeye salmon (Oncorhynchus nerka) from the Fraser River are scheduled to be assessed by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in 2013. Information on the spatial distribution of Sockeye salmon populations is required for COSEWIC quantitative criteria B and D. Fraser River sockeye populations have been organized into 24 Conservation Units (CUs) in support of Fisheries and Oceans Canada’s (DFO) Wild Salmon Policy. For each of the 24 CUs, we estimated: the number of spawning locations (defined as individual spawning streams or whole lakes or large branches of lakes); the extent of occurrence (defined as polygons encompassing all spawning areas in each CU); the area of occupancy (a measure of the habitat actually occupied for spawning, and assessed using grid techniques); and the biological area of occupancy (a measure of occupancy not constrained by grid methodology). We also estimated the extent of occurrence, area of occupancy and biological area of occupancy for the entire distribution of Fraser River Sockeye salmon spawners. Estimates were based on observations of spawner distributions from 2008 to 2011. Data on the number of locations were available from 1992 to 2011, which allowed trends in spatial distribution to be examined. We identified spawning sites that contained potential vagrants, and evaluated the effect of removing these sites from all analyses. We explored a number of alternative means for calculating extent of occurrence and area of occupancy statistics, but found in most cases the method of analysis had little effect on the value of statistics relative to thresholds defined by COSEWIC.