The GIS shapefile Range extent of southern sea otters 2019.shp is a simple polyline representing the geographic distribution of the southern sea otter (Enhydra lutris nereis) in mainland California, based on data collected during the spring 2019 range-wide census. The USGS range-wide sea otter census has been undertaken each year since 1982, using consistent methodology involving both ground-based and aerial-based counts. The spring provides the primary basis for gauging population trends by State and Federal management agencies. Sea otter distribution in California (the mainland range) is considered to comprise a band of potential habitat stretching along the coast of California, and bounded to the north and south by range limits defined by combining independent otters within a moving window of 10-kilometer stretches of coastline (as measured along the 10-meter bathymetric contour; 20 contiguous ATOS intervals each) and taking the northern and southern ATOS values, respectively, of the northernmost and southernmost stretches in which at least five otters were counted for at least 2 consecutive spring surveys during the last 3 years.