Slowly swimming towards recovery, california's sea otter numbers holding steady posted on september 22, 2014 usgs. gov read the following passage: when sea otters want to rest, they wrap a piece of kelp around their body to hold themselves steady among the rolling waves. likewise, california's sea otter numbers are holding steady against the many forces pushing against their population recovery, according to the latest field survey led by federal, state, aquarium, and university scientists. since the 1980s, u. s. geological survey scientists have calculated a population index each year for the southern sea otter—enhydra lutris nereis—a species federally listed as threatened. the annual range-wide field survey collects data on this protected marine mammal on behalf of the u. s. fish and wildlife service. trends in population index reported by usgs for southern sea otters in california. data are shown for independent otters, nonpups, and all otters for mainland california, san nicolas island, and rangewide from 2012 onward. the mainland all otters line increases gradually from 1990 to 2014 from less than 1800 to about 2800. the mainland independents mirror the all otter population curve at slightly lower numbers. the san nicolas island population begins at about 18 in 1992 and climbs to about 65 in 2014. why does the author include the chart in the article? to provide a comparison of changes in the two groups of sea otter populations to provide a depiction of the variety of sea otter populations in the area to provide a list of the groups of ecosystems that were included in the study to provide a visual representation of how sea otter populations have changed