The spring 2017 mainland sea otter count began on April 30, and although the shore-based counts were completed by May 12, 2017, the aerial counts were not completed until July 12, 2017. Overall viewing conditions this year were good, although not as good as conditions experienced during the 2016 spring census (View Score 2.4 versus 3.1, where 0=poor, 1=fair, 2=good, 3=very good, and 4=excellent). The surface canopies of kelp (Macrocystis sp.) were considered by most participants to be considerably below normal for this time of year in most areas of the mainland range. Sea otters along the mainland coast were surveyed from Pillar Point in San Mateo County in the north to Rincon Point in the south at the Santa Barbara/Ventura County line. A separate ground-based survey of the sea otter population at San Nicolas Island was completed earlier in the spring (April 21–25) under good survey viewing conditions (View Score = 2.0). Surface kelp canopies at San Nicolas were also estimated to be considerably below the seasonal norm at the time of the survey. These data support the following U.S. Geological Survey Data Series: Tinker, M.T., and Hatfield, B.B., 2017, California sea otter (Enhydra lutris nereis) census results, spring 2017: U.S. Geological Survey Data Series 1067, 9 p., https://doi.org/10.3133/ds1067.