Before and After Sea Star Wasting Disease: Subtidal Sunflower Star (Pycnopodia helianthoides) observations in the central US…

Between 2013 and 2015, a sea star wasting disease (SSWD) epidemic led to a large die-off affecting over 20 species of sea stars from Alaska to Baja California. The impacts of SSWD to intertidal populations have been well documented but we know much less about its impact on subtidal species, including the critically endangered sunflower star (Pycnopodia helianthoides). We assessed sunflower star abundance using towed underwater imagery along 50 km of the central southern Strait of Juan de Fuca. Annual surveys were completed between 2010 and 2017 and again in 2020, allowing us to compare populations before and after the advent of SSWD. The study repeated transects along 40 km of seafloor, from the shallow subtidal to -15 m (Mean Lower Low Water). The imagery was initially collected by the Washington State Department of Natural Resources to assess changes in macroalgae following the removal of the Elwha River dams. While this remote methodology is lower resolution than some other field techniques, it enabled us to assess changes in star populations over a much larger area. We classified the imagery comprehensively for sea stars. We observed nine star taxa across the study area and found similar species composition in the years before SSWD, including 2013. While the video did not show clear symptoms of wasting in individuals, the total abundance of individuals declined by more than 80% in 2014. In subsequent years, sunflower star presence increased relative to 2014, yet became highly variable. Overall, we documented continuing presence of sunflower stars, which is good news about this critically endangered species. The substantial shift in abundance that we observed raises important questions like how this change in a keystone species has cascaded through the nearshore community.