Progression of Seastar Wasting Disease Outbreak Across Habitats in Central British Columbia, Canada

Disease can cause dramatic declines in animal populations. Sea star wasting disease (SSWD) can infect up to 20 species of sea star, although the etiology of the disease is still under debate. There is variability in susceptibility and mortality among the sea star species, which could lead to species specific response to disease outbreak. We document the effect of sea star wasting disease on population size and sea star size structure in response to SSWD around Calvert Island, British Columbia. Sea star wasting disease was first documented at Calvert Island in the winter of 2015. We conducted expert intertidal and subtidal diver surveys; in the rocky intertidal (2014-2019), seagrass beds pre- and post-wasting (2014-2018), and in Nereocystis and Macrocystis beds monthly during initial disease spread (2015). Following SSWD, intertidal surveys found an increase juvenile sea stars density, and seasonal peaks in wasting signs in September. We found species specific responses to wasting across habitats. In the seagrass beds, we found a drop in P. helianthoides population density in 2015 and minimal evidence of recovery by 2018. Mean P. helianthoides size decreased in 2015, and increased incrementally through 2017, with a decrease in mean size in 2018 that might indicate a year with juvenile recruitment. Dermasterias imbricata mean star size increased from 2015-18 following wasting. In Nereocystis and Macrocystis beds we saw a loss of large bodied P. helianthoides from May to October, 2015. We saw declines in density for Solaster, Evasterias troscheli and Orthasterias koehleri. At the same sites, there was no change in mean density of D. imbricata, Mediaster aequalis, or Henricia sp. Our surveys confirm trends seen elsewhere suggesting that P. helianthoides populations are the most negatively affected, we also found that D. imbricata, M. aequalis and Henricia sp. populations show a decreased response to SSWD. Resilient populations could be an indication of reduced susceptibility to infection. Alternatively, factors such as release from competitors hit harder by SSWD could be driving patterns of resistance.