COS 159-6 - CANCELLED - How deadly was the mass mortality of sunflower sea stars in Canada?

Background/Question/Methods

Beginning in 2013, the sea star wasting disease (SSWD) epidemic swept through the Pacific coast of North America, decimating sea star populations in its wake. One of the species most severely impacted by the mass mortality event was the keystone mesopredator, the sunflower sea star (Pycnopodia helianthoides). Recently, a global assessment of the species for the IUCN Red List identified broad-scale declines across its range and it has since been classified as Critically Endangered. Although the impacts of SSWD are unquestionably large, many of the estimates of decline are clouded by high uncertainties ranging multiple orders of magnitude, due to high variability across surveys and aggregation of the data. As the Committee on the Status of Endangered Wildlife in Canada prepares to assess sunflower sea stars for Canada’s national listing process, reducing this uncertainty is integral to determining an appropriate status classification. Using 21 years of dive survey data from academic, government, and community-led initiatives compiled by the IUCN assessors, we re-examined these declines in depth for the Canadian range. We developed multivariate autoregressive state-space models to assess observation error across the data sources and to determine year-to-year variation in the underlying population trends of sunflower sea stars in Canada.

Results/Conclusions

We observed declines in both the abundance index (71.2%, 95% CI: 60.2-79.3%) and occurrence (67.3%, 95% CI: 55.8-76.9%) of sunflower sea stars across their Canadian range. Modelling year-, site-, and dive-specific variation enabled us to reduce uncertainty in our estimates substantially, and the incorporation of abundance-score data in our models, using ordered logistic regression, more than tripled the sample size used in our abundance analysis relative to the IUCN assessment. Additionally, by examining year-to-year fluctuations in the underlying population state, we were able to demonstrate the relative stability of sunflower sea star abundances and occurrence prior to and immediately following the SSWD-induced crash. While our estimate of decline is less severe than that presented for Canada in the IUCN assessment, we have greater confidence in its accuracy, and the devastating impacts of SSWD on sunflower sea stars remain clear.