Introduction

Sea Star Wasting Syndrome (SSWS) emerged as an unprecedented epidemic affecting asteroids along the Pacific coast from the Aleutian islands to Mexico beginning in 2013, affecting over 20 species of sea stars (Hewson et al. 2014, Miner et al. 2018). Early reports in 2013 and 2014 from areas south (California) and north (Salish Sea) of the Oregon Coast suggested earlier and more extreme mortality impacts to intertidal sea star populations than were observed in Oregon. However, by 2015 impacts to common intertidal and shallow subtidal sea stars along Oregon’s coastline and estuaries were obvious. Since there are vastly fewer quantitative observations of subtidal sea star densities, the relative impacts of the epidemic in Oregon’s deeper coastal habitats has been unclear. This assessment compiles the sea star density estimates derived from all available ROV video surveys at the Redfish Rocks and Cascade Head Marine Reserves and their associated comparison areas. The goal of this analysis is to qualitatively assess the density responses of the full sea star community to SSWS, in order to characterize the overall effect of SSWS and the consistency of that effect across the two subtidal rocky reef sites in the Marine Reserve system that were the most-sampled by ROV.


Methods

The sea star density data are derived from ROV video surveys of rocky reefs within each of the target sites. The data used for these density calculations were restricted to the hard-substrate portions of transects based on the substrate classes categorized during video review. Data for the soft-substrate portions of transects are available but are not presented here. Consequently the soft-sediment associated Sand Star, Luidia foliolata, is excluded from this analysis. Data were further restricted to portions of transects with view characteristics suitable for consistently detecting and assessing densities of macroinvertebrates (i.e. sections with ViewScale = 3). Please see the ROV survey reports for invertebrates for the Redfish Rocks and Cascade Head Marine Reserves for descriptions of the ROV sampling at the sites. Additional sampling and data processing details are available in the ROV methods appendix.

Species’ trends across the sampling period at each site were qualitatively categorized into groups reflecting an overall similar response trajectory. The trends and categories were not quantitatively evaluated; rather, the intent of this analysis is to provide an overall view of the diversity of species’ responses to the epidemic. Species were categorized according to the relative densities before and after 2014, the assumed year of the earliest substantial impacts of SSWS on sea stars in Oregon. For the Redfish Rocks Marine Reserve and comparison areas, the species’ density trajectories broke cleanly into three groups: decreases, delayed decreases, and increases. For the Cascade Head Marine Reserve and its comparison areas, there was more diversity in response among sites so the appropriate categories were decreases, mixed responses among sites, increases, and no change.


Results

Redfish Rocks Marine Reserve

Decreases

Seven species displayed substantial density decreases across all sites following the arrival of SSWS (Fig. 1). Among these, Orthasterias koehleri, the Rainbow Star, was the only species that persisted at any substantial density after 2016.

Fig. 1: Sea star species that declined in abundance in ROV surveys of rocky substrates at the Redfish Rocks Marine Reserve and associated comparison areas. The assumed onset of SSWS in Oregon is indicated by the dashed line at 2014. Note the variable y-axis scaling.

Fig. 1: Sea star species that declined in abundance in ROV surveys of rocky substrates at the Redfish Rocks Marine Reserve and associated comparison areas. The assumed onset of SSWS in Oregon is indicated by the dashed line at 2014. Note the variable y-axis scaling.


Delayed decreases

Two additional species also exhibited substantial decreases, but the onset of the declines was delayed until 2018 (Fig. 2). These included Henricia spp., the Blood Star, which were highly abundant sea stars prior to SSWS. Although both species exhibited some transient increases in 2016, we categorize them as decreases because they ended the sampling period (in 2018 and 2019) at densities substantially lower than the pre-SSWS observations.

Fig. 2: Sea star species that declined in abundance after 2016 in ROV surveys of rocky substrates at the Redfish Rocks Marine Reserve and associated comparison areas. The assumed onset of SSWS in Oregon is indicated by the dashed line at 2014. Note the variable y-axis scaling.

Fig. 2: Sea star species that declined in abundance after 2016 in ROV surveys of rocky substrates at the Redfish Rocks Marine Reserve and associated comparison areas. The assumed onset of SSWS in Oregon is indicated by the dashed line at 2014. Note the variable y-axis scaling.


Increases

Three sea star species exhibited density increases after the onset of SSWS (Fig. 3). Dermasterias imbricata, the Leather Star, and Stylasterias forreri, the Fish-Eating Star, both sustained an overall increase in density across the entire sampling period after SSWS. Meanwhile, Evasterias troschelii, the False Ochre Star, exhibited a transient increase but returned to its initial near-absent status by 2019.

Fig. 3: Sea star species that increased in abundance in ROV surveys of rocky substrates at the Redfish Rocks Marine Reserve and associated comparison areas. The assumed onset of SSWS in Oregon is indicated by the dashed line at 2014. Note the variable y-axis scaling.

Fig. 3: Sea star species that increased in abundance in ROV surveys of rocky substrates at the Redfish Rocks Marine Reserve and associated comparison areas. The assumed onset of SSWS in Oregon is indicated by the dashed line at 2014. Note the variable y-axis scaling.



Cascade Head Marine Reserve

Decreases

Henricia spp., the Blood Star, and Pycnopodia helianthoides, the Sunflower Star, both declined in density at the Cascade Head Marine Reserve (Fig. 4).

Fig. 4: Sea star species that decreased in abundance in ROV surveys of rocky substrates at the Cascade Head Marine Reserve and associated comparison areas. The assumed onset of SSWS in Oregon is indicated by the dashed line at 2014. Note the variable y-axis scaling.

Fig. 4: Sea star species that decreased in abundance in ROV surveys of rocky substrates at the Cascade Head Marine Reserve and associated comparison areas. The assumed onset of SSWS in Oregon is indicated by the dashed line at 2014. Note the variable y-axis scaling.


Mixed responses among sites:

Four sea star species exhibited substantially differing abundance trends among the multiple sites sampled (Fig. 5). One of the most abundant sea stars, Pisaster brevispinus, the Pink Star, largely disappeared from the Cascade Head Marine Reserve and the Schooner Creek Comparison Area, but was observed in abundance at the Cavalier Comparison Area in 2017. Other species with mixed responses among sites included Pteraster tesselatus, the Slime Star, and Solaster spp. (Sun Stars, including S. stimpsoni and S. dawsoni combined for this site). For these species, densities after SSWS onset were consistent or increasing at the Cascade Head Marine Reserve, while staying consistent or decreasing at the two comparison areas. Finally, Orthasterias koehleri, the Rainbow Star, increased at two of three sites.

Fig. 5: Sea star species that exhibited mixed abundance trends among sites in ROV surveys of rocky substrates at the Cascade Head Marine Reserve and associated comparison areas. The assumed onset of SSWS in Oregon is indicated by the dashed line at 2014. Note the variable y-axis scaling.

Fig. 5: Sea star species that exhibited mixed abundance trends among sites in ROV surveys of rocky substrates at the Cascade Head Marine Reserve and associated comparison areas. The assumed onset of SSWS in Oregon is indicated by the dashed line at 2014. Note the variable y-axis scaling.


Increases

Two species, Evasterias troschelii, the Mottled Star, and Stylasterias forreri, the Fish-Eating Star, exhibited increasing trends at Cascade Head (Fig. 6).

Fig. 6: Sea star species that exhibited increasing density in ROV surveys of rocky substrates at the Cascade Head Marine Reserve and associated comparison areas. The assumed onset of SSWS in Oregon is indicated by the dashed line at 2014. Note the variable y-axis scaling.

Fig. 6: Sea star species that exhibited increasing density in ROV surveys of rocky substrates at the Cascade Head Marine Reserve and associated comparison areas. The assumed onset of SSWS in Oregon is indicated by the dashed line at 2014. Note the variable y-axis scaling.


No change

Dermasterias imbricata, the Leather Star, exhibited no overall change in density at Cascade Head across the sampling period (Fig. 7).

Fig. 7: Density trends for the Leather Star *Dermasterias imbricata* in ROV surveys of rocky substrates at the Cascade Head Marine Reserve and associated comparison areas. The assumed onset of SSWS in Oregon is indicated by the dashed line at 2014.

Fig. 7: Density trends for the Leather Star Dermasterias imbricata in ROV surveys of rocky substrates at the Cascade Head Marine Reserve and associated comparison areas. The assumed onset of SSWS in Oregon is indicated by the dashed line at 2014.


Region comparisons

The descriptive trend classifications for each species, provided above, are summarized for the two regions in Table 1.


Table 1: Comparative density trends of sea star species in response to SSWS in the Redfish Rocks region (Redfish Rocks Marine Reserve and associated comparison areas) and Cascade Head region (Cascade Head Marine Reserve and associated comparison areas).



Discussion

Fewer sea star species (two) exhibited overall density decreases in the Cascade Head region than in the Redfish Rocks region (nine). These two species, Henricia spp., the Blood Star, and Pycnopodia helianthoides, the Sunflower Star, declined in both regions, with the Sunflower Star disappearing from both sites. Other commonalities among regions were evident. Pteraster tesselatus was still abundant at Redfish Rocks in 2016 before ultimately declining, so its 2017 abundance at Cascade Head may reflect part of the same pattern. At Redfish Rocks, Orthasterias koehleri persisted better than many of the other overall declining species, and it increased at two of three sites at Cascade Head, suggesting this species fared better across regions than many species.

Other species’ results suggested more distinct dynamics among regions. Among the other decreasing species in the Redfish Rocks region, Pisaster brevispinus was essentially eradicated at all three sites from 2016 onward, so the observation of high absolute densities at one site on the mid-coast of Oregon indicates a substantially greater geographical heterogeneity in SSWS responses than was previously understood. Additionally, both Solaster species were reduced to very low densities at Redfish Rocks by 2016, so the sustained abundance at Cascade Head through 2017 seems to indicate better survivorship in that region.

Two of the other declining species at Redfish Rocks (Mediaster aequalis and Pisaster ochraceus) were not abundant enough at any time point at Cascade Head to warrant inclusion in this analysis. For P. ochraceus, this is likely because the ROV sampling at Cascade Head did not extend shallow enough to encounter significant numbers of these shallow-dwelling sea stars, rather than an absence from the region.

The results across regions show that Evasterias troschelii, Stylasterias forreri, and Dermasterias imbricata seem to have either benefitted from or at least been resilient to the changes occurring during the sampling period, whether directly in response to SSWS or perhaps through some other ecological interaction. Orthasterias koehleri and Pteraster tesselatus could possibly be added to this list for the Cascade Head region.

Taken together, these trends suggest that although there were dramatic and wide-ranging alterations of sea star communities across the Oregon Coast, there was a greater degree of geographic variation in responses to SSWS than had been previously assumed based on qualitative observations focused on the Redfish Rocks region. The finding of relatively abundant Pisaster brevispinus (Pink Stars) in 2017 at the Cavalier Comparison Area is a significant contribution to our understanding of the spatial dynamics of a highly impacted sea star.

The data presented here comprise the most comprehensive view to date of the effects of SSWS in Oregon’s nearshore subtidal habitats. Other ROV data exist for additional sites, but among these, only observations at the Cape Perpetua Marine Reserve will provide robust before and after density estimates for sea stars. The primary additional nearshore ROV surveys that will contribute additional sea star density data were conducted at subtidal reefs along Oregon’s north coast in 2012 only, and at the expansive Cape Arago / Bandon reef complex in 2018 only. The 2018 data will be useful for assessing which species may have persisted at substantial densities at this additional site after the arrival of SSWS in Oregon.

While the ROV sampling interval is likely too great for establishing detailed timelines of recovery, future sampling will be useful in clarifying the long-term consequences of SWSS to the structure of benthic communities. Variation among species in the timing and extent of population changes may add a new element to the overall understanding of the major disruption of benthic invertebrate communities caused by SSWS.


Literature Cited

Hewson, I. et al. 2014. Densovirus associated with sea-star wasting disease and mass mortality. Proc. Natl Acad. Sci. 111, 17 278–17 283. (doi:10.1073/pnas.1416625111)

Miner, C.M. et al. 2018. Large-scale impacts of sea star wasting disease (SSWD) on intertidal sea stars and implications for recovery. PLoS ONE 13, e0192870. (doi:10.1371/journal.pone.0192870)