Subtidal roving diver survey data and ARIMA models

A total of 8,097 REEF surveys were analyzed. Insufficient data precluded evaluation of the Whidbey Basin. The total number of surveys per year in each of the 5 sub-basins included in the study and the Outer Coast were relatively consistent over the times analyzed (Table 1). Mean dive times were 46, 49, 54, 49, 51 and 52 minutes for the Strait of Georgia, Northern Straits, Central Basin, Hood Canal, South Puget Sound and the Outer Coast, respectively; with typical maximum dive depths between 12-21m. Most surveys were conducted between 0700–1900 hours and diver visibility ranged from 3—14m. Eight habitat types were represented: kelp forest, rock/shale reefs, open oceans, surf grass beds, pinnacles, bull kelp beds, cobblestone/boulder fields and walls, but most surveys were conducted in rock/shale reefs, walls and cobblestone/boulder fields. During the 8,097 dives, one or more of the 5 species analyzed were not sighted, necessitating us to include 20,264 absence records to complete the dataset.

Compared to pre-epidemic (2006–13), the SF of P. helianthoides declined markedly after onset of the epidemic (2014–15), while SF of D. imbricata and urchin species S. droebachiensis and M. franciscanus, trended upward throughout the entire time series (2006–2015; Fig 2). The SF for P. brevispinus began to decline in 2012 and continued to decline through 2015; however, the lowest SF during the decline (in 2015) was similar to the lowest SF between 2006–2011.

Species and basin specific ARIMA models and their estimated parameter values are available in S1 Appendix. After the SSWD epidemic onset (in 2013), D. imbricata abundance increased in 3 basins (Hood Canal, the Northern Straits and the Strait of Georgia). Specifically, in the Northern Straits, D. imbricata abundance increased 1.51 times by 2015, exceeding the projection for the population trajectory. Although they did not exceed the population projected in the Hood Canal and the Strait of Georgia, the abundance of this species increased 1.23 and 1.3 times, by 2015, respectively. In the South Puget Sound basin, abundance increased above projected levels in 2014, then returned to within expected trends in 2015. There were no major changes for this species in the Central Basin, while the Outer Coast had a decreasing abundance trend, although the complete 10-year abundance trend in this basin was unstable when compared to the others (Fig 3).

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larger image TIFF original image Download: Fig 3. The actual 2006–15 and projected 2014–15 abundance for D. imbricata in 5 basins of the Salish Sea and the Outer Coast. Grey line marks the epidemic onset. Note: Abundance = Density Score x SF, where Density Score = [(nSx1)+(nFx2)+(nMx3)+(nAx4)] / (nS + nF + nM + nA). Here nS, nF, nM, and nA represent the number of times each abundance category was assigned for a given species. https://doi.org/10.1371/journal.pone.0163190.g003

While historically P. brevispinus has been rarer than the other two star species monitored on roving dive surveys, abundance after 2013 did show a clear decline in all basins and on the Outer Coast, although the decline in abundance was beyond the 95% CI of projected values only in the Hood Canal and South Puget Sound basin. In the Central Basin, Northern Straits basin, Outer Coast and the Strait of Georgia basin, this downward trend started in 2012. By 2015, the abundance in these 3 Salish Sea basins corresponded to 0.39, 0.03 and 0.35 times the 2013 abundance respectively. On the Outer Coast the abundance reached 0 during 2015. Declines in abundance limited to the post-epidemic onset (2014–15) occurred only in the Hood Canal and South Puget Sound basins where the 2015 abundance corresponded to 0.38 and 0.22 times the 2013 abundance. In these 2 basins, the observed abundances post epidemic onset were not lower than the historical pre-epidemic estimated mean (Fig 4).

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larger image TIFF original image Download: Fig 4. The actual 2006–15 and projected 2014–15 abundance for P. brevispinus in 5 basins of the Salish Sea and the Outer Coast. Grey line marks the epidemic onset. Note: Abundance = Density Score x SF, where Density Score = [(nSx1)+(nFx2)+(nMx3)+(nAx4)] / (nS + nF + nM + nA). Here nS, nF, nM, and nA represent the number of times each abundance category was assigned for a given species. https://doi.org/10.1371/journal.pone.0163190.g004

Post epidemic onset, P. helianthoides declined markedly in every basin compared to the projected abundance (Fig 5). The most dramatic P. helianthoides declines were seen in the Central Basin, Northern Straits and the Outer Coast, where the abundance had been stable pre-epidemic onset, while in 2015 the observed abundances in these basins corresponded to 0.04, 0.005 and 0 times the 2013 value respectively. In the Hood Canal basin the decline was less severe (2015 abundance was 0.5 times the 2013 abundance) and the declining trend started earlier than 2013. In the South Puget Sound basin and the Strait of Georgia basin P. helianthoides abundance began to decline prior to 2013, but the decline dramatically increased between 2013 and 2015 (Fig 5). In 2015, the abundances in these basins corresponded to 0.27 and 0.11 times the 2013 abundance. Similarly, declines occurred in the Outer Coast reaching 0 abundance at the end of the study period. Estimated mean count for P. helianthoides per roving dive survey were consistent until 2013, averaging 9.89 individuals. In the Hood Canal basin, the Outer Coast, the South Puget Sound basin and the Strait of Georgia basin, the decline started in 2012 but decreased steeply during the post-epidemic period. In the Central Basin, Hood Canal basin, Northern Straits basin, the Outer Coast, South Puget Sound basin and the Strait of Georgia basin the mean count per dive decreased at rates -9.25, -8.91, -8.37, -9.41, -7.38 and -10.86, respectively between 2013–15. By 2015 there were zero or close to zero individuals per roving dive survey in the Central Basin, the Northern Straits basin, the Outer Coast and Strait of Georgia basin (Fig 6).

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larger image TIFF original image Download: Fig 5. The actual 2006–15 and projected 2014–15 abundance for P. helianthoides in 5 basins of the Salish Sea and the Outer Coast. Grey line marks the epidemic onset. Note: Abundance = Density Score x SF where Density Score = [(nSx1)+(nFx2)+(nMx3)+(nAx4)] / (nS + nF + nM + nA). Here nS, nF, nM, and nA represent the number of times each abundance category was assigned for a given species. https://doi.org/10.1371/journal.pone.0163190.g005

By 2014, M. franciscanus abundance had increased in places where it was originally present (the Northern Straits basin, the Outer Coast and the Strait of Georgia basin), but in 2015 this trend continued beyond the CI of the projected abundance only in the Northern Straits (Fig 7). During the 2013–15 period M. franciscanus abundance in this basin increased 1.52 times. As urchins are harvested by tribal fisheries in Washington’s half of the Salish Sea, we compared increases in red urchin abundance with data taken only from dive sites located within urchin harvest closure zones that are present in the Northern Straits basin [21]. We did not find meaningful differences between abundances for this species in harvested versus fishery closure areas. After 2013, S. droebachiensis abundance clearly increased in the Central Basin, the Northern Straits basin, the South Puget Sound basin and the Strait of Georgia basin beyond projections. In these basins the abundances increased 2.47, 2.13, 2.38 and 2.58 times, respectively. Remarkably, in the Central Basin and South Puget Sound basin this species was historically rare, but became more abundant in 2013, with abundance increasing at even higher rates thereafter (Fig 8).

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larger image TIFF original image Download: Fig 7. The actual 2006–15 and projected 2014–15 abundance for M. franciscanus in 5 basins of the Salish Sea and the Outer Coast. Grey line marks the epidemic onset. Note: Abundance = Density Score x SF, where Density Score = [(nSx1)+(nFx2)+(nMx3)+(nAx4)] / (nS + nF + nM + nA). Here nS, nF, nM, and nA represent the number of times each abundance category was assigned for a given species. https://doi.org/10.1371/journal.pone.0163190.g007