Two decades of change in sea star abundance at a subtidal site in Puget Sound, Washington

Long-term datasets can reveal otherwise undetectable ecological trends,
illuminating the historical context of contemporary ecosystem states. We
used two decades (1997–2019) of scientific trawling data from a subtidal,
benthic site in Puget Sound, Washington, USA to test for gradual trends
and sudden shifts in total sea star abundance across 11 species. We
specifically assessed whether this community responded to the sea star
wasting disease (SSWD) epizootic, which began in 2013. We sampled at
depths of 10, 25, 50 and 70 m near Port Madison, WA, and obtained
long-term water temperature data. To account for species-level differences
in SSWD susceptibility, we divided our sea star abundance data into two
categories, depending on the extent to which the species is susceptible to
SSWD, then conducted parallel analyses for high-susceptibility and
moderate-susceptibility species. The abundance of high-susceptibility sea
stars declined in 2014 across depths. In contrast, the abundance of
moderate-susceptibility species trended downward throughout the years at
the deepest depths – 50 and 70 m – and suddenly declined in 2006 across
depths. Water temperature was positively correlated with the abundance of
moderate-susceptibility species, and uncorrelated with high-susceptibility
sea star abundance. The reported emergence of SSWD in Washington State in
the summer of 2014 provides a plausible explanation for the subsequent
decline in abundance of high-susceptibility species. However, no long-term
stressors or mortality events affecting sea stars were reported in
Washington State prior to these years, leaving the declines we observed in
moderate-susceptibility species preceding the 2013–2015 SSWD epizootic
unexplained. These results suggest that the subtidal sea star community in
Port Madison is dynamic, and emphasizes the value of long-term datasets
for evaluating patterns of change.