Ochre star mortality during the 2014 wasting disease epizootic: role of population size structure and temperature

Abstract

Over 20 species of asteroids were devastated by a sea star wasting disease (SSWD) epizootic, linked to a densovirus, from Mexico to Alaska in 2013 and 2014. For Pisaster ochraceus from the San Juan Islands, South Puget Sound and Washington outer coast, time-series monitoring showed rapid disease spread, high mortality rates in 2014, and continuing levels of wasting in the survivors in 2015. Peak prevalence of disease at 16 sites ranged to 100%, with an overall mean of 61%. Analysis of longitudinal data showed disease risk was correlated with both size and temperature and resulted in shifts in population size structure; adult populations fell to one quarter of pre-outbreak abundances. In laboratory experiments, time between development of disease signs and death was influenced by temperature in adults but not juveniles and adult mortality was 18% higher in the 19 °C treatment compared to the lower temperature treatments. While larger ochre stars developed disease signs sooner than juveniles, diseased juveniles died more quickly than diseased adults. Unusual 2-3 °C warm temperature anomalies were coincident with the summer 2014 mortalities. We suggest these warm waters could have increased the disease progression and mortality rates of SSWD in Washington State.

Keywords: Pisaster ochraceus; climate change; epizootic; host demography; mass mortality; sea star wasting disease.

Figure 1.

Representative photographs of Pisaster ochraceus pre-summer 2014 abundance and wasting disease progression. P. ochraceus were abundant in the intertidal survey areas in May, 2014 (a). Healthy sea stars (b) develop lesions (c) that can lead to arms detaching from central disk (d) prior to extensive tissue necrosis and death (e).

Figure 2.

Site map shows peak prevalence at each region and 2014 summer sea temperature anomalies. A total of 16 sites were surveyed at (a) the Washington outer coast, (b) the San Juan Islands and (c) South Puget Sound of Washington (see electronic supplementary material, table S1 for coordinates). SST anomalies shown for the region are 5-km resolution and are average anomalies for June–August, 2014, with anomalies during each month calculated by comparing daily data to the monthly mean (calculated from 1985 to 2012). Pie graphs show maximum recorded adult SSWD prevalence over our study period. The sites with samples sizes (n) and the date when peak prevalence occurred are (1) Colin's Cove (n = 12, 7 August 2014), (2) Crescent Beach (n = 8, 9 August 2014), (3) Eastsound Waterfront (n = 15, 8 August 2014), (4) Lonesome Cove (n = 17, 10 August 2014), (5) Pile Point (n = 44, 12 August 2014), (6) Point Caution (n = 48, 7 August 2014), (7) Reuben Tarte (n = 14, 10 August 2014), (8) Richardson (n = 6, 11 August 2014), (9) Rosario (n = 104, 12 July 2014), 10) Strathmann's Beach (n = 23, 7 August 2014), (11) Yellow Island (n = 56, 12 July 2014), 12) Ruston Way (n = 12, 15 July 2014), (13) Hyde (n = 3, 14 July 2015), (14) Point Defiance (n = 43, 30 January 2014), (15) Titlow Beach (n = 47, 1 January 2014) and (16) Starfish Point (n = 143, 21 January 2015).

Figure 3.

Proportional population decline in (a) San Juan Islands (SJI), (b) South Puget Sound (SPS), (c) outer coast (OC) and disease prevalence (d) SJI, (e) SPS and (f) OC of P. ochraceus. SJI and SPS graphs are for adult (>75 mm) ochre stars; OC graphs show data including both adults and juveniles, due to data constraints. Proportional population is scaled to the maximum number of ochre stars observed over the study period.

Figure 4.

Predicted probability of SSWD in SJI from June through August 2014 as a function of temperature (a) and size (b). Predictions are based on the SJI temperature model (table 2). Temperature predictions are for ochre stars with 10 cm radius, and size predictions are for 16°C.

Figure 5.

Demography of ochre stars before and after the SSWD epidemic. (a) Light shades represent 2014 data (before outbreak) and dark shades represent 2015 data (after outbreak). Colours represent geographical regions surveyed. Blue, South Puget Sound (January 2014 and May 2015); green, San Juan Islands (June 2014 and July 2015); orange, outer coast (June 2014 and June 2015). (b) Populations of adults (dark) and juveniles (light) relative to the pre-outbreak populations.

Figure 6.

Pisaster ochraceus at four different temperatures in a controlled experiment. Differential rates of disease progression (a), survival (b) and time between onset of disease signs and death (c) compared in adult (>75 mm radius) and juvenile (<75 mm radius). Time to disease onset and survivorship are affected by reproductive stages and temperature.