An outbreak of sea urchin disease associated with a recent hurricane: Support for the “killer storm hypothesis” on a local scale

The frequency of epizootics causing mass mortality of sea urchins (Strongylocentrotus droebachiensis) along the coast of Nova Scotia, Canada has increased over the past three decades. Laboratory and field studies show that outbreaks of disease are caused by the amoeba Paramoeba invadens, and are associated with hurricane activity and warm seawater temperatures. A statistical model indicates that the probability of a mass mortality event increases with the proximity of a hurricane to the coast and the maximum sustained wind speed, and is greater when post-storm seawater temperature is above a threshold for disease propagation. To assess the reliability of the model in predicting mass mortality events on an annual scale, and to examine spatial variability in mortality (in the event of a disease outbreak) on a local scale (meters to kilometers), we transplanted sea urchins into cages in kelp beds at 6 sites around St. Margarets Bay, Nova Scotia, where localized outbreaks of paramoebiasis have been observed following hurricanes in the past. On 4 September 2010 a Category 1 hurricane (Earl) made landfall 110km SSW of the experimental area. Based on the parameters of the storm, the model forecasted a 43% probability of a disease outbreak. Morbidity of caged animals was first documented on 6 September 2010, and morbidity and mortality in the cages was ~50% by the end of September and ~85% two months after the storm. Laboratory experiments indicated that the temperature-dependent transmission or induction of morbidity was consistent with paramoebiasis. Our findings provide support for the efficacy of the model to predict the occurrence of disease outbreaks, although the source population(s) of the pathogenic agent and oceanographic mechanisms affecting its introduction and spread along the coast of Nova Scotia remain poorly understood.