Recurrent outbreaks of disease in sea urchins Strongylocentrotus droebachiensis in Nova Scotia: Evidence for a link with large-scale meteorologic and oceanographic events

Recurrent outbreaks of a disease (paramoebiasis, caused by a marine amoeba Paramoeba invadens) result in mass mortalities of sea urchins Strongylocentrotus droebachiensis in the rocky subtidal zone of the Atlantic coast of Nova Scotia (Canada). Recent epizootics in 1993 and 1995, like those which occurred in the early 1980s, were associated with unusually warm sea surface temperatures (SST) in late summer/fall of each year. Disease outbreaks were localized in 1993 and did not completely eliminate sea urchin populations, whereas a widespread epizootic caused near-complete mortality over similar to 130 km of coast (straight-line distance) in 1995. Interannual differences in the extent of mortality were related to differences in the temperature regime (peak temperatures prevailed longer in 1995), which is consistent with previously described patterns. The absence of mortality in 1994 is attributed to the inability of P, invadens to survive low (<0 degrees C) winter temperatures, suggesting it is an exotic pathogen. Recent outbreaks of paramoebiasis were associated with increased proximity to the coast of warm water masses in the summer/fall, as indicated by satellite-derived and ground-truthed charts of SST. Intrusions of Gulf Stream water as warm-core rings, which entrain and mix with shelf water, may contribute to coastal warming, although the frequency of ring formation was fairly constant among years (from SST charts, 1991 to 1995). Disease outbreaks during the past 2 decades have also been associated with years of relatively high tropical storm and hurricane activity in the northwest Atlantic (from records of the National Hurricane Center USA, 1976 to 1995), which may influence advection and mixing off Nova Scotia. These observations suggest that large-scale oceanographic and meteorologic processes may play a role in triggering epizootics by transporting the infective agent (ii P, invadens is an exotic species) and/or creating environmental conditions conducive to the propagation of the disease. Because of the stochastic nature of these external events, the dynamics of the rocky subtidal ecosystem appear to be highly unpredictable. Severe disease outbreaks which eliminate sea urchins cause major changes in community structure as barren grounds, previously dominated by sea urchins, are colonized by kelps and other macroalgae. This has important implications for coastal fisheries, particularly the rapidly expanding sea urchin fishery.