RECURRENT OUTBREAKS OF DISEASE IN SEA-URCHINS STRONGYLOCENTROTUS-DROEBACHIENSIS IN NOVA-SCOTIA - EVIDENCE FOR A LINK WITH LARGE-SCALE METEOROLOGICAL AND OCEANOGRAPHIC EVENTS

Recurrent outbreaks of a disease (paramoebiasis, caused by a marine am oeba Paramoeba invadens) result in mass mortalities of sea urchins Str ongylocentrotus droebachiensis in the rocky subtidal zone of the Atlan tic coast of Nova Scotia (Canada). Recent epizootics in 1993 and 1995, like those which occurred in the early 1980s, were associated with un usually warm sea surface temperatures (SST) in late summer/fall of eac h year. Disease outbreaks were localized in 1993 and did not completel y eliminate sea urchin populations, whereas a widespread epizootic cau sed near-complete mortality over similar to 130 km of coast (straight- line distance) in 1995. Interannual differences in the extent of morta lity were related to differences in the temperature regime (peak tempe ratures 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 tem peratures, suggesting it is an exotic pathogen. Recent outbreaks of pa ramoebiasis were associated with increased proximity to the coast of w arm 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 contribut e to coastal warming, although the frequency of ring formation was fai rly constant among years (from SST charts, 1991 to 1995). Disease outb reaks during the past 2 decades have also been associated with years o f relatively high tropical storm and hurricane activity in the northwe st Atlantic (from records of the National Hurricane Center USA, 1976 t o 1995), which may influence advection and mixing off Nova Scotia. The se observations suggest that large-scale oceanographic and meteorologi c processes may play a role in triggering epizootics by transporting t he infective agent (ii P, invadens is an exotic species) and/or creati ng environmental conditions conducive to the propagation of the diseas e. Because of the stochastic nature of these external events, the dyna mics of the rocky subtidal ecosystem appear to be highly unpredictable . Severe disease outbreaks which eliminate sea urchins cause major cha nges in community structure as barren grounds, previously dominated by sea urchins, are colonized by kelps and other macroalgae. This has im portant implications for coastal fisheries, particularly the rapidly e xpanding sea urchin fishery.