The 1997–98 El Niño event in Panama and Galápagos: an update of thermal stress indices relative to coral bleaching

Associations were examined between warm sea surface temperature (SST) anomalies (mostly associated with the occurrence of warm El Niño–Southern Oscillation events) and coral bleaching in Academy Bay, Galápagos Is. and Naos Is., Gulf of Panama, in the tropical eastern Pacific Ocean. Various SST-related metrics potentially associated with bleaching occurrence were computed. These quantities suggested that the 1997–98 event, considered one of the strongest in recent history, was comparable in intensity to the strong 1982–83 event for the regions studied. Previously established 'bleaching conditions' (function of maximum annual SST values and a degree-days index) were shown to be valid for the 1997–98 ENSO event. In Galápagos, the 1997–98 metrics fell within the previously identified 'bleaching conditions' and bleaching indeed was observed (although not so severe as earlier and populations subsequently recovered). In the Gulf of Panama, the metrics' values were outside the range of bleaching conditions and no bleaching was detected. Lower coral mortality in Galápagos in 1997–98 may have been due in part to the timing of largest SST anomalies and to SST fluctuations in early 1998. Additionally, since current coral populations in the Galápagos have originated from coral survivors and their progeny following the 1982–83 disturbance, the presently-observed lower mortality rates may also have resulted from host/symbiont combinations more resistant to high temperatures.