Mass mortality event in red coral Corallium rubrum populations in the Provence region (France, NW Mediterranean)

A mass mortality event of the red coral Corallium rubrum (L.) occur-red in the NW Mediterranean region in summer 1999. The main objectives of the pres ent study were to document the mortality suffered by the red coral populati ons in the Provence region and assess ecological correlates of the mortalit y which could help to identify the putative agent or agents of the event. T he mortality outbreak resulted in partial to complete loss of the coenenchy me. The first observations of red coral mortality in the study area were in early October 1999 and continued until November 1999, To determine the ext ent of mortality on red coral populations, the incidence (= % colonies suff ering mortality) and the virulence [= % coenenchyme loss) were quantified. Twelve surveys were carried out from November 1999 to January 2000. A total of 874 colonies of the 2375 observed showed some mortality. Incidence and virulence differed significantly among surveys. In some cases about 80 % of the colonies were affected, while in others only 5 % of colonies showed so me mortality. Depth had a strong influence on the incidence of mortality. S hallower populations had the greatest incidence, while populations deeper t han 30 m depth showed no mortality. However, virulence was similar in the 1 0 to 30 m depth range. With regard to habitat, the entrances of caves displ ayed a significantly higher incidence but not virulence than the interiors. Given the life history traits of red coral (mainly slow growth and low rec ruitment), it seems that recovery from the outbreak will be a long-term pro cess. However, further studies are urgently required to provide basic infor mation regarding red coral population dynamics as a basis for hypotheses on the actual recovery capability Of affected populations. The cause of the m ass mortality is unknown. However, during late summer 1999, the NW Mediterr anean area affected by the mass mortality event experienced high temperatur es and hydrographic stability over a period of several weeks, This temperat ure anomaly could have caused physiological stress or/and triggered the dev elopment of pathogenic agents that otherwise would have remained non-virule nt. This hypothesis is reinforced by the absence of signs of mass mortality below 30 m depth in red coral populations. Since the NW Mediterranean seem s to be affected by the global warming trend, if the temperature hypothesis is confirmed, the long-term consequences of a repetition of this kind of e vent could severely endanger the persistence of shallow-water red coral pop ulations.