Gorgonian corals are long-lived, slow-growing species exhibiting slow population dynamics. Demographic data collected over a period of 11 yr on a small population of temperate, canopyforming gorgonians dwelling near the edge of the summer thermocline in the northwestern Mediterranean Sea enabled us to assess its responses to the large mortality events that occurred in 1999 and 2003. Changes in population density, size structure and recruitment were examined. Overall, 2101 adult colonies and recruits were observed in situ and 240 photographic plots analyzed. During the first 3 years (2004 to 2006) after the mass mortalities, our measurements revealed a dramatic reduction in the density of healthy colonies (90% suffered total or partial mortality), a shift in the dominant size class towards smaller size and a significant reduction in recruitment. In the following years (2007 to 2008) a significant recovery of injured colonies was found, with almost complete detachment of the dead colonies, a reduction in mortality and a 4-fold increase in recruitment. These findings indicate clear-cut restoration trends of the population, suggesting that recovery after extensive mortality could be faster than predicted by our current knowledge of gorgonian population dynamics.

2. The population was monitored for three years after the mortality events. Availability of pre-event data (1998) allowed comparison of population density and population size structure of the healthy population with those recorded in the three years following the mortality events.3. In 1998, before the two mass mortality events, mean colony density was 33:3 AE 3:7 colonies m À2 and had fallen to 6:7 AE 1:9 colonies m À2 in 2004. 4. In the post-event period the population size structure changed and the modal class of colonies shifted from 16-21 cm to 6-15 cm height.5. In 2004 mortality affected 75 AE 6.4% of colonies. A significant, positive correlation between the extent of damage and colony size was found throughout the monitoring period.6. In the three years following the two mortality events, a small increase in density of recruits and of older undamaged colonies was recorded suggesting that the population was slowly recovering.7. The bathymetric distribution of P. clavata straddles the summer thermocline making this population particularly sensitive to temperature increases. The lack of deeper colonies (less exposed to warming) and the geographical isolation of this population is likely to prevent any substantial external larval supply.8. An increased frequency of mass mortality events associated with ever increasingly high temperature events represent a considerable threat to the persistence of a P. clavata population in the Gulf of La Spezia.

A population of the Mediterranean red gorgonian Paramuricea clavata has exhibited unexpected resilience after being impacted by 2 anomalous mortality events in 1999 and 2003. To understand the recovery mechanisms, we examined the population reproductive structure and reproductive output based on data collected via non-destructive sampling techniques. The overall population sex ratio was balanced, though the spatial distribution of sexes was significantly segregated. Dividing the population into 14 size classes on the basis of their measured average annual growth revealed a decreasing monotonic trend of abundance of larger classes. The Recruitment class was consistently dominant. The minimum size at first reproduction was 8.5 cm in height, corresponding to an age of ~3 yr. The percentage of fertile colonies increased with size, reaching 90% in size Class 9. Polyp fecundity increased with colony size and did not differ signif- icantly between healthy and damaged colonies. As the number of mature oocytes produced by a colony is a function of polyp fecundity and of the number of reproductive polyps, colony reproduc- tive output increased exponentially with size. The population reproductive output (145 × 103 mature oocytes m−2 yr−1) was one-fifth of that measured in stable, undamaged populations and came mainly from the medium size classes. After the catastrophic mortality, the population has been recovering, albeit with reduced reproductive output. Moreover, it has exhibited a 2-fold increase in recruitment rate, 3-fold greater than that measured in other, undisturbed populations. Our findings are consistent with a strict density-dependent recruitment control operating in crowded, stable P. clavata populations

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