Long‐term Assessment of Temperate Octocoral Mortality Patterns, Protected vs. Unprotected Areas

Coastal marine protected areas (MPAs) are usually established with an aim to protect areas of special ecological value. However, protected areas tend to attract more tourism and associated recreational activities, thereby exposing the biota to new risks such as high diving activity. The effects of these drawbacks are still little known for low‐dynamic systems such as one of the most characteristic and fragile Mediterranean communities, the coralligenous community. Mortality rates were assessed in both heavily dived and lightly dived areas to evaluate the effect of diving on the survival of the gorgonian Paramuricea clavata . The study was designed to distinguish human‐induced causes from natural causes of gorgonian mortality and to provide criteria for sustainable management of protected areas. We examined total and partial mortality of adult colonies (>10 cm in height) at four locations, two each representing one of the two situations, heavily dived (MPA) and lightly dived (control), annually over a 9‐yr period (1992–2000). High levels of recreational activity did not affect the degree of injury of the colonies. On average, the proportion of live tissue of the colonies remained steady at ∼91%. Colony death by detachment was the main source of mortality at the MPA, four times higher than death due to overgrowth. Mortality rate due to overgrowth remained constant at both areas with an average of 1.2%/yr. Natural mortality rate in sites with low diving activity was estimated to be about 2.7%/yr, whereas high diving activity was estimated to increase the natural mortality rate of the species by a factor of three, up to ∼7.4%/yr. The demographic characteristics of long‐lived low‐turnover structural components of ecosystems makes them especially vulnerable to disturbance events. Since an increase in diving activity seems to be unavoidably brought about by the creation of any MPA, strict regulation of recreational activity must be guaranteed in areas with low‐turnover communities.