Mediterranean red coral (Corallium rubrum, L 1758) is a colonial anthozoan endemic to the Mediterranean Sea. Due to the high economic value of its carbonate axial skeleton this species has been harvested and traded since ancient times. In the last two decades a reduction of the overall fishing yield by 2/3 has been recorded.
Data on shallow populations suggest that several coastal shallow-water populations have been depleted and colony size reduced due to the over-harvesting these populations faced. These populations need therefore conservation and restoration actions. To this aim management approaches based on a detailed knowledge of specific population demographic features are the key to ensure long-term exploitation sustainability. At the present long-term studies on Mediterranean red coral demography are scarce due to its slow growth and longevity.
Aim of this study is to investigate the demography of two different populations of Corallium rubrum located in Portofino (Ligurian Coast, Italy) and Cap de Creus (Costa Brava, Spain). Both populations dwell within two Marine Protected Areas in which Corallium rubrum management is different. Harvesting is regulated at Cap de Creus while banned at Portofino since 1999. Moreover, illegal harvesting also occurs in the Spanish area, seriously affecting the local Mediterranean red coral population.
To determine the age of the colonies of Corallium rubrum in the two areas a sclerochronological approach has been applied.
The estimated colony growth rates of the two populations (0.22 and 0.24 at Portofino and Cap de Creus respectively) are similar to those observed in other populations and CaCO3 deposition rate (measured as circular crown area) is constant overtime. These results suggest a similar growth pattern of C. rubrum over different geographic areas.
The decrease in diameter growth with age suggests that this colony descriptor may not be the most reliable descriptor of colony growth mostly for larger / older colonies. Others, such as weight or branching pattern, should be taken into account to assess the growth rate of the red coral.
Differences in the life-span of the two populations indicate a recovery trend in Portofino from the previous exploitation occurred till the 1970s. On the contrary the population of Cap de Creus, still object of both legal and illegal harvesting, shows a shorter life span by about 25% and a population structure dominated by young colonies.
Data on 1) colony density; 2) population size/age structure, 3) reproduction; 4) survival have been merged in two “static life – history tables” – one for each population - in order to describe population – specific structure. Lower densities, shorter life - span, lower larval output and higher mortality emerged for the harvested population of Cap de Creus than Portofino, despite the two populations present the same sex ratio and the same average fecundity.
The minimum size for commercial harvest of C. rubrum in Cap de Creus is 7 mm. These findings underline that such measure could represent a conservative limit allowing adult colonies to reproduce about 25 times during their life-span. Despite this, it is worth to note that colonies bigger than 7 mm in diameter are very scarce in Cap the Creus (only 4 on 163 colonies collected on whole, and not considered in population age determination because of morphological limitation). It is necessary for a sustainable fishery management, to maximise the yield ensuring an adequate number of sexually mature individuals: 7 mm in diameter could be a prudential measure, whatever respected.
The “static life – history tables” here reported will be the base to set out demographic models that will allow to project population trends over time. This approach, applied to populations with different life-histories, will allow to compare their demographic structures and trends and to set out conservation and management strategies suitable for specific local populations.