Demography of deep-dwelling red coral (Corallium rubrum) populations

Mediterranean red coral (Corallium rubrum, L 1758, Octocorallia, Gorgonacea) 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 for more than 2 thousand years. In the last two decades a reduction of the overall fishing yield by 2/3 has been recorded. Although our knowledge on the demographic features of red coral populations living between 10 and 50 meter depth has increased considerably in recent years, the main life-history traits of deeper populations (the main target of harvesting nowadays), are still largely unknown. To increase the demographic knowledge on the latter populations an ad hoc cruise was carried out during early Summer 2010 in the North and Central-East Tyrrhenian Sea, between 62 and 120 m depth. The aim of this study is to investigate the demography of deep Corallium rubrum populations located in Amalfi Coast/Ischia Island (Campania) and Elba Island (Tuscan Archipelago). The analysis of the sexual features, revealed a balanced sex ratio, a colony fertility of 90.3% and an average fecundity of 0.83 oocytes or planulae per polyp. The knowledge of these life-history descriptors is basic for our understanding of deep dwelling red coral population dynamics, to develop suitable demographic models and for matching harvesting to population growth rate. Colonies bigger than 7 mm in diameter (the minimum harvesting size in several countries) are 38% (32 on 84 colonies with a valid basal diameter). To determine the age of the colonies of Corallium rubrum, a sclerochronological approach has been applied. As red coral develops annual growth rings, composed of a dark and a light layer alternatively, colony age was assessed by counting these annual growth rings on thin sections of 69 colonies. On the basis of a 2-way ANOVA, which factors were age classes and observers, no significant difference was found between the age estimated by three independent observers. The average annual colony growth rate (basal diameter), showing some decrease with colony age increase, was 0.26 mm/yr, similar to those observed in other populations. These results suggest a similar growth pattern of C. rubrum over different geographic areas. The decrease in diameter growth rate with age suggests that diameter could be not the most reliable descriptor of colony growth for older colonies. The relationship between age and basal diameter fitted on a subsample of colonies was then applied to assess the age of a larger sample of the population. The majority of colonies (97.7%) fell in the 6 – 55 years range; about half of them (51.1 %) were in the 21-25 and 26-30 age classes. Only two colonies have an estimated age of 93 and 69 years. Effects of partial predation exerted by the gastropod Pseudosimnia carnea on reproduction of red coral were examined from a quantitative point of view. Colonies from a mesophotic population (living between 60-100 m depth) at Elba Island were collected prior to their annual spawning and were examined for their reproductive features. Female colonies were predated significantly more frequently than males (3:1 ratio); the mean number of polyps, the mean polyp fecundity and the mean colony fecundity, were significantly reduced by gastropod grazing independently from colony size. There was a dramatic effect on the main reproductive parameters of coral, especially on the colony reproductive output, reducing it by about 84 %. The consequent dramatic reduction of population reproductive output could have long-term effects on predated populations limiting their resilience to intense harvesting. The last part of my thesis deals with data collected by ROV (Remotely Operated Vehicle). The target of this part of my research was to establish a relationship between the data gathered on the small sample of colonies taken and the wider one of the colonies recorded by ROV. As red coral is an overharvested species, the number of colonies collected for research must be small. The analysis of about 3000 images recorded by ROV revealed that this method can give good results about occupancy, density, exposition, degree of sedimentation and of substrate inclination; discrete results about branching order but it is not reliable when the subject is the measurements of basal diameter at different depths.