Population Dynamics and Life History Traits of Corallium Rubrum Populations: Implications for Sustainable Management and Trade

Population demographics, especially density, abundance, size structure, and morphology (branching pattern), as well as certain life history traits (e.g. growth rates, reproductive strategy, and longevity), must be considered when developing fisheries management strategies for precious corals in the family Coralliidae, as these provide indicators of the status of these species and their vulnerability to harvest. All species of Coralliidae are sessile, modular (colonial) gorgonians characterized by slow-growth, low rates of natural mortality, and a long lifespan. They typically occur at low densities, but are capable of forming large, highly branched, tree-like structures in absence of disturbance. Most representatives in this family broadcast gametes into the water column on an annual cycle, relying on external fertilization and an extended planktonic development phase, while the endemic Mediterranean species, C. rubrum, is a brooder that releases well-developed, competent planula. This trait has allowed C. rubum to form dense assemblages in shallow water, where populations are dominated by small (< 5 cm tall), unbranched colonies. These populations exhibit high rates of early mortality, especially in environments frequently disturbed by natural stressors (e.g. temperature anomalies, bioerosion) and heavy collection pressure. However, this is not the normal situation: C. rubrum colonies are capable of achieving a much larger size (30-50 cm height) and more extensive ramifications, as reported historically, in deep water populations, and in shallow water when protected from fishing. The brooding life history strategy may have allowed the persistence of C. rubrum in marginal environments and when subjected to heavy fishing pressure, because planula display lower rates of mortality and higher settlement success than related broadcast spawning species. Nevertheless, these populations are highly susceptible to extirpation due to the selective removal of the largest colonies by SCUBA divers, followed by collection of progressively smaller colonies; these smaller colonies produce exponentially fewer larvae per reproductive event than large colonies and they reproduce only a limited number of times before being removed from the population. Furthermore, C. rubrum exhibits a single annual discrete reproductive event unlike other corals that are brooders, and local sources of planula are needed to maintain populations due to limited potential for long-distance dispersal. Deep populations may be even more vulnerable to extirpation, since they occur at much lower densities; they may experience allee effect (depensation) if harvest further reduces colony density below levels needed for successful fertilization. An increase in the minimum size of legal harvest (to at least 10 mm diameter and 10 cm height) is needed to rebuild overexploited stocks of C. rubrum and provide insurance against catastrophic disturbances. This measure would maximize reproductive output by increasing colony size, allowing colonies to develop third order branching patterns, and increasing the number of times a colony reproduces before removal by collectors. Harvest should be limited in deeper areas (> 50 m) where densities are lower until population dynamics and life history traits from the deep populations are better understood. Other management approaches, such as an increase in the number and size of no-take marine protected areas, are needed to maintain high reproductive output and compensate for losses due to fishing pressure and other stressors outside of protected areas.