Demography, reproductive biology and trophic ecology of red coral (Corallium rubrum L.) at the Costa Brava (NW Mediterranean) : ecological data as a tool for management = Demographie, Fortpflanzungsbiologie und Ernährungsökologie der Roten Koralle (Corallium rubrum L.) an der Costa Brava (NW Mittelmeer) : ökologische Daten als Werkzeug im Management

摘要

The Mediterranean red coral (Corallium rubrum, L. 1758) is a characteristical gorgonian living in semidark habitats in the coralligenous zone of sublittoral hard bottoms. Despite being harvested commercially for the use of its red calcium carbonate skeleton in the jewelery industry, its ecology is still little understood. Thus the objective of this study was to increase knowledge on the population structure, reproduction and feeding ecology of red coral and to use these data to evaluate the efficiency of current management strategies of this species.The size/age structure of a population records valuable information, summarising the effects of mortality, recruitment and growth of past years. The six studied populations at the Costa Brava (NE Spain) consisted mainly of small/young colonies, as larger colonies are commercially exploited. Average size and age were estimated at 3 cm height and 7.5 years, whereas the species is capable of reaching 50 cm and 100 years. Furthermore, these young populations consist of colonies that have developed only few branches, offering considerably less three dimensional structure to the ecosystem than unharvested populations. One of the studied populations is located in a protected area, the Medas Islands Marine Park, and consisted of significantly larger/older colonies, thus allowing to distinguish harvesting effects from environmental ones.The reproductive output of red coral was found to significantly depend on colony size, having important implications for its management. Another aspect of potential importance for optimal harvesting is mesoscale variation of reproductive output. In a comparison of eight stations, however, we did not find significant differences, indicatig a homogenous reproduction. Colonies in deep water were found to spawn slightly earlier than shallow water ones, however, this phenomenon was observed only when temperature stratification in summer was particularly pronounced. Yet depth was not found to affect reproductive output itself, again in contrast to the demographic features.The trophic ecology is one of the key aspects of the biology of an organism, helping to understand its demography and reproductive biology. The main proportion of the diet of Corallium rubrum was found to be particulate organic matter, while crustaceans such as copepods played a smaller role due to the irregular capture. Red coral appear to be able to meet their energy requirements by constant access of resuspended detritic material, while taking advantage of occasional capture of large zooplankters. Prey capture rate per polyp is higher in small colonies, and also in colonies situated in deep water. We did not find a clear seasonal pattern in red coral feeding, as observed in some gorgonians, which indicates that changes in water movement have a stronger effect on the prey capture rate than the seasonal succession of the plankton. The grazing impact of red coral populations was estimated to be lower than that by other gorgonians, which is a result of its population structure being dominated by small colonies.Ecological data like those presented in this study are the base for ecosystem management, as they allow to calculate the secondary production of a population. We therefore used the demography and reproduction data to provide managment recommendations for red coral, by using the Beverton-Holt fishery management model to calculate maximum sustainable yield. The results show that the red coral fishery at the Costa Brava would reach its maximum yield only at a considerable age of first capture, whereas the colonies are harvested at a much younger age. This results in an estimated annual catch that is only a fraction of what could be harvested using an optimised management strategy. Furthermore, harvesting at this age provides a short reproductive period before capture, compared for example to the sustainable fishery of black coral in Hawaii. The comparison of calculated and declared harvests indicates considerable poaching activities and undeclared harvest. Finally, a recent rise in mortality confirms a non-sustainable level of harvesting.Concluding, the results confirm the paramount importance of data on the reproductive biology and population structure of a species for its management. In this study we investigated the effects of harvesting, spacial mesoscale variability and depth on the population structure, reproduction and feeding ecology of red coral. The data show that red coral populations at the Costa Brava are very young and have a diminished reproductive potential due to overharvesting, yet show a considerable impact on the seston. Protecting young colonies is proposed as the central management measure.