International Congress on Universitat Autònoma de Barcelona, Bellaterra, Catalunya, Spain July 22 - 27, 2006 Evaluation of Spondylus gaederopus Linneo, 1758 mass mortality event in the Columbretes Islands Marine Reserve (Western Mediterranean, Spain). ¹Kersting, Diego-Kurt, ²García-March, Jose R., ³Templado, José

1. Columbretes Islands Marine Reserve. SGPM-Tragsa. C/ Músico Perfecto Artola, 6A, 12003 Castelló, Spain. [email protected] 2. Marine Biology Laboratory, University of Valencia. C/ Dr. Moliner, 50, 46100 Burjassot (Valencia), Spain. 3. Museo Nacional de Ciencias Naturales, C/ José Gutiérrez Abascal, 2, 28006 Madrid, Spain.

Columbretes is a group of small volcanic islands located 30 nautical miles off the coast of Castelló (Spain) on the Balearic basin at the Mediterranean Sea. The Columbretes Islands Marine Reserve was created in 1990 by the Spanish Fishing Authority, Secretaría General de Pesca Marítima, Ministerio de Agricultura Pesca y Alimentación, and has become since then one of the best preserved marine environments on the Western Mediterranean. A mass mortality episode affecting the bivalve Spondylus gaederopus was detected during the summer of 2005. Mortality was also observed, although to a lesser degree, on other bivalves (Barabatia barbata and noae). In order to evaluate the mortality of Spondylus, censuses on known surface areas were undertaken in L’Illa Grossa inlet. A total surface of 401.9 m² was surveyed and 720 S. gaederopus specimens were censed. The average density of S. gaederopus in the surveyed areas was 2.3 ± 1.6 specimen/m² and maximum density of individuals reached 6.8 specimen/m² (n=27 areas). It was found that total mortality rate reached 99.3 %. The origin of this massive mortality is still unknown. Seawater temperature does not seem to be the cause, as there was no temperature anomaly during the summer of 2005. A similar mortality event was detected during the summer of 1981, 1982, and 1983 on the North-Western Mediterranean (Meinesz & Mercier, 1983)(1). These authors suspected bacterial, fungal or viral infection to be the main mortality cause.

The Columbretes Islands Marine Reserve encircles a volcanic archipelago situated 30 nautical miles off the coast of Castelló, Spain. The Marine Reserve was created in 1990 and is managed by the Spanish Fishing Authority, Secretaría General de Pesca Marítima. The Columbretes Islands seafloor is constituted mainly by rocky substrate close to the main islets reaching down -50 m. Soft bottoms, mainly detritic gravels and maërl beds, cover wide areas between the islets. The bivalve Spondylus gaederopus is ubiquitous around the Columbretes archipelago and appears mostly on rocky substrate, preferring vertical or overhanging walls with dim light conditions in a depth range from - 3 to -35 m. A mass mortality event affecting Spondylus gaederopus was detected in the Columbretes Islands Marine Reserve during the summer 2005. Previous mortalities on this have been reported in the Northwestern Mediterranean during the summer 1981, 1982, and 1983 (Meinesz & Mercier, 1983). Materials and Methods In order to obtain the percentage of mortality on S. gaederopus, in August 2005 censuses on known surface Columbretes Islands Marine Reserve areas in SCUBA diving were undertaken. As the main objective was to evaluate the mortality, the study areas were located on rocky substrate with dim light conditions (mainly vertical walls and overhangs), where S. gaederopus is abundant. The surface coverage for each of these areas depended on the morphology and size of the wall, boulder or overhang to be surveyed. The areas were located in three different sites of the Illa Grossa inlet in a depth range of -4 to -16 m (Fig.1). The following data were obtained for each area: maximum and minimum depth, compass reading (orientation of the substrate), morphology of the substrate (overhanging, vertical, etc.) and the number of living or dead S. gaederopus individuals, the presence of other dead bivalves was noted down as well. The areas were carefully revised to make sure that all the living individuals were detected, as these are covered by sponges such as and could be easily overseen. The counting of dead specimens was much easier since the bright white colour of the inner side of the cemented valve remaining in the rock was detectable from the distance. The total number of specimens obtained for each area was also used to get information about the density of S. gaederopus in the Illa Grossa inlet. Qualitative surveys were also carried out in other parts of the Fig.1. Survey sites in archipelago to estimate the spatial coverage of the mortality in the Marine Reserve. L’Illa Grossa inlet. Since the first surveys started, in August 2005, until late October 2005 some of the living S. gaederopus Results censed were monitored so as to evaluate the possibility of a continued mortality after the first detected strike. The first dead S. gaederopus specimens were found in the Illa Grossa inlet in early June 2005. A total number of 27 areas was surveyed, ranging in surface size from 4 to 40 m², altogether the surface studied reached 401.9 m². An overall sum of 720 S. gaederopus specimens was censed and the average density of this species in the surveyed areas was 2.3 ± 1.6 specimen/m² (mean ± SD) (n=27 areas). It is remarkable that the maximum density observed reached 6.8 specimen/m². (Fig.3) Among the 720 specimens censed 715 were found dead, therefore the global mortality rate went up 99.3%. In the majority of the studied areas (24 of 27) the mortality rate reached 100%. (Fig.2) As a result of the qualitative surveys done in other parts of the Columbretes archipelago it was confirmed that the mortality spread over the whole area, independently of the type of rocky substrate or depth. Other bivalves such as Barabatia barbata and Arca noae were found dead, although in a much lesser degree than S. gaederopus. Shell accumulations of these species (B. barbata and A. noae) were to be found at the base of the rocky substrate, nevertheless many alive individuals were easily detected on the walls and overhangs. The monitoring on the living S. gaederopus, carried out after the mass mortality event until late October 2005, resulted in a 100 % survival rate of the individuals.

S. gaederopus mortality (%)

100,00

99,00

98,00

97,00

96,00

95,00

94,00

93,00

92,00 b1 b2 b3 b4 b5 b6 t1 t2 t3 t4 t5 t6 e1 e2 e3 e4 e5 e6 B1 B2 B3 B4 B5 B6 B7 B8 B9 areas Fig. 2. Mortality rate in each of the survey

areas. 2 S. gaederopus specimen affected by the S. gaederopus reached high density values S. gaederopus density (specimen/m ) mortality (before loosing the non cemented in L’Illa Grossa inlet. 8,00 valve). Dead specimens of B. Barbata and 7,00 A. noae were found at the base of 6,00 Discussion the rocky substrate. 5,00 The mortality rate on S. gaederopus found in the Columbretes Marine Reserve was extremely high (global mortality rate of 99.3%). This study shows that the mortality rate reached in Columbretes is much higher than those described by Meinesz & 4,00 Mercier in the North-Western Mediterranean for the summer 1981,1982 and 1983 (accumulated mortality rate after three 3,00 seasons of 90%). 2,00 The cause of the massive mortality described for the Columbretes Islands is still unknown. Meinesz & Mercier suspected bacterial, fungal or viral infection to be the main mortality cause in the mortality event from summer 1981 to 1983. The 1,00 observations done during the surveys point out to the possibility of an epidemic too: the specimens died independently of 0,00 depth, substrate characteristics and individual size, and the mortality spread in a short period of time affecting a very high b1 b2 b3 b4 b5 b6 t1 t2 t3 t4 t5 t6 e1 e2 e3 e4 e5 e6 B1 B2 B3 B4 B5 B6 B7 B8 B9 areas number of individuals. Fig. 3. Density of S. gaederopus in each of the There is no available data on the impact of the mortality described by Meinesz & Mercier in the Columbretes Islands during survey areas. the eighties. If S. gaederopus suffered mortality, the species would had reached a successful recovery by summer 2005 in the Columbretes archipelago, where it had reached high densities as we noted in our surveys. Aknowledgments. In the last decades several mass mortality episodes affecting different species have been described for the North-Western Our thanks to the “Secretaria General de Mediterranean (Cerrano et al. 2000; Garrabou et al. 2001; Perez et al. 2000; Romano et al. 2000). In most cases there is little Pesca Marítima” (Spanish Fishing Authority) knowledge about these events and information about the mortality cause or the spatial coverage is scarce. In order to have a and the "Conselleria de Territori i Habitatge", which are responsible of the better understanding of these aspects, studies involving longer historical series and a wider taxonomic range are needed. management of the Marine Reserve and the Natural Reserve of the Columbretes Islands. References. We are very grateful to the Marine Reserve Cerrano C., Bavestrello G., Bianchi N., Cattaneo-vietti R., Bava S., Morganti C., Morri C., Picco P., Sara G., Schiaparelli S., Siccardi A., Sponga F. 2000. A catastrophic mass-mortality episode Wardens (Valentín Tena, Roque Belenguer, of gorgonians and other organisms in the Ligurian Sea (North-western Mediterranean), summer 1999. Ecology Letters, 3:284-293. Santiago Sales, Vicente Ferrís, Mar Prados, Garrabou J., Perez T., Sartoretto S., Harmelin J.G. 2001. Mass mortality event in red coral Corallium rubrum populations in the Provence region (France, NW Mediterranean). Marine Ecology Rebeca Velasco, Jacobo Méndez and José Progress Series, Vol. 217:263-272. Gisbert) whose enthusiasm has made it Meinesz A. & Mercier D. 1983. Sur les fortes mortalités de Spondyles (Spondylus gaederpopus Linné) observées sur les cotes de Méditerranée. Trav. sci. Parc nation. Port-Cros, 9: 89-95. possible to protect effectively this paradise, Perez T., Garrabou J., Sartoretto S., Harmelin J.G., Francour P., Vacelet J. 2000. Mortalité massive d’invertébrés marins: un événemente sans précédent en Méditerranée nord-occidentale. and whose help and collaboration is crucial C.R. Acad. Sci. Paris, Sciences de la vie, 323: 853-865. to develop studies in this rough place. We Romano J.C., Bensoussan N., Younes W.A.N., Arlhac D. 2000. Anomalie thermique dans les eaux du golfe de Marseille durant l’été 1999. Une explication partielle de la mortalité d’invertébrés would like to thank Marta Molinos for her fixés. C.R. Acad. Sci. Paris, Sciences de la vie, 323:415-427. useful help and advice.