Quantitative approach of the relationship between Sarpa salpa (Teleostei, Sparidae) and the Posidonia oceanica seagrass beds in the Mediterranean
Houziaux, J.-S.; Lecloux, A.; Michel, C.; Voss, J. (1993). Quantitative approach of the relationship between Sarpa salpa (Teleostei, Sparidae) and the Posidonia oceanica seagrass beds in the Mediterranean. Belg. J. Zool. 123(Suppl. 1): 34
In: Belgian Journal of Zoology. Koninklijke Belgische Vereniging voor Dierkunde = Société royale zoologique de Belgique: Gent. ISSN 0777-6276; e-ISSN 2295-0451, meer
Sarpa salpa is the only Mediterranean seagrass grazer fish. This gregarious species can be observed from 0 to 40 meter deep, especially on rocky shores. Despite the growing interest of scientists for the Posidonia oceanica seagrass beds, very little is known about this "herbivorous" fish. Recent observations in Calvi Bay (Corsica) revealed a significant diminution of the average leave's length on the -10 m isobath (1). These results have motived us to start quantitative studies on the Sarpa salpa population of the region. Field observations were realised in Scuba diving in an area of about 40,000 m2, near Calvi. Total abundance and biomass estimation revealed a density of about 0.1 fish / m2. We noted an important vertical migration of big individuals, going deeper between May and June. This may be in relation with the important development of the epibiontic community of the Posidonia leaves, but also with the approach of the mating season (occurring in September at about 40 meter deep). We have realised an approximation of the impact of the fish on the seagrasses. Therefore, we measured the average weight of an ingested Posidonia particle (which look undigested) and the average number of bites given per time unit. We associated to this data the density value and found that about 0.02 % of the Posidonia biomass is daily eaten. Biochemical analyses (enzymatic activities) on the digestive tractus show that Sarpa salpa is able to digest the animal fraction of its alimentation (i.e. epibiontic community of the Posidonia leaves and algae). The strictly herbivorous status given to this fish may thus be very debatable. This first global approach of this species shows that it probably has an important place in the ecosystem. Future studies must be improved in this way for a better comprehension of the relations between grazers and seagrasses. (1) S. BELKHIRIA (1992). Mémoire de licence, Fac. Sc., Univ. de Liege, 56 pp., unpublisheddocument.
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