Gulf of Gabes - Ecology

A defining characteristic of the Gulf of Gabes is the Posidonia oceanica meadow which is the most important across the Mediterranean and one of the largest in the world (Batisse and Jeudy Grissac 1998). The meadow is a perfect spawning ground for all marine organisms and a nursery for juvenile fish, giving it the status of nursery (Hattour et al. 1995). The Gulf of Gabes houses 247 of the 327 fish species of Tunisia with 44 species recorded only in this gulf (Bradai et al. 2004). The biodiversity of the Gulf of Gabes consists of 46 species of Echinoderms, 6 species of Cnidarians, 57 species of Bryozoa, 11 species of Annelids, 24 species of Crustaceans, 17 species of Ascidians, 108 species of Sponges and 171 species of Molluscs (Afli et al. 2005).

This high biodiversity is threatened by three factors: unsustainable fishing, pollution and exotic species. First, fishing, and particularly bottom trawling, causes a progressive but systematic destruction of Posidonia meadows. The regression of the meadows that give the Gulf of Gabes its reputation, has been reported in the literature since 1925 (Le Danois 1925). More recently, trawling has led to bare bottoms in many areas of the gulf. Second, the Gulf of Gabes undergoes severe problems of pollution especially from the phosphate industry. Discards of chemical complexes led to the accumulation of a very thin black deposit on a completely azoic surface of 230 hectares spreading out to a distance of 3 km. The combined effect of pollution and overfishing has resulted in large changes in the Gulf of Gabes ecosystem. Large areas of Posidonia have disappeared, Cymodocea nodosa is scare and Caulerpa prolifera has almost completely disappeared. Regression of the vegetation cover by 90% led to biological communities characterized by flora and fauna of degraded environments (Ben Mustapha et al, 1999). Third, the Gulf of Gabes is being invaded by alien species. Indeed, several species, mainly Indo-Pacific, have emerged, such as the bivalve Pinctada radiata currently abundant in the gulf (Bradai, 2000), the shrimps Metapenaeus monoceros and Trachypenaeus curvirostris that strongly compete with the native shrimp Penaeus kerathurus (Missaoui & Zaouali 1995), the green alga Caulerpa racemosa (Ben Alaya, 1971) and the Brachyura Eucrates crenata that become very abundant (Zaouali, 1992). There are about twelve exotic fish species including Siganus luridus, Siganus. Rivilatus, Parexocoetus mento, Seriola fasciata and Pisodonophis semicinctus.