Les fonds chalutables de la région Nord de la Tunisie: 1.Cadre physique et biocoenoses benthiques. | The fishing grounds in North Tunisian waters. Physical factors and benthic communities.

The main characteristic of submarine morphology of the region North of Tunisia is its irregularity. The continental shelf often remains narrow, uneven and extended by a steep slope. Hydrological studies of the area reveal complex phenomena. This zone is essentially exposed to 2 opposite influences, one from surface Atlantic water which has its maximum extension during the winter and one from the eastern basin of the Mediterranean which seems to rise to the surface from spring to the summer. The contact of the 'Atlantic' and eastern Mediterranean waters forms an important area of mixed water dominant in the sector of the northern region. The heterogeneous nature of the sea bottom of this region explains the diversity of biocoenoses and their interpretation. Due to this particular type of morphology and the heterogeneous nature of the bottom of this region, the biocoenoses are numerous and broken into fragments with coral organisms being dominant. The light soil of the bottom is generally intercalated with substratum. The following areas are characterized by rich benthic population: corals on 'La Galite' sheld; algal development, northeast of Cap Zebib; abundance of Echinodermata (Antedon mediterranea) in the center of the Gulf of Tunis which is essentially populated by the biocoenose of the terrigenous mud; presence of Leptometra phalangium in the La Galite channel; existence of a prairie of Cnidaris funiculina in the high sea line and of Isidella in the middle sea line the light soily bathyal bottom, characterizes the biocoenose of the deep mud, and the branching madreporaria which are well represented in the hardbathyal substrata. Furthermore, the composition of benthos in the northern region is comparable to that in other regions of the western basin of the Mediterranean. That of the south-east compares, by many spp, with other regions of the eastern basin. This confirms the transitional character of the Tunisian coasts between the western and eastern Mediterranean basins.

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