Le plateau continental Nord-Gascogne et la grande vasiere étude bionomique

The northern Gascony area of the continental shelf runs from the southern Armorican and Vendée regions to the Rochebonne plateau, i.e. 47°50'N to 46°20'N. This vast platform 60 to 85 nautical miles wide is above all characterised by the presence of the Great Mudflats. In summarising thirty years of research on the Atlantic coasts, Le Danois (1948) defines it as "NW-SE trending; the mudflats stretch over more than 150 nautical miles in length and 30 to 40 nautical miles in width from Penmarc'h Point to the Island of Oléron". This area is heavily fished. In numerous ports from southern Brittany to Vendée, data was collected on the different fishing locations. Nevertheless, no comprehensive marine map exists to date. The Institute of Marine Fisheries provided the research vessel "La Pelagia" for dredging expeditions carried out in collaboration with two geomorphologists, Mr Pinot and Mr Vanney. This allowed us to complete the work started on board the "Kornog". The main biological data will be presented here (Glémarec, 1969). The dredging grid, whose cell size should be even further reduced in the future, shows that the Great Mudflats are not as homogeneous as previous trawling expeditions led us to believe. It is, in fact, a mosaic of different substrates, always muddy but dissimilar, and lined with rocky banks that can reach great proportions. Landside, the Great Mudflat lies between the 70 and 90 m isobaths on the coastal strip or the prelittoral zone, which is a submarine extension of the Armorican Massif. The Mudflats are located at the foot of rock cliffs that rise more than 30 m (off the shore of the southern Brittany coast), of gravel beaches stretching between prelittoral rocky massifs (in Vendée), and, more rarely, of fine sandy beaches bordered by rocky ridges (to the south-east of Groix). Offshore, from 100 to 120 m according to the region, the Mudflats are limited by a series of "hard bottoms", buried rock massifs covered with a layer of coarse sediment. This fact is important and has only rarely been demonstrated, until now. Although channels exist, the Great Mudflats are separated from the sandy stretches (130 to 160 m in width) that constitute the external edge of the continental shelf. On the map (Figure 1), the 200 m isobath drawn by Berthois and Brento (1964) is the limit of this platform. The prelittoral sea floor will not be discussed here (Glémarec, 1965). The following features will be addressed in turn: I. Great Mudflats; II. Hard-bottomed sea floor; III. Sandy stretches of the external margin; IV. Channels. (unverified OCR)