Esquisse d'un bilan écologique provisoire de l'impact de la marée noire de L"Amoco-Cadiz" sur le littoral

The wreckage of the tanker "AMOCO CADIZ" on the reefs of PORTSALL was the fourth oil spill in ten years on the coast o£ Brittany : it also is the largest known oil spill in the world. The patches at sea were driven eastward by westerly winds from March 17 to March 23. 140 Km of coast line was polluted. The oil spreaded more slowly towards the north west, south and north west where it spoiled some 375 Km of touristic sea coast and important fishing grounds. The coastline is six kilometers wide ; it includes islands and large reefs. The local tidal currents are very strong and reach more than three knots, during spring tides which the slicks and mixed the oil within most of the water column, reducing its toxicity. Because of the geomorphological conditions, the rocky, sandy and intricate biotopes are extremely varied and the settlements are diversified. The fauna and flora has been well studied for ever 100 years and reference data have been collected for all this area. Around the hull of the ship, high mortality rates were observed for all species on the first day. Many dead animals were washed ashore ten to fifteen kilometers around the wreckage. The oil spill affected the Gastéropodes living on the rocks and certain species living in sediment. The sea urchins, Cockles, Cardium edule, Solenidae, Veneridae, Ampeliscidae and others animals, such as worms (Ophelidae, Haustdidae, Arenicolidae), Lamellibranches (Lellinidae) remained in normal numbers. The settlements of species dwelling in the sediment were more affected below the tidal zone than on the beaches. The sea-weed was oily on nearly all the shores especially on the higher levels of the sheltered areas. But, they still lived, grew ans seemed to reproduce. However, they will generally be unsuitable to be used as food products. The field of Laminaria (400 Km in Tregor and Léon) has been normally harvested since May. The Plancton life is nearly back to normal. Fish and Crustaceans were slightly affected but were edible starting from the beginning of June. About 20.000 birds died. The most affected bays and estuaries are : ABER BENOIT, ABER WRACH, Baie de MORLAIX, Baie de LANNI0N. The oil settled on the infralittoral areas. There, the concentration of hydrocarbons seemed to increase into the cleaning action of the waves on exposed areas. Black mud is a geochemical trap and keeps the hydrocarbons preserved. The ecological effects observed were moderate when compared with the enormous volume of oil spilled at sea. 115 I0/ Dilution of the oil due to the high turbulence of the water. 2°/ Efficient means for collecting the oil which ran aground (10 to 20.000 tons). 3°/ Restricted use of slightly toxic detergents (2.000 tons) following the advice of scientifists.