SURFACE MEGAFAUNA RELATED TO WESTERN MEDITERRANEAN CIRCULATION

The present results come from the campaigns of the international multi disciplinary programme conducted by P. La Violette (Naval Oceans Resea rch and Development Activities, NORDA), the Western Mediterranean Circ ulation Experiment. The mesoscale instabilities of the Mediterranean c irculation induce meanders, gyres and eddies acting on the production potential of living resources. These resources are evidenced here thro ugh the cetaceans and other large organisms directly observable in the surface waters of the sea, along the track of the cruise NORDA 706 (M ay-June 1986), and through the underlying biomasses detected by an ech o sounder, 12 kHz fishfinder. Acoustics detection was continuous, set at the same level by day and night for 15 days interrupted only when t he ship stopped for hydrological measurements at 87 CTD sampling stati ons. The pattern of hydrological sampling was designed in order to ide ntify the intermediate water veins in the Algerian Basin. The three se ts of field data - daily visual census, continuous echo-sounding and h ydrological profiles - were compared and processed in statistical fact orial and multiregression analyses. These show that presence of cetace ans coincides with highly concentrated echos; other large organisms ob served are distributed at random, without any link with the concentrat ed echos. It results that cetaceans have the ability to detect acousti cally the same biomasses as humans do with a 12 kHz echo-sounder; this is an argument for the ''echolocation of the Odontoceta'' and ''vocal ization of the Mysticeta''; up to now, such processes are not accepted by all cetologists. Four examples of echogram are reported here assoc iated with fin whales, sperm whales, pilot whales and dolphins. Dynami cal analysis of the Mediterranean circulation before and during the cr uise by contemporaneous remote sensing imagery (CZCS and NOAA 9) enabl es deduction of the dynamic characteristics of the areas where high de nsities of acoustical detection co-occur with the presence of cetacean s. Such areas are enriched by the intermediate water by a shearing eff ect against the edge or the bottom of an eddy, i.e. at the contact bet ween two water bodies moving in opposite directions; it can also happe n on the edges of two eddies where they become tangential. Thus, four areas of high productivity of living resources (high trophic-chain lev els) are evidenced here in the Algerian Basin. One is linked with the frontal process of the Almeria-Oran Front; the three others, not previ ously described, concern the influx of levantine intermediate water in to the Western Mediterranean Basin.