Surface megafauna related to western Mediterranean circulation

Abstract

The present results come from the campaigns of the international multidisciplinary programmeconducted by P. La Violette (Naval Oceans Research and Development Activities, NORDA), the WesternMediterranean Circulation Experiment. The mesoscale instabilities of the Mediterranean circulation induce meanders, gyres and eddies acting on the production potential of living resources. These resources are evidenced here through the cetaceans and other large organisms directly observable in the surface waters of the sea, along the track of the cruise NORDA 706 (May-June 1986), and through the underlying biomasses detected by an echo sounder, 12 kHz fishfinder. Acoustics detection was continuous, set at the same level by day and night for 15 days intenupted only when the ship stopped for hydrological measurements at 87 CTD sampling stations. The pattern of hydrological sampling was designed in order to identify the intermediate water veins in the Algerian Basin. The three sets of field data - daily visual census, continuous echo-sounding and hydrological profiles - were compared and processed in statistical factorial and multiregression analyses. These show that presence of cetaceans coincides with highly concentrated echos; other large organisms observed are distributed at random, without any link with the concentrated echos. It results that cetaceans have the ability to detect acoustically the same biomasses as humans do with a 12 kHz echo-sounder; this is an argument for the "echolocation of the Odontoceta" and "vocalization of the Mysticeta"; up to now, such processes are not accepted by all cetologists. Four examples of echogram are reported here associated with fin whales, sperm whales, pilot whales and dolphins. Dynamical analysis of the Mediterranean circulation before and during the cruise by contemporaneous remote sensing imagery (CZCS and NOAA 9) enables deduction of the dynamic characteristics of the areas where high densities of acoustical detection co-occur with the presence of cetaceans. Such areas are enriched by the intermediate water by a shearing effect against the edge or the bottom of an eddy, i.e. at the contact between two water bodies moving in opposite directions; it can also happen on the edges of two eddies where they become tangential. Thus, four areas of high productivity of living resources (high trophic-chain levels) are evidenced here in the Algerian Basin. One is linked with the frontal process of the Almeria-Oran Front; the three others, not previously described, concern the influx of levantine intermediate water intothe Western Mediterranean Basin.