SWATH MAPPING OF THE SEA-FLOOR AND ITS APPLICATION TO DEEP-BOTTOM FISHERIES IN NEW-CALEDONIA

A bottom longline fishery operated in the EEZ of New Caledonia from 19 88 to 1991. Fishing focused on five seamounts with summits at depths r anging from 500 to 750 m. The target species was alfonsino, Beryx sple ndens. As the soundings available from marine charts were not detailed enough, the fishing masters had to make their own charts in order to set the gear in the right location. A series of 11 scientific cruises devoted to a research program on alfonsino was launched in late 1991. During the first scientific cruises, several days were spent mapping t he seabed to improve the existing knowledge of the topography of three seamounts by coupling the EDO echosounder depth measurements to the G PS positions. As this procedure is slow, it is applicable to limited a reas or, if a wider grid is used, to large zones to locate major struc tures such as ridges and seamounts. The emergence of the multibeam ech osounder has greatly improved seabed mapping performance. This tool se ems to Bt the requirement of exploratory deep-bottom fishing particula rly well, as it covers large areas while providing details of the bott om. The EM 12 was used to obtain bathymetry and imagery of the SE port ion of the EEZ of New Caledonia, covering an approximate area of 70,00 0 km(2) in two weeks. For depths ranging from 500 to 4,500 m, the resu lts were impressive. They confirmed the known major features and provi ded greater topographical detail, revealing fine unknown structures. T hey also provided information on;the type of substratum, information w hich might influence the fishing strategy. Finally, they made it possi ble to obtain an accurate estimate of the exploitable area which in tu rn led to new stock estimates.