ABUNDANCE AND DISTRIBUTION OF THE MESOPELAGIC FISH MAUROLICUS-MUELLERI IN THE SOUTHERN BENGUELA SYSTEM

Extensive sound-scattering layers of mesopelagic fish and macrozooplankton are found over the continental shelf and slope off the west coast of South Africa. They provide an abundant food resource for commercially valuable midshelf fish species such as Cape hake Merluccius spp. The sternoptychid Maurolicus muelleri (the lightfish) is a widespread and major component of the mesopelagic fish assemblage, occurring mostly between the 100 and 500 m isobaths. Estimates of lightfish abundance are derived from echo-integration data and from information on catch rate in research midwater trawls and egg abundance. Taking into account evidence for multiple-batch spawning behaviour, the most probable range of lightfish density within the regions and period surveyed was 4-10 tons.km-2. Assuming the population to be confined to the shelf and shelf-edge in most regions off the West Coast, it is proposed that the total biomass of lightfish in the Benguela system was of the order of a million tons. Diel patterns of vertical migration are shown, and factors influencing the distribution and abundance of the species are discussed.