| Summary: | The mesopelagic fauna of the western and northern Arabian Sea between Mogadisco and the Indo-Pakistanean border was studied on cruises with R.V. "Dr. Fridtjof Nansen" during 1975-1976. A deep scattering layer was observed over the whole area at depths between 250 and 350 m. In the northwestern part of the area, and sometimes in the Gulf of Aden, an additional layer was found between about 100 and 200 m. Benthosema pterotum and B. fibulatum were the most abundant species in the area, but Diaphus spp. were also numerous. The Benthosema species seemed to have a life cycle of one year or less. The biomass was estimated by using a 38 kHz echosounder and the electronic integration technique. The area was covered five times, and the estimated abundance of mesopelagic fish was about 100 million tonnes (range 60-150 million tonnes). Estimates from the spring were higher than those from summer and autumn. When using a 1360 mesh pelagic trawl, catch rates as high as 20 tonnes/hour of trawling were reached.
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