Studies on the fish fauna of the Arabian Sea carried out from R/V “Dr. Fridtjof Nansen” during the years 1975-1976 showed that the mesopelagic fishes were far more abundant than any of the other fish groups (ANON. 1977a). Examination of eggs and larvae from the area (AHLSTROM 1968, NELLEN 1973, FURSA 1973, ALI KHAN 1973) give the same indication. The studies of eggs and larvae and of the adult fish indicate that the myctophids is the dominant group of mesopelagic fish.
Taxonomy and distribution of myctophids from the Arabian Sea have been studied by NAFPAKTITIS & NAFPAKTITIS (1969) and by KOTTHAUS (1962). But their life history, ecology and abundance are largely unknown. There are, however, several studies of the ecology of the mesopelagic fauna in the eastern Indian Ocean (e.g. LEGAND & RIVATON 1967, 1969, LEGAND et al. 1972) and in the more southerly parts of the western Indian Ocean (BRADBURY et al. 1970, MAKSHTAS & RYABTSEV 1973). In these waters some acoustical work has also been carried out, but no abundance estimates have been made (e.g. HALL 1971, 1973).
A general description of the hydrography of the Arabian Sea has been given by e.g. WYRTKI (1973). WOOSTER et al. (1967) pointed out that there is “extremely high rate of primary productivity and zooplankton in the Arabian Sea, especially along the western side”. They also stated that the primary productivity is “as large as or larger than that encountered in such upwelling areas as the eastern boundary currents along the coast of Peru, or off West Africa”. A review of studies of primary production and an analysis of the transfer between primary and secondary production was published by CUSHING (1973).
The present study is based on the cruises with R/V “Dr. Fridtjof Nansen”, covering the area between Mogadisco in Somalia and the Indo-Pakistanean border twice in 1975 and three times during 1976. It aims to give an indication of the abundance of mesopelagic fish and their production in the western and northern parts of the Arabian Sea. The behaviour of this fauna and various aspects of the ecology of some of the more important species are also treated.
The cruises of R/V “Dr. Fridtjof Nansen” mainly covered the near-shore areas. Only the upper 450 meter layer was sampled with the echo integrator system and most trawling was also carried out in this layer. The deeper living mesopelagic and bathypelagic fishes are therefore not covered representatively and the neretic and surface migrating myctophids are dominating in the material.
