Longnose lancetfish (Alepisaurus ferox Lowe, 1833) is a typical component of by-catch in the tuna longline fishery. It is widely distributed in the tropical epi- and mesopelagic zones of all the oceans. It is an obligatory predator feeding on all the accessible marine animals. The up-to-date knowledge about biology of this species is based on information which remains rather scarce till now. Longnose lancetfish was studied in order to widen present notions about biology of these species and their role in the trophic chain of the oceanic pelagic zone in the Indian Ocean. Data were collected in longline research cruises in the Western Indian Ocean from the early 1970s till the late 1980s. In the Western Indian Ocean, longnose lancetfish was observed from 10 degree N to 35-40 degree S. In the areas under study the species is closely related with lower layers of thermocline and the minimum oxygen layer. It seems to be an epi-mesopelagic species able to migrate close to the surface. Size of the species in longline catches varied from 55 to 185cm and weighed from 0.3 to 11.0kg. Pelagic crab Charybdis smithi, crustaceans Hyperiidea, longnose lancetfish, barracuda Paralepis elongata, hatched fish Sternoptyx, diaphana, omosudids Omosudis lowei and Polychaeta were predominant lancetfish foods. Differences in species composition of food were observed for small (FL less than 100cm) and large (FL above 100cm) individuals. Food composition of large individuals differs from smaller ones by cannibalism intensification, appearance of other large-sized preys in the stomachs and the presence in the food of Sargassum seaweeds, which normally float at the ocean surface. Other peculiarities of seasonal or regional character were also found.
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