Regional Feeding Patterns of the Longnose Lancetfish (Alepisaurus ferox Lowe, 1833) of the Western Indian Ocean

biogeographic provinces Abstract—The food composition of the longnose lancetfish (Alepisaurus ferox Lowe, 1833) was studied in two areas belonging to different biogeographic provinces of the western Indian Ocean: Indian Monsoon Gyres Province (MONS) and Indian South Subtropical Gyre Province (ISSG). A total of 158 lancetfish were sampled during two research longline cruises in 1986-1987: 50 within the MONS, from the western equatorial area (between the EEZs of Kenya and Seychelles) and 108 within the ISSG, from the northern part of the EEZ of Mauritius (Saya-de-Malha Bank – Agalega Islands area). In the equatorial area, 135 prey items (of 18 families or higher taxa) were found in the lancetfish stomachs. In the waters of Mauritius, 476 prey items of 53 taxa were recorded. Regional variability in the feeding habits reflects province-specific differences in the prey abundance and the structure of pelagic communities of the intermediate trophic levels. Large predatory crustaceans dominate in the waters of MONS, while they are absent in the poor waters of ISSG. Foraging success of the lancetfish on non-evasive prey was 3.2 times higher in the MONS than in the ISSG. In overall, swimming crab, Hyperiidea, conspecifics, barracudina, hatchetfish, hammerjaw, and Polychaeta were the predominant prey of the lancetfish. Great differences in the food composition of ‘small ’ (FL < 100 cm) and ‘large ’ (FL ≥ 100 cm) lancetfish were recorded. For large lancetfish, the cannibalism rate and the occurrence of large evasive prey and Sargassum algae floating at the sea surface were greater than for small lancetfish