THE PELAGIC DISTRIBUTION OF SEA‐BIRDS IN THE WESTERN INDIAN OCEAN

Abstract

SummaryThe present paper summarises observations made during the International Indian Ocean Expedition on board the R.R.S.' Discovery, from August to November 1963 and from February to September 1964 in the Indian Ocean north of 20° S and west of 70° E. In 1963 work was carried out in the Somali Basin. In 1964 a series of transects were made over the ocean, the main purpose of which was to investigate the system of equatorial currents and the changes in the sea associated with the onset of the Southwest Monsoon in May.The Tropical Indian Ocean is briefly described. Except for local concentrations, the surface layers are poor in nutrient salts and plankton. There is a seasonal reversal of winds and surface currents in the Arabian Sea, but seasonal changes become less marked further south.Previous ornithological observations in the western Indian Ocean are mostly confined to the Arabian Sea or to the island groups. Thus, existing information on the pelagic range of pantropical species is incomplete.Observations made on each transect across the Indian Ocean in 1964 are summarised and compared with oceanographic data collected at the same time. General conclusions are not possible on the basis of so little information, though there appeared to be some relationship between the distribution of certain species and wind or current belts. The only marked discontinuity recorded was a concentration of Puffinus pacificus and Gygis alba at the northern edge of the Equatorial Counter‐current at 58° E in March. Observations made on transects that were repeated before and after the onset of the Southwest Monsoon suggested that Sterna fuscata concentrates in the equatorial region as the monsoon develops. In general, both plankton and sea‐birds were more abundant at 58° E than at 671/4° E.An analysis of the presence or absence of sea‐birds during each observation period, which lasted an average of one hour, established the difference between pelagic species and those largely restricted to within 50 miles of their breeding stations. There was no evidence of any correlation with zooplankton abundance, though birds of all species taken together were commonest where flying‐fish were most abundant and S. fuscata appeared to be commonest in cool‐water areas with strong winds, i.e. the Trade‐wind belts. It was not possible to sample the food organisms of sea‐birds quantitatively.Sea‐bird observations in the Indian Ocean more than 200 miles from the continental coasts are summarized and compared with previous observations.The little information collected in the Indian Ocean agrees with previous work in the Pacific Ocean, where sea‐birds are commonest in areas of convergence, and not where local upwelling and an associated concentration of plankton occur, such as on the equator. This may be due to the fact that populations of organisms on which sea‐birds feed develop or concentrate a considerable time after upwelling of nutrient‐rich water occurs.Since few sea‐birds were seen feeding, it seems likely that available food is scarce and that much time is required to locate areas where it is abundant.Finally, evidence is presented to indicate how some sea‐bird species may avoid or reduce competition by feeding at different distances offshore, or in different geographical areas.