THE SEA‐BIRDS OF THE SOUTHEAST COAST OF ARABIA

Abstract

SUMMARYThe present study of an upwelling area off the southeast coast of Arabia, between 52° and 60° E. is based primarily on observations on board the R.R.S, `Discovery’ during the International Indian Ocean Expedition. During oceanographic surveys off the Arabian coast from June to August 1963 and in early March and late May 1964 I made 234 counts of birds of about an hour each, the results of which have been used as an index of bird density. In this area, the only offshore islands known to be important to sea‐birds are the Kuria Muria Islands, which were apparently at the centre of an upwelling area. The upwelling of cold water along the Arabian coast during the southwest monsoon from May to September greatly increases organic production in the sea, and large populations of organisms at every level of the food chain develop there.Previous work in Arabia and the Arabian Sea is reviewed. Little is recorded about the ecology of sea‐birds there, but the composition of the fauna is quite well‐known.The distribution of every sea‐bird species recorded from 52° E. to 60° E., and within 200 miles offshore, is described, with particular reference to my own observations. Details are given of the distribution of common species during the southwest monsoon of mid‐June‐mid‐August 1963 and in March and May 1964. Specimens collected on the `Discovery’ are recorded together with their measurements. Of 12 sea‐bird species seen commonly off Arabia during the southwest monsoon in 1963, 11 were seen in late May 1964, but only five in March.There is certain evidence that two species have bred on the Kuria Muria Islands and suggestive evidence for a further three. Information on breeding seasons in the Arabian Sea is based on short visits to breeding colonies, mostly outside the breeding season, and on the plumage condition of specimens. Nevertheless, the sum of the evidence suggests that breeding is regular at the same time every year, and in most species seems to be in the northern summer.The food of sea‐birds in the Arabian Sea, determined from stomach contents and visual observation, only serves to stress the lack of knowledge on this subject. A brief summary of feeding methods recorded by me suggests that they may be important in avoiding interspecific competition, for there seems to be little overlap.The density of sea‐birds during the southwest monsoon 1963, expressed as the number of each species seen per hour, was analysed in relation to: (i) the sea surface temperature, which was inversely correlated with the zooplankton density from 0–200 m. (ii) the distance from the Kuria Muria Islands, the only likely breeding station; (iii) the distance from the nearest land which may have been important to land‐tied sea‐birds.All the abundant species were commonest close to land in the cool‐water area and, with the exception of migrants from the southern hemisphere, most were concentrated around the Kuria Muria Islands in the centre of upwelling. A multiple analysis suggested that the islands were of real importance to some species. A separate analysis also demonstrated the existence of a correlation between sea‐bird numbers and the abundance of zooplankton from 0–200 m. depth, but not the abundance of zooplankton at the sea surface.During the southwest monsoon the southeast Arabian coast is inhabited by a distinctive cool‐water fauna within the tropical zone of the Indian Ocean. The high degree of endemism, the high proportion of migrants, including some from the southern hemisphere, and the absence of most pantropical species, suggest that the marked changes in the environment off southeast Arabia demand considerable adaptation. A brief discussion of the possible origins of the Arabian coast sea‐bird fauna shows that it is not typical of either a subtropical or a tropical community in other regions.The likelihood that sea‐birds breed in the summer, the concentration of most species in the upwelling area at that time and their absence during the winter, clearly demonstrates the importance of the upwelling. However, the results of the analysis suggest that some species were concentrated around the Kuria Muria Islands in the centre of the upwelling because the islands had a real importance to them, possibly as a breeding station.The correlation between sea‐bird density and zooplankton abundance in the top 200 m., but not at the surface, may be explained if sea‐birds concentrate in areas of high productivity rather than in areas of abundant surface plankton, which is largely irrelevant as food.