52
R. I. CURRIE et al.: Arabian Sea Upwelling
about 8 ° 30' Nand 9° 30' N. There the shoals occurred at an average depth of 73 ± 36 m.
The region north of this where upwelling was taking place proved to be surprisingly
barren. Further south between about 4° and 6° 30' N, a section well offshore showed
echoes to be present some 100-300 miles out from the coast and, although at a lower
level of abundance, they did occur over a fairly wide area. The depth of occurrence of
these latter shoals was similar to that off the Arabian coast - an average of 220 ± 33 m.
4. Distribution of Fish in Relation to Upwelling
Nothing, of course, can be said on the species of fish responsible for these echoes
but it seems well established that the South-east Arabian coast shows substantial
quantities of a resource which would warrant further investigation. Off Somalia, however, so far as the evidence goes, it would appear that fish of the type responsible for the
echoes are comparatively less abundant. Sea birds, which were also abundant on the
Arabian coast, were notably absent in the Somalia upwelling area.
It seems probable that the time scale of the biological events in the Ras Hafun area
is such that the higher levels of the food web may be reached only in the area north of
that surveyed and possibly in the vicinity of Socotra. The distribution of birds is, of
course, so closely linked to the availability of nesting sites that its significance may be
somewhat irrelevant.
llt Discussion
The obersvations of these cruises demonstrate the existence of extensive areas of
upwelling on the western margin of the Arabian Sea during the SW monsoon. BANSE
(1968) showed that upwelling also occurs at the same time of year along the eastern
margin of the Arabian Sea. Thus it appears that during the SW monsoon the Arabian
Sea is potentially an exceptionally fertile area.
The upwelling on the Arabian coast seems to conform in general principle and shows
no great deviatioll from the classical pattern of wind-induced upwelling characteristic
of the western continental coasts. Nevertheless, there are significant differences. The area
of divergence appears to be very much wider than usual and the depth of upward vertical
motion much greater. The wind stress in contrast to the other major upwelling regions
is poleward rather than equatorward.
By contrast, the upwelling on. the Somali coast in the Ras Hafun area appears to
result from the density structure of the Somali Current rather than from the direct effect
of the local wind stress on the sea surface. SWALLOW and BRUCE (1966) point out that, if the
flow of the Somali Current is in fact geostrophic, then the more water is carried in the
current, the further South would be the point at which the thermocline comes to the
surface at the coast. The extent of the Ras Hafun upwelling may thus be closely associated
with the volume transport of the Somali Current and subject to fluctuations in the latter,
and this could account for the absence of some features characteristic of possibly more
stable upwelling areas-the abundance of fish and seabirds, and for events such as the
fish mortality reported by FOXTON (1965). Further observations may in time clarify the
reality of this dependence.
R. I. CURRIE et al.: Arabian Sea Upwelling
about 8 ° 30' Nand 9° 30' N. There the shoals occurred at an average depth of 73 ± 36 m.
The region north of this where upwelling was taking place proved to be surprisingly
barren. Further south between about 4° and 6° 30' N, a section well offshore showed
echoes to be present some 100-300 miles out from the coast and, although at a lower
level of abundance, they did occur over a fairly wide area. The depth of occurrence of
these latter shoals was similar to that off the Arabian coast - an average of 220 ± 33 m.
4. Distribution of Fish in Relation to Upwelling
Nothing, of course, can be said on the species of fish responsible for these echoes
but it seems well established that the South-east Arabian coast shows substantial
quantities of a resource which would warrant further investigation. Off Somalia, however, so far as the evidence goes, it would appear that fish of the type responsible for the
echoes are comparatively less abundant. Sea birds, which were also abundant on the
Arabian coast, were notably absent in the Somalia upwelling area.
It seems probable that the time scale of the biological events in the Ras Hafun area
is such that the higher levels of the food web may be reached only in the area north of
that surveyed and possibly in the vicinity of Socotra. The distribution of birds is, of
course, so closely linked to the availability of nesting sites that its significance may be
somewhat irrelevant.
llt Discussion
The obersvations of these cruises demonstrate the existence of extensive areas of
upwelling on the western margin of the Arabian Sea during the SW monsoon. BANSE
(1968) showed that upwelling also occurs at the same time of year along the eastern
margin of the Arabian Sea. Thus it appears that during the SW monsoon the Arabian
Sea is potentially an exceptionally fertile area.
The upwelling on the Arabian coast seems to conform in general principle and shows
no great deviatioll from the classical pattern of wind-induced upwelling characteristic
of the western continental coasts. Nevertheless, there are significant differences. The area
of divergence appears to be very much wider than usual and the depth of upward vertical
motion much greater. The wind stress in contrast to the other major upwelling regions
is poleward rather than equatorward.
By contrast, the upwelling on. the Somali coast in the Ras Hafun area appears to
result from the density structure of the Somali Current rather than from the direct effect
of the local wind stress on the sea surface. SWALLOW and BRUCE (1966) point out that, if the
flow of the Somali Current is in fact geostrophic, then the more water is carried in the
current, the further South would be the point at which the thermocline comes to the
surface at the coast. The extent of the Ras Hafun upwelling may thus be closely associated
with the volume transport of the Somali Current and subject to fluctuations in the latter,
and this could account for the absence of some features characteristic of possibly more
stable upwelling areas-the abundance of fish and seabirds, and for events such as the
fish mortality reported by FOXTON (1965). Further observations may in time clarify the
reality of this dependence.