tides causing an up and down migration of the fish. During the 

 southwest monsoon the mixed surface layer will be rather thick off 

 Bombay and the mlnimuiu oxygen correspondingly deep due to the piling 

 up of surface waters. During the northeast monsoon the minimum 

 oxygen layer will gradually ascend to shallower depths because the 

 surface water will be blown seaward and cause upwelling from below. 

 A rich plankton bloom is associated with the upwelling; however^ the 

 tuna and sardines do not come into the area to feed on the plankton. 

 They remain outside the upwelling zone because of the low oxygen 

 content of its water. 



The origin of the layer of minimum oxygen in the Arabian Sea is 

 not yet fully understood, but it is associated with a high salinity 

 and high temperatures and for this reason many authors believe that 

 it originates through the outflow from the Red Sea and Persian Gulf. 

 Surface Water Properties . The surface water of the Arabian Sea 

 generally occupies the layer from the surface to a depth of IOO-I5O 

 meters, although in some areas where seasonal upwelling occurs the 

 normal surface water is replaced by water which rises from the sub- 

 surface. The average temperature in the surface layer ranges from 

 SO^C to 29 °C varying according to the seasons. The highest surface 

 temperature in the Indian Ocean, excluding the Red Sea and Persian 

 _Gulf, is found in the northern sector of the Arabian Sea during 

 summer. Here the surface temperature will reach 30°C during August. 

 In winter, under the Influence of the northeast monsoon, the surface 

 temperature in the northern peak will drop to 22-23°C. The summer 

 monsoon causes upwelling in the flox^/■ of the relatively cool waters 

 of the Somali Current along the African and Arabian coasts. This 



12 



