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Why does upwelling make parts of the Arabian Sea productive fishing grounds?

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Indian Ocean The Indian Ocean, with depth contours and undersea features.

Coastal upwelling in the Arabian Sea is a primary driver of its high biological productivity. This process circulates nutrient-rich water from the seafloor into the euphotic zone, the upper layer of the sea where sunlight is available for photosynthesis.1

The influx of inorganic nutrients, such as phosphates, triggers the rapid reproduction of phytoplankton. These organisms serve as the essential foundation of the marine food web, supporting large populations of zooplankton and, subsequently, commercially valuable pelagic fish such as tuna, sardines, and billfish.123

While this upwelling is vital for sustaining rich fisheries, the process can occasionally have negative consequences. If strong upwelling brings subsurface water that is naturally poor in oxygen to the surface, it can lead to mass fish mortality due to hypoxia.1