Why is tuna fishing mainly done in the southern oceanic part of the Bay of Bengal?

Tuna fishing in the Bay of Bengal is primarily concentrated in the southern oceanic sector, specifically south of latitude 15° N, because of the environmental conditions in the northern part of the bay.1

Large rivers drain into the northern region, resulting in significant freshwater runoff that heavily influences the nearshore waters. This influx of fresh water creates conditions that are less suitable for the tuna species found in the bay, which are better adapted to the true oceanic environment found further south.1