The visitors at Rushikonda and Sagar Nagar beaches in the city were surprised to see thousands of dead fish lying on the shore, on Wednesday.

The entire 2-km sea shore stretch from Sagar Nagar to Rushikonda was littered with dead fish and it was quite an eyesore to the visitors, with stink emanating.

D.E. Babu, a senior professor from the Department of Zoology, Andhra University, said that there could be a number of reasons for this. But the primarily it could be due to depletion of oxygen in the water or growth of harmful phytoplankton.

Speaking to The Hindu, he said, “During this period, especially when there is depression or monsoon, there is a possibility that healthy phytoplankton might die due to the stress condition in the water. And the depletion of phytoplankton can reduce the oxygen level in the water, which can lead to death of fish in large numbers.”

But observing that the variety of dead fish are predominantly the perch fish with one or two more species in smaller quantity, he said that the second reason could be growth of harmful phytoplankton.

The present condition favours the growth of harmful phytoplankton, and the fish that have died such as the perch fish inhabit the inshore water (near to coast) and feed on phytoplankton. “If they have eaten the harmful phytoplankton, which is poisonous, they might have died. Most of these fish move in schools and that is why there is mass mortality,” said Prof. Babu.

Such conditions were seen earlier, and it happened about five years ago, he added.

However, fishermen claimed that the fish have died due to increase in the pollution level, as effluents were being pumped into the sea unchecked and in huge quantity by chemical and pharma companies.

These fish have some commercial value in the local markets and small fishermen depend on it, said a fisherman.