Kottayam: The seasonal migration of freshwater prawns (aattu konchu) to waters in the Vaikom area has begun. The prawns travel through the lake floor from Kuttanad carrying their eggs which is a sight that only scientists and fishermen notice. This variety of prawns lives in fresh water, but lays its eggs in salty water.

According to Dr K G Padmakumar, project director at Kuttanad backwater research centre, a minimum salt content of 11 PPT (parts per thousand) is required to facilitate the hatching of the eggs. This is one-third of the salt content in the sea.

However, the young prawns hatched from the eggs cannot survive in salt water for more than a month. As a result, they return to Kuttanad from the Vaikom area. This migration is a unique scientific phenomenon that local people term as the ‘pilgrimage’ of freshwater prawns to take part in the festival at Vaikom temple.

Freshwater prawns are among the most ecologically important species in the Vembanad Lake. But its population has been found to be shrinking drastically over the years.

In the 1970s, the catch of the prawns came to an average of 429 tonnes. That figure has fallen sharply to 35 tonnes during the last few years. The lowest catch so far was recorded in 2013 – 17 tonnes. The fall in the prawn numbers has been attributed to the construction of the Thanneermukkom bund which has upset the balance that existed between fresh water and salt water in the Vembanad lake.