The mass beaching of fish noticed along the Alibag coast earlier this month was not a result of pollution but a natural incidence, according to a report prepared by the Central Marine Fisheries Research Institute (CMFRI) after studying the samples. “Salient observations indicate that there was no physical observation of oil or toxic algal blooms in the gills of fish samples. Water quality parameters were at optimum level. No indication of diseases were observed in the fish samples,” reads the report that has been submitted to the state’s fisheries department. Four species of stingrays, six species of Sciaenid, three species of prawns, five species of low value fish, four species of catfish, Bombay Duck and Golden Anchovy washed ashore on the Alibag coast in the first week of October.
The report attributes the incident to the ‘presence of high productive zone’, or simply a large quantity of fish near the shore, along with a rise in sea temperature. “As there is less oxygen in warmer waters with the rising sea surface temperature an upwelling occurs, where the bottom dwelling fishes come to the shore for oxygen. The force of the winds and the current then wash them ashore,” explained an official from CMFRI. “Current satellite derived data also shows a low oxygen profile in the region and varying circulation pattern. This also is known to drive fishes to near the shore where the oxygen is at optimal level resulting in mass beaching of live and healthy fishes,” added the report.
The beached fish comprised mainly young commercially important species of croakers and catfish, low value varieties and stingrays. Stingrays are known to be seen along Mumbai and adjacent coasts during the end of the monsoon season. The report suggests it could be a result of ‘seasonal hypoxia’ — a deficiency of oxygen — in the ocean after the southwest monsoon. “We have been noticing this phenomenon along Mumbai coast for the last five years now where these fishes wash ashore between September 1 and 15. It usually occurs during Ganesh Chaturthi and warnings are issued to the devotees to be careful during visarjan (immersion). However, this time the festive season came early,” added the CMFRI official.
However, Dr Vinay Deshmukh, former chief scientist, CMFRI, and a marine biologist, explained that this phenomena had been observed since the 1960s. “The Arabian Sea has the thickest Oxygen Minimum Zone (OMZ) in the world going from 200 m to 1000 m, which is about 800 m thickness. This is the area in seawater where the oxygen saturation is the lowest and the marine creatures find it difficult to breathe here. This layer rises to the surface during monsoon all along the west coast of India owing to Southwest wind drift that pushes the coastal waters southwards by a process called ‘upwelling’, as a result mid-sea water deficient in oxygen surfaces every year with varying intensity. They often wash ashore in their attempt to escape the zone. Owing to global warming, this zone is expanding and such fish kills are going to increase in future.”