The area is known for oil spills that have polluted the waters and left fish and other wildlife inedible.
The massive die-off was first reported in February when community people in Delta State complained of the schools of dead fish floating and littering their shores.
Samples of the fish were taken by the National Oil Spill Detection and Response Agency (NOSDRA).
Idris Musa, head of NOSDRA, declared the die-off had nothing to do with the continual oil leakages from offshore platforms as claimed over the years by Amnesty International, the U.N. Environmental Program, the Fishnet Alliance, and dozens of other groups in and outside of Nigeria.
Meanwhile, the Shell Petroleum Development Company (SPDC) this month said that the dead fishes floating and littering the Niger Delta coastline had nothing to do with its operations.
