A dead dolphin was recovered from Barua Para area of Modunarghat of Halda River on Thursday morning.
Halda River is known for its natural breeding of carp-like fishes in South Asia.
Mizanur Rahman, officer-in-charge of Sadarghat River Police, said the dolphin weighed around 40 kgs and 59-inch in length and the mammal seemed to have died a day ago.
He said the exact cause of death of the dolphin could not be ascertained.
Professor Manzoorul Kibria, an eminent researcher on the Halda river, said Halda lost a total of 30 dolphins since 2017.