More rare freshwater dolphins in Brazil have been found dead in a new location along the Amazon River, in the latest grim fallout from the longest drought in the rainforest's recorded history.

Sea Shepherd Brazil, an international non-profit marine life conservation organization, said on Friday that 16 of the rare, usually pink-colored dolphins in the Coari region of the river had died, along with seven other deceased water-based mammals.

Their deaths were "an extremely worrying development that could indicate that there are dolphins dying in other parts of the river that are not yet being monitored," Sea Shepherd said.

Parts of the Amazon River recently fell to their lowest level in over a century as a punishing drought upends the lives of hundreds of thousands of people and damages the jungle ecosystem.

Low river levels during the months-long drought have heated water in some areas to intolerable temperatures for the dolphins, researchers believe. Thousands of fish have also died in recent weeks on Amazon tributaries due to a lack of oxygen in the water.

Aside from the 16 pink dolphins found dead on Sea Shepherd's monitoring trips to Coari on Sunday and Monday, teams found three tucuxis, another kind of freshwater dolphin, and four more dolphins of an unidentified species.

A few living dolphins were also observed, all showing normal behavior, the organization said.

A total of 178 of the pink dolphins have been reported dead since September, mostly in the Tefe region of the river, according to Sea Shepherd, which it said represents 10 percent of the population.

Slow reproductive cycles make the dolphins' populations especially vulnerable to threats.