In September 2023, an extreme drought shrank Lake Tefé in the Brazilian Amazon, and the sun did the rest. The water level fell by half, and the water warmed by about eight degrees above normal. The botos, the pink dolphins of the Amazon, were in the lake, along with tucuxis, their gray relatives. Bodies began appearing on September 23, and on the 28th, with the water at 39,1 °C, about 70 were counted in a single day.
Miriam Marmontel, a biologist at the Mamirauá Institute, said that the dolphins were swimming in circles that day and could not get down to the bottom. Some died in front of her. The first reports spoke of more than 120 dead; the tally from Mamirauá and WWF later rose to 153 or 154, close to 10% of the lake’s dolphins.
Heat is the most likely cause, though a biotoxin was also considered among the hypotheses. You know what’s most unsettling: a river dolphin navigates among roots and logs using echoes, but no echo warns it when the water itself starts to burn.
