RIO DE JANEIRO (AFP) - An "unprecedented" thousands of dead fish washed up in Rio's heavily polluted Guanabara Bay this week, deepening fears for the health of competitors at the 2016 Olympics and alarming fishermen.
Detritus including dead animals, television sets, sofas and shoes have all been found in the waters, which will host sailing competitions at next year's Rio games.
A slew of dead fish was found in October and 12.3 tonnes of small silver fish were removed Wednesday and Thursday alone, according to the local city cleaning service, suggesting clean-up efforts are failing.
Authorities are scrambling to fix the problem but have conceded they will only be able to clean up the bay by 80 per cent by the time the games start in August 2016.
Nearly 70 per cent of Rio's waste ends up in Guanabara Bay.
"I flew over the bay yesterday and saw thousands of dead fish," biologist Mario Moscatelli told AFP.
"It the same species (shad) as in October, in the same spots.
"We still don't know if it's an environmental phenomenon or if it's because fishermen throw unwanted fish back into the water.
"But the quantities are unprecedented."
Janete Gomes of the Comlurb municipal cleaning service on Ilha do Governador beach, in the bay, said that large amounts of dead fish have usually been spotted in the Southern Hemisphere summer.
"It's as though there was a thermal shock in the water that triggers this massive death," Gomes said.
The Rio state environment agency said no toxic chemicals, or abnormal variations of potential hydrogen (pH), salinity or oxygen were found in the water during 252 tests conducted on 33 separate locations of the bay.
Examinations of the dead fish also failed to reveal a cause for the deaths, it added.
Brazil is spending around US$14 billion (S$19 billion) of public and private money on preparations for the games.