Possibly the last things a city preparing to host one of the most significant sporting events in the world wants to wake up to is a blanket of dead fish floating on the surface of one of its would-be venues. But that's the rather unsavory reality in Rio de Janeiro.

The rotting animals, which were found coating the surface of Rodrigo de Freitas lagoon, where the rowing and canoeing events in next year's Olympic Games will be held, have left many wondering if the host city is up to the challenge.

The authorities have long said they would use the advent of the games as a trigger to clean up Rio's waterways, which all too often contain raw sewage and garbage. But with the race is on, and last week Governor Luis Fernando Pezao said there was "not going to be time" to finish the job before the opening ceremony in August 2016.

Safety concerns

Fifty tons of twait shad, which is a small silvery fish, have been pulled from the lagoon in the past few days, but those involved in the clean up are still removing more. The mass fish die-off has sparked fresh fears about the safety of the water for competing athletes.

Opinions over the cause of the mass death are divided. A statement issued by Rio's environmental secretariat attributed it to a sudden drop in water temperature.

"The intense rains that happened last week and a rise in the sea levels led to a spike in the [sea] water entering the lake, causing a thermal shock," it read.

But some scientists put it down to the chemicals, sewage and waste that flow into the lagoon from rivers and starve the fish there of oxygen.

Guanabara Bay, which appears on iconic images of the city, and which will be the scene of sailing races in Olympics, also struggles with major pollution issues. In February, it too was the scene of a mass die-off of twait shad.