Thousands of dead fish have washed up over the past week in a lagoon in Rio de Janeiro that will host rowing and canoeing events at next year's Olympic Games.

The latest headache for organizers comes amid concerns for the health of participants in the Olympic sailing events, who will take to the waters of a bay where masses of dead fish have similarly washed ashore in recent months.

It was not known whether the cause in that case was an environmental problem or if fishermen had thrown the fish aside because they had no commercial value, but garbage and sewage regularly clog Rio's waterways.

Residents reported a foul stench in recent days at the Rodrigo de Freitas Lagoon because of the tons of dead silvery shad fish -- the same type of fish found in Rio's Guanabara Bay.

Rower and local hopeful Felipe Xavier said: "I have to come train but these dead fish bother us because the smell is very strong and the huge amount of fish slows the boat."

Environmental officials said the shad fish, which are sensitive to temperature change, had died of "thermal shock."

Alexandre Fernandez, rowing coach of Botafogo club, told AFP that he had no worries about the phenomenon and that he believed the cause was natural, not because of pollution.