The deaths of thousands of fish on the banks of the River Plate may have been caused by lack of oxygen rather than water pollution, the Uruguayan National Water Resources Administration, or Dinara, said.

"The evidence increasingly points to a massive concentration (of fish) in surface waters that may have caused mortality due to anoxia," the Dinara said in a statement.

Almost 200 tons of Brazilian menhaden were collected in four days earlier this month on the River Plate, and experts have not yet determined the cause of the fish kill.

A fish kill occurred on the Argentine shores of the estuary, south of San Borombon bay.

Most of the dead fish were females that had laid eggs and sample necropsies did not find evidence of infectious parasitosis, which might explain the massive fish kill.

The Dinara reported a rise of up to 2 C (3.6 F) degrees in water temperatures during February, a level above historical records in the region.

A phytoplankton analysis did not reveal the presence of species that could produce toxins harmful to fish, the agency said.

The fact that only one species of fish seems to have been affected by increased mortality led Dinara specialists to rule out the possibility of any pollutants in the water.

The fact that fish kills occurred on both banks of the wide River Plate makes it "very unlikely" that the cause was waste dumped by fishing vessels, the agency said.

"For these reasons, there is no health risk in the consumption of fish both from coastal and deep waters," the Dinara said.