“Some of the water used to extinguish the fire has reached the Besos river, greatly affecting its fauna,” the Catalan Water Agency which manages water resources in the northeastern region of Catalonia said in a statement.

The agency said it had declared a state of emergency “to be able to act in the basin of the Besos river”, which flows into the Mediterranean.    

Spanish television showed images of people collecting dead carp and eels from the river, as well as of fish lying on their sides in shallow water, struggling to breathe.

“The phenomenon is not widespread but we found many dead specimens scattered between the site of the fire and the mouth of the river,” the statement said.

Officials from the water agency were taking samples of water and sediment from the river to assess the extent of the damage.   

The agency said the “recovery of the affected stretch of river” would be done “in a natural way” but could not estimate how long this could take.   

Nearly 30 teams of firefighters were deployed to tackle the blaze, which broke out before dawn on Wednesday at a firm that recycles solvents and industrial waste in Montornes del Valles, some 15 kilometres (10 miles) northeast of Barcelona, sending vast plumes of black smoke into the air.   

The regional civil protection service threw up a security cordon around the area, and urged local residents to stay at home with their windows closed for several hours.

Green group Ecologists in Action noted that it had previously warned of the dangers the plant posed.

It said the impact of the water used to put out the blaze on the river's biodiversity was “very serious”, coming after serious efforts were made to restore its fauna.

READ MORE: 

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• 'Stay indoors': Smoke warning as huge blaze rips through industrial waste plant near Barcelona