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LA CORONILLA, Uruguay — The day the yellow clams turned black is seared in Ramón Agüero’s memory.

It was the summer of 1994. A few days earlier, he had collected a generous haul, 20 buckets of the thin-shelled, cold-water clams, which burrow a foot deep into the sand along a 13-mile stretch of beach near Barra del Chuy, just south of the Brazilian border. Agüero had been digging up these clams since childhood, a livelihood passed on for generations along these shores.

But on this day, Agüero returned to find a disastrous sight: the beach covered in dead clams.