MONTLUEL, France—A rainy, cool climate has sustained fisheries in this region north of Lyon for eight centuries, filling up hundreds of man-made ponds that are France’s main source of freshwater fish. This year, however, many of the ponds are nearly empty after an exceptionally dry and warm stretch of weather that has lingered across much of Europe since last summer. The farmers expect to raise half as much fish as they did last season and lay off workers who usually help with the catch.

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