By Kevin Damasio HUMAITÁ, Brazil — At dawn, 20 fishers received a tip that matrinxãs were swimming down the Aripuanã River toward the Madeira, the Amazon Basin’s largest tributary. It was their opportunity to interrupt a week of unproductive fishing in Novo Aripuanã, a municipality in southern Amazonas state, in the Brazilian Amazon. So, they divided themselves among three wooden canoes and went to the river. It was just a matter of waiting for the shoal, which swims on the surface, and throwing the net. At 11 a.m., with no sign of the matrinxãs (Brycon melanopterus), half of them gave up and,…