And if the river is polluted to such a degree that oxygen levels are low, fish cannot breathe properly. Their behavior becomes erratic—at first they swim faster, moving frantically in search of pockets of air. Then they slow down, falling progressively into a lethal torpor. Massive die-offs can occur in a matter of minutes. Mother fish will exhaust themselves to bring oxygen to their eggs, even as the effort drains them. Still they will carry on fanning and ventilating to keep their offspring alive, until their own reserves of energy run out. Down in the murky depths, the glow from the eggs
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