Domesticated salmon, after several generations, are fat, listless things that are good at putting on weight, not swimming up fast-moving rivers. When they get into a river and breed with wild fish, they can damage the wild fish's prospects of surviving to reproduce.

The End of the Line: How Overfishing is Changing the World and what We Eat (ed. Univ of California Press, 2008) - ISBN: 9780520255050