Looking out over the low green mountains jutting through miles of placid waterways here in southern Chile, it is hard to imagine that anything could be amiss. But beneath the rows of neatly laid netting around the fish farms just off the shore, the salmon are dying.

A virus called infectious salmon anemia, or I.S.A., is killing millions of salmon destined for export to Japan, Europe and the United States. The spreading plague has sent shivers through Chile’s third-largest export industry, which has left local people embittered by laying off more than 1,000 workers.

It has also opened the companies to fresh charges from biologists and environmentalists who say that the breeding of salmon in crowded underwater pens is contaminating once-pristine waters and producing potentially unhealthy fish.

Cramped cages are less healthy than open rivers? Never would have thought it, durrrrrr, durrrr.