The Pacific herring—an oily, silvery, schooling fish—is rarely high on the list of marine animals people fret about.

But for the second straight year, the Canadian government has ignited a skirmish in British Columbia by moving to let fishing nets scoop up spawning herring, despite objections from scientists, Native people, and even commercial fishing groups.

"Last year it almost got to a war—locals were geared up to block fishing boats in port," said Tony Pitcher, a fisheries scientist with the University of British Columbia. "There were more police on the dock than there were local people."

This unusual battle is part of a global debate about the future of some of the oceans' most important fish: the abundant schools of sardines,