With nets and divers, sonar and surveys, scientists around the world grapple with one of Earth's great unknowables: how many fish in the sea. Fish counts are the science behind regulations from Virginia's Northern Neck to the South Pacific, dictating a charter boat's take and an island nation's diet. But this is a science so inexact that some call it an art. And when the counting ends, the fighting often has just begun. That's what happened this winter when Maryland tried to open the Choptank River to commercial yellow perch netters for the first time in nearly two decades. Counts had documented a 530 percent increase in the Eastern Shore river since 1988, Piavis said.