On the muddy edge of the Wicomico River, where tall grass hides all sorts of vermin, Stephen Kendrot and his patrol of federal wildlife biologists tracked the dirty rat that’s destroying precious Chesapeake Bay marsh. A near-decade-long federal and state effort to get rid of nutria, or swamp rats, killed 13,000 of the giant rodents but failed to eradicate them from the Delmarva Peninsula.