Some lakes have large populations of fish like Blueback Herring (Blueback Shad), which are schooling fish and plankton eaters. They grow to 16" long, so a large school of them will look tempting on the sonar, but they're not interested in anything in your tackle box or bait bucket and you can waste a lot of time fishing for all those fish the sonar shows to be under the boat.
[Since they aren't typically caught by fishermen, lots of folks don't know they exist. Lake Champlain had a massive die-off of these fish a few years ago (it seems to happen about once a decade or so), and the locals called the news media because nobody seemed to know what they were or where they came from! ?]
