Tips for reading sonar at last ice

Being last is rarely a good thing. I mean, how often do you hear someone brag about being lapped by the rest of the field? No, “last” and “best” rarely go together—unless you’re talking about ice fishing. Simply, there’s no better time to be hitting the hardwater than last ice, when the spring days are gloriously warm and the fish are biting like crazy.
Those same fish can also throw you a curve ball, however, by showing themselves on the sonar screen yet refusing to bite. At least, that’s how it appears. In reality, most fish are always in a biting mood at last ice—it’s just that the ones you mark on the sonar screen never see your baits. But how can that be, especially when you carefully monitor your flasher and appear to be putting your lure right in front of their noses?

In short, because of the way these sonar units take three-dimensional information and convert it into two-dimensional output, things are not always as they appear. And the problem is most pronounced at last ice, when aggressive spring fish have a tendency to suspend in the water column.

To understand what’s happening, you need to visualize how the signal from your transducer spreads out below your hole in the shape of a cone. Anything entering the cone will show up as a red bar on the flasher, indicating its distance from the transducer. This is key: It’s the distance from the flasher that is indicated, not necessarily the depth the fish is at.
For example, let’s say you mark a fish at 30 feet. Naturally, you drop your lure to 30 feet. But while the fish you see on your flasher is indeed 30 feet away from the transducer, it’s on the outside slope of the cone, not straight down. As a result, it’s only about 24 feet down. So, by fishing your lure 30 feet below your boots, you’re actually positioning it six feet below the fish, where it has no chance whatsoever of being seen (see diagram).

Trust me on this: I spend between 50 and 100 days ice fishing each winter, and I’ve done so for many years. Yet in all that time on the ice, I can recall watching only one fish—a lake trout—on my sonar screen as it swam down to hit my lure. And how many times have you watched a fish swim onto your flasher screen, rise up, then smack your bait? Many times, I’m sure. But I guarantee you, most of those fish didn’t swim up to strike your bait—they swam straight across the water column to get it instead. That’s what the fish in the diagram below is likely to do, provided the lure is presented at the 24-foot mark, instead of 30, where the fish would otherwise appear to be holding according to the flasher.

That’s exactly what happened late last winter during a scorching whitefish bite, when my buddies and I were catching upward of 100 dandy fish a day. I caught so many fish, in fact, that I drilled a second hole to drop down an underwater camera. That way, I could watch how the fish were reacting to my lure as I jigged it 20 feet off the bottom. I was astonished to see not one whitefish fly up to clobber my bait, as the flasher suggested they were doing. Instead, they were scooting laterally across the water column to hit my lure.

I was truly mesmerized by the way the fish resembled great white sharks, with their pectoral fins splayed out to the side, gliding in effortlessly from the edge of the video screen to devour my lure. Meanwhile, my flasher displayed every one of these side-swiping whitefish as if it were roaring up from the bottom, but it just wasn’t so.

And that’s why you need to keep your bait above the fish, especially at last ice.

Illustration by Mike Del Rizzo