Why Does My Fish Finder Show More Marks at Night?

Why Does My Fish Finder Show More Marks at Night? The Great Lakes Sonar "Explosion" Explained

Quick answer: That sudden explosion of marks on your sonar screen at dark isn't new fish showing up out of nowhere. And the majority aren't gamefish.  It's baitfish, mainly alewife, that were there the whole time. Dusk triggers a vertical shift and a schooling breakdown, the alewife scatter, and your sonar just shows that transition happening in real time.

And We're Off...

Trolling for king salmon out of Ludington on Lake Michigan the other night, right on cue at dark, the sonar screen blows up with hooks. Marking what looks like tons of fish - mostly smallish hooks, but they fill the screen, often top to bottom. There are often wispy clouds of smaller critters mixed in, and occasionally larger hooks too.

The sonar screen changed. Drastically. Often the entire screen fills top to bottom with marks. This usually happens right after the dusk flurry, when the kings are really snapping and then shut down, and the screen explodes with marks. What's actually happening?

Where Do All the Fish Come From at Dusk?

I believe the majority of these hooks are baitfish, mainly alewife. I think they were under the boat all day, and then darkness falls and they appear everywhere. But where were they 10 minutes before, and why wasn't I marking them? A couple places, and a couple reasons:

1) They were on the surface. Seldom marked due to the tiny cone angle at shallow depth, fish skirting the boat, or getting lost in the noise and clutter. Feeding on surface and near-surface critters in the warmer water, as dark sets in they disperse deeper to feed on critters rising from the bottom, or maybe to reduce their profile for hungry salmon.

2) They were in balls. On most graphs, tight schools show up as blobs, not separate hooks. When darkness falls, they disperse ready to feed.

3) They were on bottom, spending the day feeding on whatever bugs were available. As darkness falls, they rise in time with the Mysis shrimp (up to an inch long), feeding as they go. From what I've read, these tend to be the larger adults.

Why Do the Hooks Appear?

With most of us running skimmer-type transducers, target separation isn't great. Fish on bottom aren't marked at all, and bait balls just show up as a cloud of pixels. Then the alewives disperse and the sonar can pick up each fish as a separate hook, filling the screen.  Suddenly a garage-sized ball of bait is hook after hook, spread over a quarter mile.

Read more about skimmer transducers and target separation: Transducer Comparison

But If They Are Bait, Why Do They Look So Large?

These hooks can also appear larger due to dwell time. This phenomenon is driven by time spent under the sonar cone. In the simplest terms, an object that spends more time in the sonar ping returns a larger echo than one that's moving through the cone (swimming), assuming depth is maintained. A fish that's moving slower returns a stronger, cleaner echo than one darting quickly through the beam. So these alewife are spreading out and eventually becoming still, or at least "stiller" than in daylight. At the same time, the larger adult alewife are suddenly in the water column (up from bottom) and returning their own pings, bigger pings.

TVG (Time Variable Gain) could also be affecting many sonar screens. It's a feature that helps keep a target appearing the same size regardless of depth. I think, if it's set incorrectly, smallish hooks could appear larger than they actually are. Still trying to wrap my head around that one.

So Where Did All These Fish Come From?

They were alewife, and they were there the whole time, just balled tight, hugging bottom, or flitting around on top - difficult to resolve as individual marks, or to mark at all. Dusk triggers a vertical shift and a schooling breakdown, and the sonar is just showing you that transition happening in real time.

Now the question is: Why did the kings shut down after this dispersal of bait?!

FAQ (gleaned from the web)

Why does my fish finder show more marks at night?
Baitfish like alewife undergo diel vertical migration, moving from tight schools near bottom during the day to dispersed, individually-swimming positions higher in the water column at dusk. That shift makes them far easier for sonar to mark as separate targets.

What is diel vertical migration?
It's the daily up-and-down movement many baitfish and zooplankton make in the water column, typically staying deep during daylight and rising toward the surface at night to feed, usually tracking their own food source as it migrates.

Why do sonar marks look bigger at dusk?
Partly because fish that are moving slower or holding still return a stronger, cleaner echo (dwell time), and partly because dispersed individual fish are easier for sonar to resolve separately than a tightly packed school, which usually reads as one solid blob.

Does TVG affect fish size on sonar?
TVG is meant to normalize echo strength across depth so the same fish looks the same size no matter how deep it is. If it's set incorrectly, it can distort how large or small marks appear, though the main dusk "explosion" effect is more about fish behavior than TVG.