What Is a Lateral Line? | Understanding How Trout Feel Vibrations Can Improve Your Fly Fishing

If you're someone who spooks a lot of fish, this is probably the reason why.

See the faint horizontal line in the middle of the body? That's it! | Photo by Jasper Taback

If you’ve ever spooked a trout and wondered how the hell it knew you were there, odds are it felt you. Trout have a lateral line – a sensory system that runs along the sides of their body, allowing them to feel vibrations and movement underwater. Pretty cool. But it also means you can do everything right – approach the trout from downstream, stay low, minimize your shadow – and still spook it. 

What a lateral line is.

The lateral line system is a line of small cells that runs along each side of a trout’s body, from gills to tail. When something moves nearby, it creates pressure waves in the water – these waves enter the trout's pores and send a signal to the brain that there’s something moving close by. 

How Do Trout Use It?

Trout rely on their lateral line system to detect the movement of predators, prey, and each other. They register all kinds of underwater movement – insects floating by, larger animals splashing, and other trout swimming – and can differentiate between the vibrations that each causes. 

How to Approach a Fish That Can Feel You

It’s not just a cool thing that trout have – it can and should inform how you approach them. A trout judges whether nearby movement is a threat based on the intensity of the vibrations – in other words, keep your steps small. If you splash around, you’re sending strong signals that the trout needs to relocate. But if you keep your movements to a minimum, those vibrations blend in with those of the countless other living beings moving around underwater. 

Use It to Your Advantage

At first glance, it might appear that the lateral line system isn't great for anglers, but it can be pretty helpful. When you find yourself fishing stained water, keep in mind that the trout are going to be making decisions about what to eat using feeling rather than their eyesight. If you are fishing a nymph or streamer, imparting natural movement can trigger a trout to strike.

See It In Action

The best way to get a sense of how trout use their lateral line system is to get out there and witness it. With time, you’ll get a sense of the kind of movements you can and can’t get away with – and how to present your fly in a way that creates vibrations that feel natural. Experiment, and don’t beat yourself up when you spook a fish – it happens.