Like hot race cars, fish sport a pin strip down the length of their bodies. It’s the lateral line–lateral meaning “side.” It may be distinctly darker than the side of the fish or it may only be seen as a raised area along the side. The  lateral line consists of a series of interconnected “U” shaped tubes filled with a jelly like material. The open end of the “U” is facing to the exterior of the body. The lateral line extends forward around the fish’s eyes and on its lips, although in these areas the structure is much more difficult to discern with the naked eye. Nerve endings line the bottoms of the tubes. Waves in the water generated by displacement (a minnow swimming, an angler wading, and so on) create pressure on the material in the lateral line. This pressure is sensed by the nerves. It’s a very sensitive structure that not only allows the fish to detect prey and predators but also allows the fish to align itself most advantageously in the currents, to space itself in a school of fish, and to swim with the synchrony of the school. The lateral line can detect differences in currents between the two sides of the fish’s body  as small as 1/25 of an inch a second.

Have you ever wondered how fish are able to just sit there on the bottom is a fast stretch of water and not have to swim to do so? Well, they can easily find the places where the currents will allow them to park without having to work. The very first and single most important rule about reading waters is that fish cannot hold in fast currents. They simply cannot swim endlessly. If they could, one would never land them without simply forcing them to shore. Fish rely on the lateral line to help them find that just right spot to stay out of the currents while looking for food or hiding from a predator.

Speaking of which, Mr./Ms. Predator, trudging along the stream bank like an elephant in a congo line is not smart. The pounding vibrations of one’s feet transfer directly into the water as displacement waves, and fish pick up on it immediately. They either flee or they zip up their lips and sink to the bottom. Walk like a stealth heron, and stay hidden. Always assume that the fish can both see and hear you. And for goodness sake, wade quietly. There’s a saying that goes like this: Fish know you’re in the water, big fish always know. If they can hear you hiking on shore, they can certainly hear you waltzing around in their living room.

A South American Payara—note the very easily seen lateral line; the better to hear you with , my dear. Note also its very large eyes; the better to ...