Water movement and pressure changes are detected by the lateral-line system. This is a network of sensory cells, often in water-conducting canals, in the skin.
This system is common to all fish but is only visible to the unaided eye on some. On black bass, crappie, sunfish and some minnows, the lateral line can be seen as a conspicuous line on the side of the fish running from behind the gills to near the tail. What you are seeing is not the lateral line sensory cells or canal but a row of modified scales that allow water to flow from outside the fish to the lateral-line sensory cells.
The lateral-line system is not confined to the side of the fish. Indeed, the system is especially well developed on the head of the fish. Because the lateral-line system is not restricted to the side of the fish, knowledgeable biologists refer to the lateral-line system as the lateralis system.
The lateralis system on the head of the fish (the head canals) is difficult to see. If you look at the underside of the jaw of a largemouth or smallmouth bass, you will see some pores. These are a few of the many openings to the head canals of the lateralis system. You can find these pores on other fish, but they are not as conspicuous.
By detecting water movement, the lateralis system allows the fish to sense water currents but also allows the fish to detect objects, including forage and predators.
Everything moving through the water displaces water and creates currents or pressure. The change in pressure may be from the movement of the fins or the undulating body of another fish. The lateral line on the side of the fish may help detect a predator approaching from the side or behind, but the lateralis system on the head allows the fish to detect what is in front, a much more important direction when the fish is searching for prey.
The head canals can also detect objects that are not moving. Even though fish are hydrodynamically "slick" to be able to swim efficiently, they still push water ahead of themselves, like the bow wake of a boat. Changes in the "bow wake" that result from this pressure wave reflecting back from an object are detected by the sensory cells in the head canal and interpreted to sense the environment. The system works just like the depth finder on your boat, except the high-frequency sound pulses are changes in water pressure.
The function of the lateralis system in detecting objects, including potential prey, has been well understood for decades. However, recent studies on muskie and northern pike are making biologists take another look at the role of the lateralis system in feeding. Pike and muskie are aggressive fish-eating predators. They typically live in clear-water lakes and rivers and have always been regarded as sight-feeders. Vision is undoubtedly involved in detecting prey, but the lateralis system is involved in prey capture.
The head canals of pike and muskie have numerous openings on the top of their duck bill-like snouts. When the sensory cells in these canals are disabled, the pike still strike at prey but capture success decreases. Apparently, the lateralis system is important to increase the accuracy and timing of strikes at prey. To what extent the lateralis system is involved in prey-capture efficiency in other fishes awaits further study.
Many fish are considered sight feeders. However, sight is an effective sensory system only in clear water, and all fish have to deal with muddy water sometime. Many anglers I know put a lot of stock in the fish's sense of smell to detect bait or lures. Fish have very acute abilities to detect chemicals in the water, and the sense of smell can help a predator detect prey. The diffusion of chemicals in water is a very slow process. While smells may alert a predator to the presence of prey (or alert prey to the presence of predators), chemicals in the water would not allow the quick detection and pinpointing of prey needed for successful feeding. The lateralis system is a perfect backup system for feeding in turbid water.
Anglers can, indeed should, consider the lateralis system when fishing, especially in turbid water where fish are "feeling for prey." Lures that move a lot of water, like crankbaits and large-bladed spinnerbaits, are good choices for turbid water; but don't overlook slow-moving baits. Large soft-plastic baits, skirted jigs, and curl-tail grubs, although usually fished slowly, also move a lot of water.