What sensory system lets many fish detect water movement and vibrations?

The sensory system that lets many fish detect water movement and vibrations is the lateral line.

The lateral line is a network of sensory organs, usually visible as a faint row of pores along each side of a fish. Inside those pores are clusters of hair cells, which bend when nearby water moves. The signals help fish detect pressure changes, vibrations, and the direction of disturbances.

This system is useful when visibility is poor. Fish can use it to orient themselves, avoid obstacles, locate prey, sense predators, and coordinate movements within a school. A predator may detect vibrations produced by a struggling fish even when it cannot see the prey.

The lateral line is not a fin, gill structure, or buoyancy organ. It is also distinct from electroreception, although some lateral-line-related sensory structures evolved into electroreceptors in ancient vertebrate lineages. The system is evolutionarily old, appearing in fish groups that diverged more than 400 million years ago.