shoals

Shoal is a term used to describe two related phenomena in marine and freshwater environments. In physical geography, a shoal is a natural rise in the seafloor or riverbed—a sandbank, reef, or other shallower area that stands higher than the surrounding bottom. Such features can extend as banks or patches and are shaped by sediment transport, tides, waves, and currents. Shoals are common on continental shelves, in estuaries, and at river mouths, and they can reduce depth and influence local currents, sometimes creating navigation hazards.

A shoal, in biology, refers to a group of fish or other aquatic animals that form a loose aggregation while moving. In many contexts, a fish shoal simply means individuals swimming together for social reasons, whereas a school denotes a tightly coordinated, synchronized group with uniform movement. Shoaling provides benefits such as increased vigilance against predators and improved foraging, though it does not require the precise alignment of a schooling swarm.

Physical shoals arise from sediment deposition and erosion and may migrate slowly due to changes in currents or sea level. They may be composed of sand, mud, gravel, or biogenic material and may be permanent or ephemeral. Hydrographers chart shoals and publish navigational notices; sailors avoid shallow areas using depth soundings, sonar, and markers.

Ecologically, fish shoals can be temporary aggregations or long-term social groups depending on species, season, and food availability. Shoaling behavior is a subject of behavioral ecology, with cues such as vision and chemical signals influencing cohesion, orientation, and the decision to join or leave groups.