shoal

A shoal is a natural submerged or exposed elevation on the seabed that is shallower than the surrounding area. In nautical usage, a shoal can be a sandbank, sandbar, reef, or rocky outcrop that reduces depth and may pose a hazard to navigation. The term also applies to shallow areas in rivers and estuaries created by sediment deposition. A related sense refers to a large group of fish or other marine organisms that swim together.

Formation and characteristics: Shoals form through the transport and deposition of sediments by tides, currents, and waves. They can be relatively permanent features or dynamic, migrating with seasons and storms. Some are exposed at low tide, others remain submerged. Shoals influence local wave patterns, currents, and tidal flow, and they often create specialized habitats for birds, invertebrates, and juvenile fishes. Their geometry and depth can change due to dredging, coastal engineering, and natural sedimentation.

Navigational and ecological importance: Shoals affect harbor entrances, sailing routes, and anchorage areas, requiring regular charting and, in some cases, dredging or the installation of navigational aids. They can serve as nurseries or feeding grounds, contributing to local biodiversity, but also increase the risk of grounding for vessels with insufficient depth clearance.

Fish shoal: A separate usage of the word refers to a large group of fish or other mobile marine organisms that swim together, often for feeding or protection. The verb form shoal can describe waters becoming shallower or, in some contexts, the act of forming a shoal.