Coasts cover a sliver of the ocean yet sustain most commercial fish
Coastal habitats account for only about 7 percent of the world's oceans, yet at least 85 percent of commercially caught fish depend on them at some stage of life. Nearly half of humanity lives within 150 km of the sea. And nobody can say exactly how long Earth's coastline is.
A coast is the boundary zone where land meets sea or a large lake, shaped by the surrounding terrain, by wave erosion and by the rocks and soils underfoot. Earth has roughly 620,000 km of it, but because shorelines keep shifting, no exact figure is possible, a puzzle known as the coastline paradox. Definitions vary too: marine scientists count wet habitats like seagrass and salt marsh as coastal, some land-based scientists count only shoreline plants, and governments draw coastal boundaries to suit their own policies.
Tides and waves do much of the sculpting. Geologists sort coasts by tidal range: macrotidal above 4 metres, mesotidal between 2 and 4, microtidal below 2. Macrotidal coasts lack barrier islands and lagoons and have funnel-shaped estuaries with sand ridges lined up along the currents. Tides alone rarely erode, though tidal bores surging up estuaries can. Big waves release more energy, and on short, cliffed shores that energy compresses air and water into cracks and prises rock apart. Rivers supply most sediment where estuaries exist, though dams now trap much of it inland, while coral reefs feed the beaches of tropical islands.
Sheltered shores support salt marshes, mangroves and seagrass beds that serve as nurseries for fish and shellfish, while exposed rocky coasts host mussels, barnacles, starfish and seaweeds. Mangroves and seagrasses lock away carbon far more effectively than many land ecosystems, and coastal ecosystems buffer communities against tsunamis and rising seas. In many countries mangroves are the main source of fuelwood and building timber.
A 2013 UN atlas put about 44 percent of people within 150 km of the sea, relying on ports, fisheries, aquaculture and tourism. That concentration leaves coastal communities exposed to erosion, flooding, saltwater intrusion and stronger storms, while pollution, overfishing and development have degraded ecosystems worldwide. The UN has named 2021 to 2030 its Decade on Ecosystem Restoration, though coasts have received too little of that attention.
Source: Coast
