Human activities can change the seabed directly (by dredging, trawling, and boat groundings) and indirectly (by damming, construction of sea walls, and other things that increase sedimentation and/or cut off the natural flow between land and sea). Coastal landscape changes that replace natural coastal ecosystems— like the building of major urban centers, resorts, hotels, golf courses, ports, and factories—can be huge.
As coastal areas are developed, seabird habitats, marine mammals, and other marine species are decreased or wiped out. Shallow harbors are dredged for the passage of larger ships. Sea walls stop the natural exchange between land and sea. The damming of rivers cuts off important migration routes for salmon and blue crab, which need fresh- and saltwater habitats to survive.
Coastline physical changes can also impact open ocean species and systems, as some species spend part of their life cycle near shore, onshore, or inland.
Another widespread source of physical marine change is caused by bottom trawling. Bottom trawlers drag weighted fishing nets across the sea floor, dis turbing whatever rocks, coral, and organisms are in their paths. Fish that hide among rocks and coral reefs are left homeless, coral that needs clear water to survive becomes choked with silt, and fishing ecosystems are altered to a large extent.
Bottom trawling is often compared to clear-cutting of forests. However, a forest is clear-cut only once over many years; a specific sea floor area may be bottom-trawled 100 times in a single year. Scientists believe roughly 70 to 95% of the organisms collected by bottom trawlers are not wanted and usually thrown away.
Undersea mining operations, offshore oil exploration, other types of resource extraction activity, ship groundings, and anchorages also disturb coral reefs, ecosystems, and the sea floor.
Dynamite fishing is another method that devastates coral reefs and reef communities. Fishers set off dynamite on coral reefs to kill and stun fish that can then be scooped up with nets. The explosions kill everything near the blast, including corals. Another technique, called muro ami, involves pounding reefs with heavy weights to scare fish out of their hiding places, destroying the coral heads in the process.
Fortunately, many of these physical impacts can be prevented or changed through better education on marine environments and newer, less damaging technologies.
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