<P> Marine debris is any solid object that enters coastal and ocean waters . Debris may arrive directly from a ship or indirectly when washed out to sea via rivers, streams, and storm drains . Human - made items tend to be the most harmful such as plastics (from bags to balloons, hard hats to fishing line), glass, metal, rubber (millions of waste tires), and even entire vessels . </P> <P> Plastic debris kills several reef species . Derelict (abandoned) fishing nets and other gear--often called "ghost nets" because they still catch fish and other marine life despite being abandoned--can entangle and kill reef organisms and break or damage reefs . Even remote reef systems suffer the effects of marine debris . Reefs in the Northwestern Hawaiian Islands are particularly prone to the accumulation of marine debris because of their central location in the North Pacific Gyre . From 2000 to 2006, NOAA and partners removed over 500 tons of marine debris there . </P> <P> Dredging operations are sometimes completed by cutting a path through a coral reef, directly destroying the reef structure and killing any organisms that live on it . Operations that directly destroy coral are often intended to deepen or otherwise enlarge shipping channels or canals, due to the fact that in many areas, removal of coral requires a permit, making it more cost - effective and simple to avoid coral reefs if possible . </P> <P> Dredging also releases plumes of suspended sediment, which can settle on coral reefs, damaging them by starving them of food and sunlight . Continued exposure to dredging spoil has been shown to increase rates of diseases such as white syndrome, bleaching and sediment necrosis among others . A study conducted in the Montebello and Barrow Islands showed that the number of coral colonies with signs of poor health more than doubled in transects with high exposure to dredging sediment plumes . </P>

What are some natural threats to coral reefs