Habitat Destruction or Alteration
Conversion of coastal ecosystems for agriculture or aquaculture has adverse effects on marine fisheries because it destroys the habitat of exploited fish stocks. For example, conversion of mangroves in a number of South and Southeast Asian countries during the mid 1990s caused an increased risk of diseases in wild stock. It also significantly reduced the recruitment and survival rates of the stocks. Since some 90% of fish stocks depend on coastal habitat for at least parts of their life cycle, this habitat is critical. (Perrings 2000)
Areas of special concern include wetlands, coral reefs and mangrove swamps.
Wetland soils are saturated long enough during the growing season to create an anaerobic (low oxygen) state. The wetland soil becomes so saturated with water that it cannot hold much, if any, oxygen.
Wetlands are often called "nurseries of life", meaning they support thousands of species, both terrestrial and aquatic. But they do more than just provide a habitat for these animals. When rivers overflow, wetlands help absorb the flood waters, which can help reduce property damage and loss.
Rainwater runs off and brings exposed soil particles toward larger bodies of water. In water, sediment may either settle to the bottom or remain suspended in the water column. Settled sediments may destroys the spawning grounds for fish and may suffocate fish eggs. Sediments may also smother macro-invertebrate benthos (bottom dwellers)---an important source of food for fish. Suspended sediments also affect aquatic organisms. Sediment makes water more opaque so the water temperature increases. It can also abrade fish gills and make feeding difficult for fish that rely heavily on sight to find food.
Wetlands generally slow water velocity, which allows much of the suspended sediment to settle out (slow-moving water can transport a smaller sediment load). Plants within the wetland also mechanically slow sediments. This helps prevent the sedimentation (or mud-clogging) of streams, lakes, or rivers.
Runoff entering wetlands contains much more than just sediments. Pesticides, excess nutrients from fertilizers, bacteria, salts from winter road maintenance, and other chemicals also wash from the land and enter our waterways. Studies have found that after this polluted water has flowed through a wetland it becomes much cleaner. Wetlands, with their dense plant life and unique anaerobic environment, can protect downstream waters from these substances by using the extra nutrients for plant growth and by storing and breaking down the chemicals. This filtering process improves the quality of the water for wildlife and humans. (Dietz)
From an economic perspective, about 75% of the nation's commercially important species of marine fish and shellfish, and 80 to 90% of recreationally important species, are dependent on shallow inshore waters, such as bays, estuaries, and rivers flowing to the sea, for their survival (Vymazal, 2007). However, the importance of wetlands is perhaps best shown by example:
Three Gorges Dam in China
In China, the Yangtze River branches out into a broad estuary that stretches 655 kilometers into the East China Sea, and forms one of the largest continental shelves in the world. Over half of the Yangtze's annual sediment load is deposited in the estuary. The health of the estuary depends on the delivery of this sediment because a significant relationship exists between intertidal wetland growth rate and riverine sediment supply. Yet, due to the Three Gorges project and other dams, the sediment accumulation rate in all reservoirs on the river has increased. This is causing erosion of the wetland habitat there, which provides nurseries for fish and resting areas for migratory birds and is considered one of the world's most important wetland ecosystems. There is also concern about the impact the project will have on biological diversity. The baiji dolphin, the ancient river sturgeon, and the finless porpoise depend on the Yangtze for their survival. The population of Siberian cranes in Poyang Lake will also be affected by the dam (Cleveland 2007).
Currently, wetlands are threatened by many natural and anthropogenic forces, particularly pollution from land (NOAA). Eutrophication--the overfertilization of aquatic ecosystems--affects coral reefs particularly badly because the algal growth can smother the coral (Jones and Endean, 1976). Oil spills can negatively affect coral spawning (Bryant, et al, 1998), and practices such as harvesting for aquariums, blast fishing, careless diving, cyanide fishing, and trawling also destroy coral reefs (NOAA).