The marine ecosystem is under severe stress; in many areas the conditions necessary to sustain life are either in jeopardy or nonexistent. The destruction of marine habitats is especially prevalent along coastlines where human populations have increased. Habitat loss, pollution, overfishing, destructive fishing practices, and global warming are all undermining the marine environment.
Habitat loss, pollution, runoff, and increased salinity are destroying coral reefs, sea grasses, and other habitats for birds and fish. As coastal wetlands are filled in to accommodate growing human populations, the damming of rivers decreases the flow of freshwater, slows nutrient runoff, and inhibits fish migration. Less freshwater means increased salinity in wetlands and estuaries, which harms the grasses that purify water as it flows to the sea. Erosion caused by deforestation sends silt into rivers, streams, and eventually the ocean, blocking the sunlight needed for coral reefs to survive.
Maximum sustainable yield is calculated by fisheries biologists to estimate the amount of fish that can be harvested from a population without risking its long-term viability. Between 1974 and 1999, the proportion of fisheries that surpassed the maximum sustainable yield for cod tripled, from 10 percent to 30 percent. According to the Center for Ocean Solutions, since the early 1990s total catch in one of the world’s most productive fisheries, the Okhotsk Sea, has decreased betwen two and two and-a-half times due to overfishing. In the Pacific ocean, more than half of the island nations do not sustainably manage their coral reefs.
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Employing a practice known as bottom trawling, commercial fishing vessels drag large nets attached to heavy weights across the sea bottom. Targeted species include shrimp, cod, sole, and flounder, but everything along the seafloor is captured. Bottom trawling can leave the marine ecosystem permanently damaged and the bycatch (non-target species like sea turtles, seabirds,and mammals) are simply thrown overboard. Bycatch can amount to 90% of the total catch and endangered fish and deep-sea corals are often killed.
As the climate warms, the ocean is absorbing more carbon dioxide, making it more acidic. Increased acidity inhibits the ability of marine organisms to develop shells, and this includes the tiny animals called plankton that form the base of the ocean's food web. Some researchers suggest this will also cause some marine species to emit fewer of the sulfur compounds that promote cloud formation, which cools the Earth. Climate models predict this will cause 0.5 degrees Celsius (0.28 degrees Fahrenheit) of additional warming during this century.