Humans Impact Oceans Worldwide: Fishing, Farming, Pollution Harm Sea Life

Scientists are increasingly realizing how much the world's oceans are being impacted by human activity. And, in almost every case, that impact isn't good.

Just last week, researchers at the annual meeting of the American Association for the Advancement of Science (AAAS) presented a study demonstrating how the effects of ocean trawling are so dramatic, they can be seen from space. The huge plumes of sediment kicked up during trawling expeditions are visible by satellite.

Trawling boats in search of shrimp use giant nets that scour the sea bottom, scooping up vast quantities of not only shrimp, but fish, corals, sponges and many other types of marine life as well. The result is vast swaths of ocean floor devoid of almost all life.

"Bottom trawling is the most destructive of any actions that humans conduct in the ocean," said Les Watling, one of the study's authors and a professor of zoology at the University of Hawaii. "Ten years ago, Elliott Norse (president of the Marine Conservation Biology Institute) and I calculated that, each year, worldwide, bottom trawlers drag an area equivalent to twice the lower 48 states. Most of that trawling happens in deep waters, out of sight. But now we can more clearly envision what trawling impacts down there by looking at the sediment plumes that are shallow enough for us to see from satellites."

Images and videos of trawling's after-effects are posted on the Marine Conservation Biology Institute's Website. A "virtual tour" of satellite imagery can also be seen at SkyTruth.

"For years marine scientists have been telling the world that fishing has harmed marine biodiversity more than anything else," Norse said. "And it's clear that trawling causes more damage to marine ecosystems than any other kind of fishing. Now, as the threats of ocean acidification and melting sea ice are adding insult to injury, we have to reduce harm from trawling to have any hope of saving marine ecosystems."

Another recent study, this one from the National Oceanic and Atmospheric Administration (NOAA), analyzed how much various human activities are affecting the Earth's oceans. The researchers found some 40 percent of oceans are showing clear signs of human impact and that no part of the planet's hydrosphere is untouched by civilization.

"The extent of human influence was probably more than any of us expected," said Kenneth Casey, a co-author of the study and a researcher at NOAA's National Oceanographic Data Center.

Among the seas most affected by humans: the East Coast of North America, the Caribbean, some parts off the western Pacific Ocean, the Bering Sea, the North Sea, the Mediterranean, the South and East China seas, the Red Sea and the Persian Gulf. In those regions, the habitats most threatened by human activities are coral reefs, seagrass beds and mangroves.

The Earth's polar oceans remain among the least impacted.

"This project allows us to finally start seeing the big picture of how humans are affecting the oceans," said Ben Halpern, the study's lead author and a researcher at the National Center for Ecological Analysis and Synthesis at the University of California, Santa Barbara.

Activities that are leaving unmistakeable marks on the oceans include fishing, shipping, pollution and agriculture, which causes fertilizer runoff that enters waterways and contributes to growing marine dead zones around the world.

Dead zones are areas of the sea so depleted of oxygen that they can't support marine life. These areas form when fertilizer-rich runoff from farmlands fuels masssive blooms of algae in the seas. As the algae die, they provide food for bacteria that multiply and take in oxygen from the water, causing the area to become inhospitable to other forms of life.

The U.S. Geological Survey (USGS) last month reported that, in the U.S., nine farm-heavy states along the Mississippi River Basin are the main source of nutrients flowing into the Gulf of Mexico. Those runoffs threaten both the fishing industries and ecological well-being of the Gulf, USGS researchers said.

The nine states -- Arkansas, Illinois, Indiana, Iowa, Kentucky, Mississippi, Missouri, Ohio and Tennessee -- account for more than 75 percent of the nitrogen and phosphorus runoff entering the Gulf, the study found. It also found that manure from animals on pastures and open ranges contributes nearly as much phosphorus to farm runoff as do croplands.

"This study is important because it reveals new details about sources of phosphorus," said Richard Alexander, a USGS scientist and lead investigator in the study.

Such details are critical in efforts to reduce the extent of the dead zone in the Gulf of Mexico. The U.S. Environmental Protection Agency's Science Advisory Board has recommended reducing nitrogen and phosphorus runoff by at least 45 percent to cut the Gulf's dead zone in half by 2015.

Growing areas of low oxygen have also been observed off the coasts of Oregon and Washington. A team of NOAA scientists last week reported seeing "low oxygen levels that are much more widespread and far more intense than what has been recorded in the past," according to William Peterson, one of the researchers involved in the study.

"The fish have simply moved out of these areas and are probably doing fine elsewhere," Peterson said. "But animals that can't move to better waters like Dungeness crabs, sea urchins and starfish will perish."

In the summer of 2006, scientists observed an ocean area completely devoid of oxygen -- a condition known as anoxia -- off the coast of Oregon for the first time ever.

Many marine researchers this year are focusing special attention on coral reefs, as 2008 marks the International Year of the Reef. Reefs are among the habitats feeling the greatest impact from human activity. In fact, one study last year warned that climate change could cause 98 percent of the world's oceans to become too warm to support coral reefs by 2050.