Historic Overfishing Led to Modern Ocean Problems
By Cat Lazaroff
WASHINGTON, DC, August 14, 2001 (ENS) - A new study identifies overfishing as the cause, historically and currently, of many of the problems facing coastal ecosystems today. Researchers linked the ecological extinctions of marine megafauna - vast populations of whales, manatees, dugongs, monk seals, sea turtles, swordfish, sharks, giant codfish and rays - to overfishing at a global scale never before realized.
Recognition of what has been lost, however, also shows what could be gained. The scientists claim this revolutionary historical perspective is essential to wildlife management because historic data provide a framework for restoration that is otherwise invisible.
"Every marine ecosystem I have ever studied during my entire 30 year career looks unrecognizably different from the way it used to be, and I wanted to know why," added Dr. Jeremy Jackson of Scripps Institution of Oceanography in San Diego. Jackson, a renowned marine ecologist, instigated the two year study of human impacts on oceans over time.
Jackson convened an international team of 19 leading marine researchers at the National Center for Ecological Analysis and Synthesis (NCEAS) in Santa Barbara, California. Drawing on paleoecological, archeological and historical data, the scientists uncovered past evidence of seas teeming with large animals as well as abundances of oysters and shellfish so vast they posed hazards to navigation.
The scientists examined records from marine sediments dating from about 125,000 years ago; archaeological records from human coastal settlements occupied after about 10,000 years ago; historical records from documents of the first European trade based colonial expansion in the Americas and South Pacific in the 15th century to the present; and ecological studies from the past century to help calibrate the other records.
The scientists also were able to determine whether changes occurred due to human impacts or changing climate. They compared the function and structure of kelp forests, coral reefs and estuaries before and after fishing occurred.
The scientists found that as human disturbance occurred over time, ecosystem structures and functions changed in response to overfishing. While few species like the Steller's sea cow of the North Pacific and the sea mink of the Gulf of Maine were fished to extinction, many became ecologically extinct like the sea otter, which did not make a comeback from intense exploitation until afforded protection in the 20th century.
"Ecological extinction caused by overfishing precedes all other pervasive human disturbance to coastal ecosystems," begins the study, published in the July 27 issue of the journal "Science."
"Comparing the magnitude of the mass ecological extinctions in the ocean to those on land may not be enough," said study coauthor Dr. Roger Bradbury of the Australian National University in Canberra Australia. "On the land, as we killed off the giant mammals and destroyed the ancient forests, we replaced them with a new suite of farmed species. In the coastal seas, we took out animals and replaced them with nothing."
The new data also show that historical overkill of this marine life triggered current ecological collapses - many of which have been mistakenly attributed to pollution.
"We started out to study everything that people had ever done to oceans historically and were astounded to discover that in each case we examined, overfishing was the primary driver of ecosystem collapse," said Jackson.
Removal of key predators and entire layers of the food chain set off sequences of events that are now culminating in toxic algal blooms, dead zones, outbreaks of diseases and other symptoms of ecological instability.
One example of the negative chain reaction brought on by overfishing is the Chesapeake Bay, which is now an impoverished ecosystem dominated by bacteria. Historically, oysters filtered the entire water column of microscopic bacteria and debris every three days. Records describe a lost cannon, "clearly visible in over 30 feet of water."
Overfishing the oysters removed the top down control of phytoplankton. Grey whales - now extinct in the Atlantic - dolphins, manatees, river otters, sea turtles, alligators, giant sturgeon and hammerhead sharks were all once abundant inhabitants of Chesapeake Bay, but are now virtually eliminated.
Other examples include the overfishing of large fish, which has led to overgrowth of algae on coral reefs, smothering the reefs and jeopardizing the approximately three million species that they harbor.
The recent die off of turtlegrass beds in Florida Bay can be attributed to the ecological extinction of green sea turtles. Overkill of the green sea turtle and other seagrass grazers such as dugongs and manatees has contributed to outbreaks of disease and die offs in seagrasses.
Scientists have also long suspected that overfishing has caused the well publicized collapse of sea lion and sea otter populations in the Bering Sea. New research suggests that vast depletion of the great whales by humans has also contributed to this collapse.
The researchers warn that responding only to current events on a case by case basis cannot solve the ocean's problems because impacts of human disturbance are synergistic and have deep historical roots. Ecological extinctions make ecosystems more vulnerable to other natural and human disturbances such as pollution, disease and climate change.
Instead, the scientists say, problems need to be addressed by a series of bold experiments to test the success of integrated management on the scale of entire ecosystems. With few exceptions, such as the Steller's sea cow, and Caribbean monk seal, most species that are ecologically extinct probably still survive in sufficient numbers for successful restoration with proper management.
This optimism is in stark contrast to the situation in many terrestrial ecosystems, where many or most large animals are already extinct.
Rod Moore, executive director of the West Coast Seafood Processors Association, questioned the interpretation of the new data by some researchers, who blame marine fishing for many modern marine problems.
"Every naysayer and prophet of doom needs a scapegoat," Moore said. "The villain du jour seems to be 'overfishing' with the American fisherman as the star evildoer."
But the scientists advocate major changes to management practices, such as calling for massive restoration of the once vast oyster reefs of Chesapeake Bay. This would result not only in cleaner water but in an economic mainstay. Current plans for remediation of eutrophication of estuaries are still based on the belief that it is caused only by increased nutrients without regard to overfishing of suspension feeders such as shellfish.
"Clearly we have allowed too much fertilizer to enter bay waters, but we have also removed the major biological filters in the bay to only one percent of historical levels in the Chesapeake Bay and North Carolina's Pamlico Sound system," said Charles Peterson of the University of North Carolina. "Oysters filter and clarify bay waters. Oyster reefs provide habitat for blue crabs, rockfish and many other valued fishery resources."
Other recommendations include the restoration of coral reefs and seagrass beds by protection of fishes, sharks, turtles and sirenians in very large reserves on the scale of all of Florida Bay and the Florida Keys. The potential for reducing diseases of corals and turtlegrass by restoring natural levels of grazing is unproven but consistent with historical evidence.
Historical data not only help clarify underlying cause and rates of ecological change, but they also demonstrate achievable goals for restoration, management and exploitation of coastal ecosystems.
The scientists state that fisheries regulators and marine managers need to move beyond their fixation on quotas and boundaries and devise ways to restore the productivity and function of coastal seas.
"We need to change the way we think about our coastal seas: not pristine, but damaged, and equally not hopeless, but salvageable," explained Bradbury. "Our research points the way."