Public Release: 

Overfishing sets the stage for other problems in marine ecosystems

University of Maine

ORONO, Maine -- Overfishing of key marine animals such as cod, oysters, sea turtles and other species is the primary cause leading to a variety of problems that have appeared recently in coastal waters around the world, according to an article published this week in the journal Science. In locations ranging from the Chesapeake Bay to the Great Barrier Reef in Australia, increases in disease, invasions of non-native species and declining water quality can be traced back to a loss of species that exert a controlling influence over marine ecosystems, the authors conclude after a review of historical data that stretches back thousands of years.

In the Gulf of Maine, evidence for the loss of large predatory fish is clear, says co-author Robert Steneck of the University of Maine Darling Marine Center in Walpole. While a cause and effect relationship between the loss of those species and disease, water quality problems or the spread of non-native species has not been demonstrated for the Gulf, it can't be ruled out. "Taking a longer look at ecosystems gives us perspective on how much things have changed," says Steneck.

"The big news here is that we haven't considered that fishing impacts may have begun thousands of years ago and that some of the ecosystem level changes may be indirect," he adds.

The article, "Historical Overfishing and the Recent Collapse of Coastal Ecosystems," is co-authored by scientists from 14 universities and scientific organizations around the world, including Steneck and Bruce J. Bourque, a lecturer in anthropology at Bates College in Lewiston. Jeremy B. C. Jackson of the Scripps Institution of Oceanography in La Jolla, California, is the primary author. Science is published weekly by the American Association for the Advancement of Science.

"In Maine, we have evidence from Indian middens dating back 5,000 years that our coastal zone was dominated by large predatory fish such as cod. The average size of cod for thousands of years was about a meter long which is impressive considering the fishers used crude hooks made of deer bone with line made of deer intestines. Coastal food webs were probably dominated by large predators and many of the invertebrates such as lobsters and sea urchins may have been much less abundant than they are today," says Steneck.

"By any measure the rate of change in the world's coastal oceans is accelerating. Whereas cod were harvested by humans as a primary food for over 5,000 years and early Europeans saw cod as an inexhaustable resource, it wasn't until the last half century that cod became rare in coastal zones. Since then, increased lobster and sea urchin abundances may be the result of those former prey species living in a new predator-free world," he adds.

The article notes that a lack of historical perspective has led to erroneous conclusions about the causes of problems in marine waters. Such problems often have "deep historical roots," the article says. For example, land use activities around Chesapeake Bay have been considered the primary cause of water quality problems there. However, the loss of massive oyster beds had severe consequences for water quality because oysters kept the growth of water clouding algae in check. It was the loss of oysters that made the ecosystem vulnerable, the article states.

In other examples, the article cites the elimination of predators such as sea otters on the West Coast and of predatory fish from the Gulf of Maine as the primary factor in the loss of kelp beds. Otters and predatory fish kept sea urchins, which eat kelp, in check. Kelp "forests" are important habitats that support diverse marine communities. In recent years, urchin harvesting by humans has allowed kelp to expand in some areas of the Gulf of Maine, Steneck points out.

In addition to overharvesting, disturbance of underwater habitats is cited as a primary cause leading to the collapse of fish populations around coral reefs in the tropics, the article states.

The paper is based on four types of records: modern ecological records, historical information about fishing activities starting from the 15th century, archeological records from human coastal settlements occupied as far back as 10,000 years ago, and paleoecological data going back as far as 125,000 years ago.

In many cases, evidence for large populations of marine creatures in the past stands in stark contrast to relatively small numbers of such animals today, the authors note. Examples of overfishing are cited in aboriginal, colonial and modern cultures, but modern technology has allowed harvesters to eliminate what had been considered an inexhaustible marine resource, say the authors.

The historical perspective can be used to develop modern management strategies that could help fish populations recover, the authors also note. With some exceptions, most populations exist at levels that can be restored through the use of "bold experiments to test the success of integrated management for multiple goals on the scale of entire ecosystems."

"Food webs are ghosts of what they once were," adds Steneck. "When food webs are altered, the ecosystem changes. In Maine we have a very unstable coastal ecosystem today with changes occurring at the scale of years to decades, not centuries to millennia as was formerly the case.

"We don't understand all of the changes, but today there are areas carpeted with blue mussels where they used to be rare. Rock crabs have exploded to extraordinary density in some areas. Last year we relocated tens of thousands of sea urchins to bring them back to an area where harvesting has eliminated them, but they were all eaten by crabs. Change is common in all ecosystems, but the rate and magnitude of change we are currently seeing is unprecedented."

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In addition to UMaine and Bates, scientists contributing to the report are affiliated with the University of California, the Smithsonian Institution, the University of Florida, the University of Chicago, the Australian National University, James Cook University and the National Center for Ecological Analysis and Synthesis in California.

Research contacts: Robert Steneck, University of Maine Darling Marine Center, 207-563-3146, ext. 233; steneck@maine.edu; Bruce J. Bourque, Bates College, bbourque@abacus.bates.edu

Media contact: Nick Houtman, Dept. of Public Affairs, 207-581-3777, houtman@maine.edu

Note: Robert Steneck, who contributed to the report, is available July 24 at 207-549-3062 between 8:30 a.m. and 10:00 p.m. After July 24, he will be conducting a research cruise in the Gulf of Maine and is available by cell phone, 207-557-4505.

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