For most of the past 4,500 years, cod was king in the Gulf of Maine's coastal waters. Today, cod have given way to the Jonah crab with potential long-term consequences for coastal fisheries, according to a University of Maine research report published in the journal Ecosystems.
With crabs and lobsters at the top of the proverbial heap, the Gulf may have entered a new stable phase marked by the presence of expansive kelp beds and the near absence of sea urchins. These findings could signal the likelihood of significant biological changes in other heavily fished parts of the world's oceans as well.
The authors of the report are Robert S. Steneck, professor of marine sciences at UMaine's Darling Marine Center, and former UMaine graduate students John Vavrinec and Amanda Leland. They received support for their research from the Pew Foundation for Marine Conservation, Maine Sea Grant, the Maine Department of Marine Resources and the National Undersea Research Program.
The researchers analyzed fishing records and previous studies to gather evidence for the changes brought on by fishing pressure in marine ecosystems. Ancient coastal middens, for example, have revealed evidence suggesting that Native American fishing activities were beginning to affect near shore ecosystems several thousand years ago. Analysis of colonial and modern fish landing records shows that such changes accelerated with the adoption of new fishing technologies.
It is a revolution of sorts, an overturning of the established order brought on by fishing pressure, that leads to major changes in the coastal marine ecosystem, according to the article, "Accelerating Trophic-level Dysfunction in Kelp Forest Ecosystems of the Western North Atlantic." In the Gulf of Maine, the revolution was brought on by the drastic reduction in the number of cod and other top predators over the past century.
"To understand how these changes are accelerating, we looked at archaeological data for coastal Maine over the past 4,500 years. The long dominance of predators has given way to many species playing 'king of the hill,'" says Steneck.
"While there is no fear of these species going extinct," he adds, "entire sections of the food web have become so rare that they no longer perform critical ecological functions in the marine community. This is called food web (or trophic level) dysfunction."
When such species as cod were no longer able to perform their function of keeping their prey species in check, the ecosystem entered a new phase marked by abundant sea urchins and a lack of kelp beds. Urchins ate so much kelp that they created areas known as "urchin barrens" where only low growing algae could survive.
In turn, the harvesting of urchins during the 1990s has led to the re-emergence of kelp beds and the dominance of crabs and lobsters. The report cites an experiment in which adult urchins were stocked in an area to see if they would survive and reproduce. Crabs ate most of the urchins.
"The problem is, this 'trophic level dysfunction' is accelerating. Ecosystem changes persist for shorter and shorter periods of time because the 'driver' species increasingly fall below functional population densities," Steneck explains.
"When a threshold is reached, the system changes fundamentally. Everything that came before it is thrown out the window. What this does in the long run is make the system unpredictable."
For the first time, adds Steneck, the low diversity of marine organisms, including Maine's fabled groundfish, have left the system too reliant on a single species (lobster) and too vulnerable to continued and unpredictable large-scale fluctuations.
Materials provided by University of Maine. Note: Content may be edited for style and length.
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