Overfishing of Sharks Key Factor in Coral Reef Decline

Newswise — Their position at the pinnacle of the marine food chain is legendary.

Now, understanding sharks and their significance as top predators—and theconsequences of human activity towards them—has taken on new importancethrough a new study by scientists in San Diego and Spain.

Jordi Bascompte and Carlos Melián of the Integrative Ecology Group, EstaciónBiológica de Doñana, Consejo Superior de Investigaciones Científicas, inSevilla, Spain, and Enric Sala of Scripps Institution of Oceanography at theUniversity of California, San Diego, developed an unprecedented model of aCaribbean marine ecosystem and details of its intricate predator-preyinteractions. This food "web" covered 1,000 square kilometers to a depth of100 meters and included some 250 species of marine organisms. The study,published in the April 12 edition of the Proceedings of the National Academyof Sciences, included an intricate network of more than 3,000 links betweenthese species.

The project was one of the largest and most detailed investigations ofmarine food webs and the first study to integrate food web structure,dynamics and conservation.

One of the most striking products of the study is a stark picture of humanimpacts on marine ecosystems and the consequences of targeted fishing. Inthe Caribbean, overfishing of sharks triggers a domino effect of changes inabundance that carries down to several fish species and contributes to theoverall degradation of the reef ecosystem. Overfishing species randomly, thestudy shows, is not likely to cause these cascading effects.

"It appears that ecosystems such as Caribbean coral reefs need sharks toensure the stability of the entire system," said Sala, deputy director ofthe Center for Marine Biodiversity and Conservation at Scripps.

When sharks are overfished, a cascade of effects can lead to a depletion ofimportant grazers of plant life. This is because there are fewer sharks tofeed on carnivorous fish such as grouper—causing an increase in theirnumbers and their ability to prey on parrotfishes. The removal ofplant-eating animals such as parrotfishes has been partly responsible forthe shift of Caribbean reefs from coral to algae dominated, the authorsnote. Thus overfishing of sharks may contribute further to the loss ofresistance of coral reefs to multiple human disturbances.

"The community-wide impacts of fishing are stronger than expected becausefishing preferentially targets species whose removal can destabilize thefood web," the authors conclude in their report.

Because of their comprehensive approach in developing the intricate foodweb, the authors say their study and its results address more thanindividual species protection and speak to larger ecosystem protectionissues.

"The paper presents a community-wide approximation of conservationproblems," said Bascompte. "We cannot asses all of the implications ofoverfishing by only looking at the target species or a few others. Speciesare embedded in a complex network of relationships and this network has aparticular shape. This has large implications for the propagation of theconsequences of overfishing through the whole food web."

Funding for the study was provided by the Spanish Ministry of Science andTechnology and a grant from the History of Marine Animal Populations Programof the Census of Marine Life, which was sponsored by the Alfred P. SloanFoundation.

Scripps Institution of Oceanography: http://scripps.ucsd.edu

Scripps News: http://scrippsnews.ucsd.edu

Scripps Institution of Oceanography, at the University of California, SanDiego, is one of the oldest, largest, and most important centers for globalscience research and graduate training in the world. The National ResearchCouncil has ranked Scripps first in faculty quality among oceanographyprograms nationwide. Now in its second century of discovery, the scientificscope of the institution has grown to include biological, physical,chemical, geological, geophysical, and atmospheric studies of the earth as asystem. Hundreds of research programs covering a wide range of scientificareas are under way today in 65 countries. The institution has a staff ofabout 1,300, and annual expenditures of approximately $140 million fromfederal, state, and private sources. Scripps operates one of the largestU.S. academic fleets with four oceanographic research ships and one researchplatform for worldwide exploration.