Without Sharks, Food Chain Crumbles

Sharks hold a legendary spot at the top of the marine food chain. A new study suggests that overfishing of sharks by humans can cause that chain to crumble.

The project - a coordinated study by the Integrative Ecology Group in Sevilla, Spain and the Scripps Institution of Oceanography at the University of California, San Diego - was one of the largest and most detailed investigations of marine food chains and the first study to integrate food chain structure, dynamics and conservation.

The study showed the adverse effects targeted fishing can have on an ecosystem. In the Caribbean, overfishing of sharks triggers a domino effect of changes that carries down to several fish species and contributes to the overall degradation of the reef ecosystem. Overfishing species randomly, the study shows, is not as likely to cause these cascading effects.

When sharks are overfished, the carnivorous fish they feed on increase in number. The carnivorous fish, in turn, prey on parrotfish, which normally graze for plant life on the reef. Since there are less parrotfish to eat the algae off the coral, the entire identity of the reef changes from coral to algae dominated, according to the authors. Thus overfishing of sharks may contribute further to the loss of resistance of coral reefs to multiple human disturbances.

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

Because of their comprehensive approach in developing the study, which covered 1,000 square kilometers to a depth of 100 meters, the authors say their results address more than individual species protection and speak to larger ecosystem protection issues.

The study, published in the April 12 edition of the Proceedings of the National Academy of Sciences.

Bjorn Carey

Credit: Scripps Institution of Oceanography