Predators Gone, Small Fish Get Bolder

A seaweed-eating surgeonfish (Acanthurus nigricans) grazing
on reef algae while keeping a watchful eye out for predators.
Photo Credit: Elizabeth Madin

By: Sindya N. Bhanoo
The New York Times, October 21, 2010

Little fish are cautious and timid around big, hungry fish, and rightly so. But when populations of predators like tuna and shark shrink because of human fishing, small prey are more adventurous, according to a forthcoming study in The American Naturalist.

“They took bigger excursions to feed, or to go to find mates,” said Elizabeth Madin, the study’s lead author and a post-doctoral fellow at the University of Technology in Sydney, Australia.

Dr. Madin did the research in the remote Line Islands, located in the central Pacific Ocean, while pursuing her Ph.D. at the University of California, Santa Barbara.

Using snorkels, scuba equipment and video cameras, she and her colleagues captured the behavior of more than 600 parrotfish, damselfish and sturgeon.

Two of the Line Islands are owned by the United States, which has designated them a protected no-fishing zone.

Three other islands are part of the Republic of Kiribati and are home to active human settlements that fish.

In waters untouched by humans, the researchers found that the little fish were spotty in their feeding habits. Some parts of the coral reefs, where fish feed on seaweed, were heavily grazed. In other areas, perhaps where predators lurked, there were large tufts of untouched seaweed.

But where fishing was happening, their grazing formed a more even pattern across the reefs.

Further study is needed, but this feeding pattern could affect coral growth, since coral grows where seaweed is not present, Dr. Madin said.

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