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 coming 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 postdoctoral fellow at the University of Technology in Sydney, Australia.

Dr. Madin did the research in the remote Line Islands, 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 surgeonfish.