Without predators, reef fish get fearless

UC SANTA BARBARA (US) — When the number of predators on coral reefs is reduced by fishing, their prey move greater distances, take more risks, and change feeding behaviors.

These changes can scale up to drive significant ecosystem changes, new research shows.

“In more pristine areas, predators abound, and small prey fish are cautious and move only short distances from their hiding places,” says Robert Warner, professor of ecology, evolution, & marine biology at the University of California, Santa Barbara.

“On heavily populated islands, where fishing has dramatically reduced numbers of predators, small fishes are much bolder. What we’ve seen is that these behavioral responses can lead to cascading effects through the ecosystem.”

Madin coral reef dive_1

Elizabeth Madin studies the coral reefs off Palmyra Atoll, Line Islands. (Credit: UC Santa Barbara)

In a pair of studies published in the journals American Naturalist and Ecology, the scientists report that when hunted by large predators, such as sharks and snapper, small fish tend to hide in reefs and move around less.

The scientists studied coral reefs of the central Pacific Ocean’s Line Islands, a small equatorial archipelago, from 2005 to 2008. The islands are thousands of miles from the nearest landmass. Predators have been heavily fished near some islands and never fished near others. The team saw firsthand how fishing had decimated populations of sharks and other predators.

“Like many of my generation, the movie Jaws instilled in me an unfortunate, somewhat irrational fear of being eaten, and I began to wonder what would happen to the reef if the small, seaweed-eating fish had nothing to fear,” says Elizabeth Madin, the lead author of both studies. “What if they could graze more like cows in a pasture than wildebeest on a lion-infested African plain?”

By removing predators and changing the grazing behavior of small fish, there were dramatic changes in the seaweed patterns on coral reefs, giving the reefs a new look.

Seaweed is important because areas of lush seaweed growth inhibit growth of coral, the important engineers of the reef. By changing where seaweed grows, fishing may inadvertently also be changing where coral can grow.

Fishing can have important consequences, the researchers say, not only for predators, but also for prey and ultimately for entire reef communities by changing the behavior of small fish.

When small fish are able to move freely over the reef without fear of being eaten, the scientists found that prey fish may have more feeding and mating opportunities. They also found that prey fish may consume their food, such as reef seaweed, more evenly across the reef, so seaweed is evenly grazed and less patchy.

Coral, which competes for space on the reef with seaweed, could potentially find fewer bare patches of reef that it can settle in, so the places where new coral can grow could be more limited.

“What our results show is that fishing can have surprising, but very clear, effects throughout coral reef ecosystems,” Madin says. “Hopefully, these results will help conservation practitioners and resource managers move toward true ecosystem-based management, where the full suite of ecological interactions and human impacts guide policy decisions.”

Researchers from Macquarie University in Sydney, Australia, contributed to the study.

More news from UC Santa Barbara: http://www.ucsb.edu/