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Around Lizard Island in Australia, small reef fishes—gaudy damselfish and parrotfish—nip around, while the larger fishes—grunt, emperor, snapper, and grouper—rest on top of the corals. The smaller fishes are prey fishes; the larger fishes, mesopredators, in the middle of the food chain. But, unexpectedly, all the fishes react in one of three ways when a snorkeler swims by.

Prey or mesopredator, fishes dart off fast past the snorkeler, make a 180-degree turn to escape in the opposite direction, or slide into hiding under a coral plate, crevice, or nook. To the fish, the seemingly harmless snorkeler may as well be a shark, or another alarming predator, according to new research.

Using a remote-controlled underwater 3D video camera, study author Andrea Asúnsolo Rivera, who conducted the research at the University of Western Australia, and her colleagues observed how mesopredators reacted when they experienced approaches from five different objects: a snorkeler, a life-size model of a blacktip reef shark, a replica of a non-threatening green turtle, a white PVC pipe, and a clear rectangle of acrylic.

The turtle, pipe, and acrylic shape went by barely noticed: most of the time, they elicited no response. But about 90 percent of fishes reacted with “antipredator behaviors” such as darting off or hiding when the fake shark came by, and a whopping 96 percent of them did so when met with the snorkeler.

“It’s not so much striking that the fish reacted, but it’s the fact that they reacted equally,” says Asúnsolo Rivera.

When the blacktip was around, fishes darted off more than 60 percent of the time. When it was a snorkeler, they had this same flight behavior for about half of the encounters, and they hid one-third of the time. Plus, the mesopredators tended to swim away rather than hide, according to Asúnsolo Rivera, probably because it’s harder for them to find a hole big enough to shelter in.

This graph shows how fishes reacted to approaches by a shark, snorkeler, turtle, pipe, and clear acrylic object. The responses were darting off (flight), doing a 180 and swimming in the opposite direction (C turn), sneaking under a coral or rock (hide), or not reacting. Graph by Asúnsolo Rivera et al.

Recent research into how predators control environments suggests their presence has an effect on an ecosystem beyond preying on animals. The new study supports this idea: as long as the fishes are scared—whether the threat is real or not—they have less time to feed, compete for territory, and mate. In addition, if fishes are afraid when researchers are around, it may lead to biased observations about fish behavior or skewed population estimates.

Fish behavior in areas teeming with snorkelers might be fundamentally different, too, Asúnsolo Rivera says, although that hypothesis calls for more research. “Sometimes we forget that we are a predator, and we have the features of a predator,” she says. “We’re definitely going to cause a reaction, and we’re going to alter the behavior of a prey.”