Ocean Racket Makes for Rascally Wrasses

Motorboat noise spurs rise in fish-on-fish crime.

Published August 4, 2017

Cheaters never prosper, unless there’s a motorboat buzzing by.

That’s what a new study found when looking at the interactions between bluestreak cleaner wrasses and other fish on coral reefs in the South Pacific. It’s the first time scientists have seen ocean noise impact a fish’s relationships with other species.

The typical cleaner wrasse makes its living by nibbling parasites off other fish. A wrasse hangs out at an established “station” on a reef that other fish visit for cleaning. By eating the parasites, the wrasse gets a meal and the client gets a clean bill of health. Everyone leaves happy unless the wrasse cheats and takes an extra treat—a nibble of tasty skin mucus or even a living scale. When that happens, the client usually punishes the wrasse by instantly giving chase. The wrasse, in turn, is more “honest” in its subsequent interactions with that fish, taking fewer extras, at least for a while.

Cheating isn’t random. Wrasses will cheat predatory fish far less often, for instance. And they’re more likely to steal extra snacks off client fish that live in a small section of the reef rather than from species that move around the reef and have their choice of cleaners. Wrasses are canny entrepreneurs with nuanced relationships.

But, as Isabelle Côté and her colleagues have found studying wrasses and their clients near the South Pacific island of Moorea, when there’s ambient noise from a nearby motorboat, the cleaner-client interaction changes. The wrasses take advantage of the noise and skim more mucus and scales, and they seem to get away with it.

“Normally when a fish cheats, the immediate reaction is to chase, to punish the cleaner wrasse,” says Côté, a marine ecologist at Simon Fraser University in Burnaby, British Columbia. “All else being equal, we didn’t see an increase in chasing.”

Côté thinks the clients don’t punish the wrasses around boats because the noise confuses them. It’s not yet clear if the wrasses cheat more because they’re also distracted by the noise, or just because they’re not being punished.

The effects of noise on fish is a new field of study; most work to date on the noise in the ocean has focused on whales. But as the number of motorboats on the reefs in Moorea grows, there is concern over the potential effects on fish and reef ecology.

Thomas Adam studies coral reef ecology on Moorea as an assistant research biologist with the University of California, Santa Barbara. His graduate work explored cleaner wrasses and their complex relationships.

“This [study] is really novel in the sense that it looked at the impact of noise pollution on the interaction between two different species,” he says. “Or in this case, more than two different species.” The relationship it’s affecting is significant, says Adam. A wrasse cleans hundreds of clients a day, and can eat 1,200 parasites in that time. That’s a lot of creepy-crawlies that could potentially zap the energy and health of a lot of fish. When wrasses are experimentally taken off reefs, Adam says, the diversity and number of other fish goes down.

Adam cautions that there are more immediate threats to reefs: sedimentation, nutrient pollution, and especially climate change. While marine protected areas may shield reefs from destruction, pollution, or boat noise, “you cannot protect reefs from climate change,” he says. And the reefs need to be there for the fish to conduct their intricate, complex marketplace.