New research has revealed 2-stroke engine noise may be making reef fish easier targets for predators.
A team of scientists from ARC Centre of Excellence for Coral Reef Studies at James Cook University and The University of Exeter, have recently found that noise produced by recreational motorboats can prevent fish from responding to danger and puts them at greater risk of falling victim to predators.
“It’s one of the first field studies using noise from real motorboats to look at how they affect coral reef fishes,” said Professor Mark McCormick from James Cook University. “When damselfish were placed on habitat patches near motorboats they used space differently, but this was dependent on the motor type.”
There are two main types of outboard motors used by recreational boat users – those powered by noisy 2-stroke motors, and those powered by quieter but more expensive 4-stroke engines.
“We found that our damselfish no longer hid when they smelt odours from other threatened fish when boats with 2-stroke engines were passing. This is important because these smells are a reliable indicator that there is a predator hunting nearby. This was not a problem when boats powered by 4-stroke engines were used,” said Professor McCormick.
Noise expert Professor Stephen Simpson from the University of Exeter explains: “2-stroke engines rattle while 4-stroke engines hum, and this appears to distract the fish so that they make inappropriate decisions.”
“This is a very important finding because we have found that responding to odours from injured fish are fundamental to the way small fishes learn what represents a threat.
“A lab experiment we conducted showed that the damselfish can’t learn the identity of new predators in the presence of noise from 2-stroke boats,” said Professor McCormick. “What’s more, when we placed fish that had learnt under 2-stroke noise on the reef, they died much faster than experimental controls.”
“Almost everything we do in the sea is noisy,” he said.
“The numbers of boats and ships using the Great Barrier Reef for transport and recreation is increasing dramatically, but at the moment we know very little about the influence of the noise they produce.”
This is a problem because marine animals, whether they are invertebrates, fishes or whales, use sound to communicate and inform their decisions.
“By understanding how marine organisms are affected by boat noise we hope to inform the development and adoption of engines that have a low impact on reef animals. This may also give managers tools with which to mitigate the detrimental effects of human noise.”
McCormick MI, Allan BJM, Harding HR, Simpson SD (2018). Boat noise impacts risk assessment in a coral reef fish but effects depend on engine type. Scientific Reports doi: 10.1038/s41598-018-22104-3
Ferrari MCO, McCormick MI, Meekan MG, Simpson S, Nedelec S, Chivers DP (2018). School is out on noisy reefs: the effect of boat noise on predator learning and survival of juvenile coral reef fishes. Proceedings of the Royal Society of London B 285:20180033