The noise of motorboats over the Great Barrier Reef can stress its inhabitants, stunting the growth of young fish and leaving them less likely to live to adulthood
Some young fish that live in coral reefs exposed to motorboat sounds have stunted growth and may be half as likely to survive as fish on quieter reefs, probably because the noise pollution changes the way their parents care for them.
Spiny chromis (Acanthochromis polyacanthus) are fish that fan water over their eggs, creating streams of inflowing oxygen that helps the embryos grow. But in reefs with motorboat noise, the parents fan their eggs less and seem more agitated – swimming around more and possibly exposing their hatchlings to more predator attacks – than those hearing only minor motor noises or none at all.
“Any kind of unexpected noise can cause a rise in the stress response. And I think that’s what’s going on here with the parenting behaviour,” says Sophie Nedelec at the University of Exeter in the UK.
She and her colleagues snorkelled every other day along the Great Barrier Reef in Australia to observe and photograph wild spiny chromis nests throughout the breeding season from October 2017 to January 2018. The team tagged 59 nests, with an average of 126 eggs per nest, in six experimental sites off the coast of Lizard Island, Australia, and counted surviving offspring every four days.
Three of the sites were “limited boating” zones, in which the scientists requested that motorboat drivers keep at least 100 metres away from reefs or, if necessary, get no closer than 20 metres away with no wake. The three other sites were “busy-boating” zones, in which the research team drove outboard engine aluminium motorboats – usually at full throttle – within 10 to 30 metres of the reef’s edge. The team made these passes in the boats about 180 times a day, totalling about 75 to 90 minutes, to mimic a port harbour or popular tourist or fishing areas.
They found that breeding pairs in the limited boating sites were twice as likely to still have living offspring by the end of the three-month breeding season, she says.
To better understand this finding, the team captured spiny chromis adults to pair them in a laboratory where the scientists could study their parenting behaviour. They played natural coral reef sound recordings through loud speakers for 12 hours a day for 13 fish pairs; for another nine pairs, the recordings included 100 minutes of boating noise recorded from the reef and spliced intermittently into the audio in 20-minute segments.
Boat noise didn’t change the number of eggs the fish laid, says Nedelec. However, in the laboratory, the scientists noted that adults frequently stopped fanning their eggs when they heard motorboat recordings, and they didn’t fan more during quiet periods to make up for that. These fish also became more active, swimming greater distances – including away from the nest – during playback of motorboat recordings, compared with the adults that heard recordings of normal reef sounds. “This could be a sign of stress,” she says.
Embryos in the limited boating conditions were 1 per cent longer than those in busy boating conditions, and 21-day-old hatchlings were 4 per cent longer. Smaller offspring might be more susceptible to predators in the wild, says Nedelec.
The change in parenting behaviour also affected survival of the hatchlings, she says. In the laboratory, the chances of the young fish surviving to 3 weeks old almost doubled without boat noise.
These findings suggest that fishermen, tourists, recreational boaters, cruise operators and even researchers could help reef-nesting fish populations recover from hurricanes and heat waves simply by slowing down their boats or, better yet, avoiding reefs altogether, says Nedelec.
“Coral reefs go through these intense climate shocks, hit by bleaching or cyclones for example, and the populations that live there have to regenerate,” she says. “This an exciting solution. It can’t replace action on climate change, and it won’t save coral reefs. But it could potentially support their resilience.”
Journal reference: Nature Communications, DOI: 10.1038/s41467-022-30332-5
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