Are noisy reefs causing fish to be bad fathers?

Written by Danie Barnes

Edited by Bobbie Renfro

If you’ve ever been snorkeling or diving on a coral reef offshore, chances are you’ve taken a motorboat to get there. Zooming from reef to reef on a boat seems like a fun and efficient way to navigate the reefs, but have you stopped to consider the potential consequences that boating noise can have on the animals below the waves?

Motorboat use for fishing, recreation, and tourism is extremely common and increasing in prevalence. An estimated 500,000 recreational motorboats will be using the Great Barrier Reef by 2040. Boat motors generate thunderous noises that negatively affect the behavior and physiology of many marine creatures including whales, invertebrates, and fish.

Little is known, however, on how noise pollution can impact reproductive behaviors such as parental care. In many reef fish, parental care is necessary to protect vulnerable and tasty eggs on a ruthless reef. Any interference of this attention could cost their babies their lives. A team of researchers led by Kieran McCloskey and Katherine Chapman recently conducted a study on the impact of motorboat noise pollution on parental care of a reef fish on the Great Barrier Reef.

In their experiments, they looked at the behavior of an abundant reef fish found throughout the Indo-Pacific, the Ambon damselfish. Females of this species lay their eggs on a nesting structure on the ocean floor and quickly flee the scene, leaving the entirety of the parental duties to the males. Fathers will fiercely guard their eggs by warding off potential predators until the eggs hatch and the larval fish swim away.

The team of researchers created 120 artificial nests made of overturned PVC pipe halves and placed them around the reefs in the shallow lagoon off of Lizard Island on the Great Barrier Reef. Each nest was adopted by an individual male Ambon damselfish and guarded for the rest of the breeding season.

Unlike previous studies on this topic that used motorboat-noise playback from an underwater speaker, their experiment is the first to test the impact of the noise from the actual motorboat. Audio playback from artificial sources misses out on different components of sound from real noises.

In their first experiment, they tested the effect of motorboat noise on parental care behaviors occurring inside the nest. They compared behavior on a quiet reef where no boats were overhead with behavior on a noisy reef where a motorboat was being continuously driven at full speed above the nests. They recorded the fish’s reactions underwater on video.

They discovered that with motorboat noise, the fish spent more time being stationary in the nest entrance and looking out, which is considered a vigilance behavior. The males may exhibit this change in behavior because they are trying to rely on the visual cues since their hearing ability has been interrupted. The acoustic information that they would have otherwise received to warn them of an existence of a threat is now being covered up by the loud roar of the motorboat. Alternatively, it is possible that the males may be attempting to identify the source of the noise if they are threatened by the loud noise.

They also found that in the presence of the motorboat noise the males spent less time fanning their eggs. Fanning is an important task, because it ventilates the eggs and supplies oxygen vital to their development and survival. This result suggests that their increased vigilance may come at a cost to their other parental care behaviors.

Figure 1. Diagram detailing the main findings of this study. The panel on the top left describes the role of male Ambon damselfish in parental care. The panel on the top right describes how unmitigated noise from motorboats disrupts this parental care. The bottom panel describes how in mitigated noise treatments (when the motorboat was further away or slower), the male Ambon damselfish exhibits healthy defensive behaviors to protect their nest.

In their second experiment, they looked at the impact of motorboat noise on the male Ambon damselfish’s response to intruder fish introduced outside the nest. Again, they compared behaviors in a quiet reef to a noisy one in which the motorboat was driven directly overhead at full speed, but this time they additionally compared it to a third scenario of a ‘mitigated boating regiment’ in which they drove the motorboat outside of an acoustic buffer zone that extended 20 meters beyond the edge of the reef. They drove the boat at quarter speed in the first zone (20 meter buffer to 120 meters from the reef edge) and they drove at full speed in the second zone (beyond the 120 meters).

When the motorboat noise was present, males spent more time seeking refuge, including both inside the nest and outside the nest under coral cover. They devoted less time to interacting with the intruder. This behavior may leave the eggs vulnerable to predation.

The mitigated motorboat treatment, on the other hand, was shown to lower the noise-exposure levels that the nesting males were experiencing and diminish the effects of the motorboat noise on these defensive behaviors, showing the same results as if the boats were not there.

Human-generated noise in the ocean, not only from motorboat noise but also from energy production, resource extraction, and construction, has been a subject of concern for policymakers recently. This study provides further incentive to change how we use motorboats around coral reefs, as the noise negatively impacts parental care of reef fishes. This study also showed a promising result: a simple mitigation plan was effective in eliminating the effects of motorboat noise.

The authors conclude their paper by advocating for the alteration of driving practices around coral reefs. Spatial management strategies with speed restrictions near these vulnerable areas could be extremely effective. This plan may be more reasonable than the installation of quieter engines onto boats, which can be quite expensive.

This study, along with a growing body of literature, provides evidence for the harmful impacts of motorboat noise on reef-dwelling organisms. Behaviors such as parental care are vital to the success of populations in the long term. Management solutions that reduce these effects need to be considered by lawmakers and implemented in the near future. The next time you are boating around a reef, consider how your motor noise may be turning fish into fickle fathers and maybe slow down and give the reef some space.

Citations

McCloskey, K. P., Chapman, K. E., Chapuis, L., McCormick, M. I., Radford, A. N., & Simpson, S. D. (2020). Assessing and mitigating impacts of motorboat noise on nesting damselfish. Environmental Pollution266, 115376.

Cover Photo Credit

BBC (https://www.bbc.co.uk/programmes/articles/29MzQ4DX205F3M3cBPs50d2/recording-sound-in-a-not-so-silent-world)

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