A new study into the effect of motorboat noise on the predatory dynamics of aquatic ecosystems has revealed that some species of fish may become stressed out by the sounds produced by these vessels, making them more vulnerable to predators.
To come to this conclusion, a team of researchers set up a series of experiments at the Lizard Island Research Station in the Great Barrier Reef, using ambon damselfish and their natural predator, the dusky dottyback. Publishing their findings in Nature Communications, the study authors suggest that the ability of the damselfish to react to the presence of the dottybacks was severely hindered by motor noise, resulting in a significant increase in the number that were caught.
First, the team sought to determine how this noise may or may not affect their subjects’ stress levels. To investigate this, they placed each damselfish in a container of water and measured the concentration of dissolved oxygen. The fish were then subjected to 30 minutes of motorboat noise, after which the oxygen levels were recorded once more.
Results showed that the damselfish used 33 percent more oxygen after the experiment, suggesting a significant increase in their metabolic rate, thereby indicating high stress levels.
To discern the effect this had on their anti-predation behavior, the researchers conducted another test in which an artificial predator – in the form of a black disk – was thrust towards the fish, and their reaction time measured. Damselfish who had been subjected to motorboat noise were six times less likely to become startled by the presence of this disk than those who were not.
Furthermore, of those that did become startled, this reaction tended to occur 22 percent slower when motorboat noise was present, allowing the "predator" to get much closer before any evasive action was taken.
Taking their study a step further, the researchers then sought to investigate the real-world effect of man-made noise on predation rates in the field, by counting the number of damselfish that were actually killed by dusky dottybacks when motorboat noise was present or absent.
Over a 72-hour observation period, their findings indicate that 2.4 times as many fish were caught by the predators in noisy as opposed to ambient conditions. Additionally, the researchers noted that the dottybacks required 74 percent fewer attempts to catch their prey in the presence of motorboats.
Commenting on these results, study co-author Dr. Stephen Simpson said that humans must now do more to limit the amount of noise pollution they generate in marine environments in order to avoid upsetting their delicate balance. “For example, we could create marine quiet zones or buffer zones, and avoid known sensitive areas or times of year when juveniles are abundant,” he suggests.