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Cod behave differently in noisy environment

Underwater noise from seismic surveys affects the behaviour of Atlantic cod. These are the results of research by Leiden biologists in collaboration with colleagues from Belgium. During such surveys the fish are less active than usual and their circadian rhythm is disrupted; soon after exposure they appear to leave the area more quickly. This could have an effect on the species. Publication in Current Biology.

The days when the seas and oceans were an oasis of calm are long gone. On the contrary: it’s a right racket underwater, with the noise, for instance, of ships’ engines or the construction of wind turbines and oil platforms. Leiden biologists have studied how Atlantic cod responded to one specific noise, that of seismic surveys. This is a method that uses loud bangs to explore the seabed, to find out, for example, whether there are oil or gas deposits.

The research showed that the cod did not flee their habitat en masse during the period of exposure, which lasted 3.5 days. Cod tend to be homebodies, despite an increase in noise. What did seem to be the case was that the fish left soon after the noisy period. Researcher Inge van der Knaap: ‘Based on the existing data you would expect between 20 and 30 percent of the cod to leave the area at a certain point. Our research showed, however, that two weeks after they had been exposed to the noise, a whopping 36 of the 37 fish fitted with radio transmitters had found another home.’

The cod were also less active than normal when exposed to the noise. They are usually fairly active, often making small movements that could indicate that they are hunting and catching food such as small crustaceans, crabs and shrimps. During the seismic survey the number of small movements decreased significantly, and the fish were less active in general. The cod were also less active around sunrise and sunset, the times of day when they usually move and eat most.

The question is what exactly these results mean for the cod as a species. Van der Knaap explains: ‘With this study we have demonstrated that the fish exhibit different behaviour. The question now is whether the increased inactivity also leads to a biologically relevant decrease in foraging behaviour and food intake. Only when you have established this can you say with any certainty that noise undermines growth and reproduction and forms a threat to the fish stocks.’

For the study Van der Knaap caught 37 cod with a line around wind turbines in the North Sea. She then anaesthetised them and placed a radio transmitter in their abdominal cavity. Within an hour they were back swimming with the shoal. Receivers on the seabed enabled Van der Knaap to determine how much each fish moved. She did this for a long period before, during and after the seismic survey. This made it possible to show the effect of the noise on the animals’ behaviour.

Text: Merijn van Nuland
Photos: Inge van der Knaap

Read also: noise pollution affects marine life worldwide

Human noise in and around the oceans stresses marine life and has an impact on marine species and ecosystems by changing the underwater acoustic climate. Leiden biologist Hans Slabbekoorn has called for technical solutions to reduce noise pollution. Science review paper on 4 February.

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