All of a sudden, lots of people in Europe decided that they wanted to know everything about underwater sounds and animal behaviour. In 2008, new European regulations were introduced, stating that the North Sea should have a “good environmental status” by 2020.
“Since then, everyone has been trying hard to discover what that means”, says behavioural biologist Hans Slabbekoorn with a smile. One of the criteria for the status is noise: if you want to make a racket in the North Sea, you need to find out whether it will do any harm, and if so, compensate for it.
All this is good news for Slabbekoorn as he has been studying fish and sounds for years. And there is plenty of noise in the sea – noise that may or may not be harmful: boats’ engines, explosions for seismic research, wind turbines and piles driven into the bottom of the sea for offshore platforms. “Those piles are eight metres wide, so a bit larger than the ones they slap into dry land, and the sound of the pile-driving resonates throughout the whole North Sea”, the biologist claims.
Slabbekoorn, Research Institute Imares and TNO have formed a partnership funded by NWO (Netherlands Organization for Scientific Research) to assess the consequences of all that noise – not an easy task.
“We know that if a sound is loud enough, it can be lethal.” Although, surprisingly, scientific research into methods for killing people by sound is still quite new, we know that the fatal volume for humans is approximately 200 decibels.
It’s suspected that the limit for fish and acoustically sensitive mammals such as dolphins could be lower, but that’s not Slabbekoorn wants to find out.
“People are willing to believe that noise can have serious consequences when whales are beached or when dead fish come floating to the surface, but far more moderate levels of sound can be detrimental too. On average, the entire North Sea has become noisier and that is impacting all sorts of animals. They use sound to catch their prey, avoid being caught by predators or to attract a partner. Potentially, there could be any number of effects. Just think: if we cause fish to scatter during the mating season and disturb the reproduction cycle of a cohort, we’d only realise something had happened much later when the fish stocks dwindle. If we can find out where and when we can create a din, it could make a difference.”
Accordingly, Slabbekoorn and his colleagues are doing various experiments to discover the effects of non-lethal sound. They play sounds to cichlids and zebra fish in aquariums and in Zeeland, the researchers are working with bass in larger pools. “For instance, the zebra fish have more difficulty catching water fleas if their surroundings are noisy, which is very strange as they don’t need their ears to hunt. On the other hand, I switch off my radio when I’m reversing into a parking space.”
In a recent article in the scientific journal Biological Conservation, Slabbekoorn’s PhD student Errol Neo describes an experiment with bass in a pool: after forming groups of four fish, he played a sound to them. The video footage of their reactions was then compared to the behaviour of fish that had not been disturbed. When the bass heard the row, they would swim closer together and move to deeper parts of the tank. The scientist was struck by the discovery that different sounds had different effects: following a continuous noise, the fish would resume swimming “normally” sooner that they would after an interrupted noise. The boom-boom-boom of a pile driver evidently had more impact that the whine of a passing container ship.
“Obviously, that’s not what the policy-makers want to hear”, continues Slabbekoorn. “They’re hoping for a straightforward answer: this number of decibels is too much. But it’s not just the volume that’s relevant; the nature of the sound has an effect too. Tolerance has a part too and as the creatures grow accustomed to noise, the damage might accumulate, just as we cause more damage to our ears the longer and the nearer the front we are at a concert.
“Besides, we’ve observed behavioural responses of some species in a certain season that do not occur in the next. Chronic stress could affect the reproduction success rates – we haven’t observed the effects of exposure to noise on that yet, but we expect there will be a connection.”
To make the research even trickier, below the surface, sound does not act in the same way it does above and a reaction in an aquarium could be influenced by its specific situation.
“At present, Neo is carrying out experiments on a floating island with a large cage made of fishing nets underneath. We’ve observed more or less the same responses, but actually I would like to repeat the experiments in real open water.”
Is there any point to worrying about the effects of underwater noise on the behaviour of fish when whole shoals of the creatures are caught at once?
“Compared to the impacts of trawling, the effects of sound on fish populations aren’t so bad, I think. But on the other hand: fishing has a purpose, because fishermen earn a living from it and many people like to eat fish. Noise is merely a by-product of what we do at sea, not the object. Perhaps we could screw piles into the ground, for instance. We hope that our study produces the information we need to make sensible assessments for planning and permitting activities that cause noise.”