To building windmill parks at sea, innumerable foundation poles must be pounded into the seafloor. Fish in a large radius of this marine construction site are thus exposed to strong sound waves. Because of the intensity of the sound and the fact that most juvenile fish do not swim away from danger very quickly, it has been assumed that certain segments of the fish population near the places where poles are being hammered into the seafloor can experience negative or even deadly effects. A series of carefully-executed experiments done by ILVO (the Institute for Agricultural and Fisheries Research) in collaboration with Ghent University and the Royal Institute for Natural Sciences has proven that even when they are close to the construction areas, no seabass died from the sound waves.
Underwater sound and need for a field experiment
The strength of the underwater sound close to a foundation pole being pounded into the seafloor is more than 200 decibels. Sound carries differently in water than in air, but even so, the strength of this underwater sound led scientists to think that it could have deleterious effects on sea creatures, including fish.
Prior to this field experiment, research on the effects of the pounding noise on (young) fish was based on mathematical models and experiments done in specially-designed acoustic rooms where the hammering noise could be emulated. The results of several of these studies already pointed in the direction of limited effects of the hammering sound on fish health.
This is the first time that the lab results have been confirmed via field experiments. With the collaboration of the construction companies Northwind NV and GeoSea NV (the companies now building the windmill park on the Lodewijckbank in the Belgian Part of the North Sea), the researchers were able to perform unique field experiments on the building platforms during the summer of 2013, under the supervision of an ethical committee.
Experimental setup and results
The species selected was seabass, a commercial species with a swim bladder (unlike the commercially-important flatfish such as sole, plaice or dab). Having a swim bladder or not determines the sensitivity of the fish for changes in pressure such as those created by sound waves.
During the experiments, young fish (seabass between 60 and 120 days old) were put in a metal cage with water sacks and sound detection equipment, then lowered from the construction platform down to 2.5 metres under the surface only 45 metres from a pole that was then pounded into the seabed. This procedure was repeated near four independent poles, with different batches of fish. This procedure exposed the fish to more than a thousand peaks of the hammering sound. The sound measurements showed that the sound peaks exposed the fish to a sound pressure of 210 dB at frequencies of 125 to 200 Hertz. Between bouts of hammering, a comparison was made with groups of young seabass left for the same amount of time in the same place but without the noise. Comparison with the exposed and non-exposed groups showed no difference in mortality immediately after the experiment nor after two weeks of monitoring in tanks. The researchers concluded that even in the worst-case scenario - close to the hammering site with the most sound-sensitive fish (and sensitive age) - the impact of sound is not as serious as initially imagined.
This research on mortal effects is only a first but important step. Next the researchers will refine their examination to injury, changes in behavior and observations of stress in fish.
The results of this experiment at sea greatly confirmed what the researchers already expected via the lab experiments with simulated sound. (Less expensive) experiments in such sound chambers can thus be deemed reliable for future experiments. This is important for future studies of the effect of sound at sea. The field experiments such as performed here by ILVO were logistically and financially challenging.
ILVO’s opinion is that the new knowledge of the effects of underwater sound can function to refine the regulation related to animal welfare and the protection of, and continued existence, of fish populations (and the related fisheries). Advice can be formulated about norms for underwater noise in the European Framework for Guidelines on Marine Strategy. These norms indicate which level of underwater noise is dangerous for wildlife and thus when regulations are needed to either minimise damage or ban certain activities or construction techniques.
A scientific article has been published about this experiment and the results of measuring the sound impact of the windmill park:
Greet Riebbels, ILVO Communication, Greet.Riebbels@ilvo.vlaanderen.be, tel. +32 9 272 25 05 GSM +32 486 26 00 14
Elisabeth Debusschere, researcher at ILVO – Elisabeth.Debusschere@ilvo.vlaanderen.be, tel +32 59 56 98 63
Sofie Vandendriessche, researcher at ILVO – Sofie.Vandendriessche@ilvo.vlaanderen.be, tel +32 9 272 25 28
Kris Hostens, Research Group Leader at ILVO - Kris.Hostens@ilvo.vlaanderen.be, tel +32 59 56 98 48
Steven Degraer – Steven.Degraer@mumm.ac.be, tel +32 27 73 21 03