01 March 2018 - Updated 25 July 2018

Tapping on the Aquarium

Knock on Aquarium

Two researchers carried out an investigation into the noise exposure of one of the aquariums due to the visitors in the Schönbrunn Zoo in Vienna, and found some surprising results.

They hid an underwater microphone in a one cubic metre aquarium to register the effects of visitors tapping on the glass. Helmut Kractochvil and Harald Schwammer investigated this activity for four weeks, counting the taps per 100 visitors between midday on Monday to midday on Friday, and then from midday to Friday to midday on Monday. They then introduced three different signs with the inscriptions: “Tapping on the glass kills the fish”, “Only idiots tap the glass”, and “Please do not tap the glass”.


Recorded taps were higher during the week than on the weekend, which was probably due to the many school classes visiting during the week. There were, on average, 2 taps per 100 visitors. On the busiest days, with roughly 15,000 visitors, this comes to 300 taps on the glass. There were between 2-4 taps for each visitor who knocked on the glass. In one case, there were 72 consecutive taps on the aquarium, which is a serious disturbance to the fish. The same noise level in the air could lead to serious hearing damage over time. And fish have it even worse: since the sound pressure is four times higher under water than it is in the air, a good portion of deaths in public aquarium can be traced back to repeated tapping on the glass.


The vast majority of knocks on the glass are meant to stir an inactive animal or grab its attention. The researchers wanted to find out what the best way of stopping this practice was, so they hung up three different signs for two weeks each. The standard “Please do not tap the glass” sign reduced the number of taps by 72%, while the sign “Tapping on the glass kills the fish” managed an even more impressive 82%. However, there was a reduction in noise disturbance by over 90% with the sign that read: “Only idiots tap the glass”.


The results at hand should be carefully considered in all aquariums, and especially in the planning process for any new builds. Not only do the different signs lend an interesting insight into how the human psyche is best influenced, but it allows us to find ways to completely protect the aquatic animals.

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