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Fig. 6.7
Acoustic diversity
(Shannon index,
average + standard
deviation) of five habitat
types (pool, runs, runs with
streambed sediments, and
pools) in 12 rivers of the
Swiss Confederation
(Reproduced with
permission from Tonolla
et al.
2010
)
4
3
2
Habitat type
The use of acoustic cues is very important for detecting predators, prey, and
mates and for orientation (coastlines, torrents, water currents). For some fishes the
acoustic signals must be 50 dB higher than background noise to communicate
efficiently.
Twelve European freshwater habitats were selected to measure underwater
background noise by Wysocki et al. (
2007
). In this investigation the linear equiva-
lent sound pressure level (
L
Leq
) was measured where the quieter environment was a
stagnant habitat (lakes and backwaters) with a noise level of 100 dB re 1
Pa (
L
Leq
)
under absence of wind. The majority of noise was concentrated in the lower
frequencies below 500 Hz. The noise in fast-flowing waters was greater than
110 dB with peaks at 135 dB. A large amount of sound energy was concentrated
in a frequency range
μ
1 kHz. Considering the acoustic capacity of fishes, noise was
moderately masking the fishes' hearing in stagnant waters, but in a fast-flowing
habitat this noise strongly affects the hearing capacity of fishes.
Recently Tonolla et al. (
2010
) investigated five common habitat types in
12 rivers of the Swiss Confederation using a couple of submerged hydrophones.
Significant differences in acoustic signature or soundscape were found, confirming
the importance of the aquatic soundscape for environmental characterization as
perceived by aquatic life (Fig.
6.7
). In a preceding experiment Tonolla et al. (
2009
)
measured the noise in a flume, changing hydromorphic drivers such as water
velocity, relative submergence, and flow obstructions. Noise emerged when the
water velocity and submergence ratio of obstacles increased, and this effect resulted
in a higher midrange of frequencies (125 Hz to 2 kHz). Thus, the obstruction of flow
and air bubble formation produce a specific sound signature.
In the inshore Lake Jyv¨sj
¨
rvi (Finland), the most common underwater noise
was that produced by motor vessels (ships, motorboats), especially along the
waterways. Sepp¨nen and Nieminen (
2004
) have measured such noise at
50-100 m from the waterways, recording a noise of 120-140 dB (linear SPL, re
1
>
Pa) where the majority of noise was concentrated at the frequencies 1-5 kHz.
μ
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