84
with acoustic noise from abiotic and biotic sources that spans the frequency range
of butterfl yfi sh sounds and their auditory sensitivity.
6.1 Sources of Ambient Noise
Low frequency background noise (<10–100 Hz) originates primarily from abiotic
sources such as wind, waves, and tidal streaming (Wenz 1962 ; Urick 1983 ). The
complex physical structure of a coral reef also contributes to multiple sources,
forms and features of acoustic noise within the sub-habitats inhabited by different
butterfl yfi sh species. For example, wind-driven ocean swells with surface chop pass
over the outer reef where planktivorous butterfl yfi shes feed in the water column during daylight hours, and where corallivores and other benthic invertebrate feeding
butterfl yfi sh species forage in long-term territories and in larger home ranges on the
bottom. Onshore swells break onto the shallow outer reef crest and impact the substrate with severe broadband acoustic noise, substrate vibrations, and strong low
frequency hydrodynamic turbulence that can affect resident butterfl yfi sh species
that inhabit shallower areas of the reef. Wave action and surge then fl ow over the
reef fl at into back reef lagoons where other butterfl yfi sh species have long-term
home ranges. Thus, ambient noise (from both hydrodynamic and sound pressure
sources) is expected to vary considerably among different habitats and will have
differential effects on the perception of sounds produced for social communication.
In addition, ambient noise at the higher end of the hearing range of butterfl yfi shes
(e.g., 100 to >1000 Hz) is generated by biological sources such as other reef fi shes
(Cato and McCauley 2002 ; McCauley and Cato 2000 ; Tricas and Boyle 2014 ),
snapping shrimp, and other invertebrates (Cato 1978 ; Lammers et al. 2008 ).
Seasonal ambient sounds generated by migratory marine mammals such as the
humpback whale in offshore Hawaiian waters have intense fundamental frequencies
(reviewed in Au and Hastings 2008 ) that are also in the hearing frequency range of
butterfl yfi shes. Thus both abiotic and biotic sources of ambient noise are expected
to vary across time in intensity and spectra among different habitats on a coral reef,
and to have potential effects on butterfl yfi sh acoustic communication.
6.2 Ambient Noise, Sound Production and Hearing
in Butterfl yfi sh Territories
Acoustic recordings on Hawaiian reefs show that the intensity of ambient noise near
the substrate on the outer reef overlaps in the frequency domain with sounds used
for communication by butterfl yfi shes (Tricas and Boyle 2015b ). Field measurements of average ambient noise levels during an afternoon with modest onshore
winds were 10–15 dB higher in shallow water territories of C. multicinctus (at <6 m
T.C. Tricas and J.F. Webb
with acoustic noise from abiotic and biotic sources that spans the frequency range
of butterfl yfi sh sounds and their auditory sensitivity.
6.1 Sources of Ambient Noise
Low frequency background noise (<10–100 Hz) originates primarily from abiotic
sources such as wind, waves, and tidal streaming (Wenz 1962 ; Urick 1983 ). The
complex physical structure of a coral reef also contributes to multiple sources,
forms and features of acoustic noise within the sub-habitats inhabited by different
butterfl yfi sh species. For example, wind-driven ocean swells with surface chop pass
over the outer reef where planktivorous butterfl yfi shes feed in the water column during daylight hours, and where corallivores and other benthic invertebrate feeding
butterfl yfi sh species forage in long-term territories and in larger home ranges on the
bottom. Onshore swells break onto the shallow outer reef crest and impact the substrate with severe broadband acoustic noise, substrate vibrations, and strong low
frequency hydrodynamic turbulence that can affect resident butterfl yfi sh species
that inhabit shallower areas of the reef. Wave action and surge then fl ow over the
reef fl at into back reef lagoons where other butterfl yfi sh species have long-term
home ranges. Thus, ambient noise (from both hydrodynamic and sound pressure
sources) is expected to vary considerably among different habitats and will have
differential effects on the perception of sounds produced for social communication.
In addition, ambient noise at the higher end of the hearing range of butterfl yfi shes
(e.g., 100 to >1000 Hz) is generated by biological sources such as other reef fi shes
(Cato and McCauley 2002 ; McCauley and Cato 2000 ; Tricas and Boyle 2014 ),
snapping shrimp, and other invertebrates (Cato 1978 ; Lammers et al. 2008 ).
Seasonal ambient sounds generated by migratory marine mammals such as the
humpback whale in offshore Hawaiian waters have intense fundamental frequencies
(reviewed in Au and Hastings 2008 ) that are also in the hearing frequency range of
butterfl yfi shes. Thus both abiotic and biotic sources of ambient noise are expected
to vary across time in intensity and spectra among different habitats on a coral reef,
and to have potential effects on butterfl yfi sh acoustic communication.
6.2 Ambient Noise, Sound Production and Hearing
in Butterfl yfi sh Territories
Acoustic recordings on Hawaiian reefs show that the intensity of ambient noise near
the substrate on the outer reef overlaps in the frequency domain with sounds used
for communication by butterfl yfi shes (Tricas and Boyle 2015b ). Field measurements of average ambient noise levels during an afternoon with modest onshore
winds were 10–15 dB higher in shallow water territories of C. multicinctus (at <6 m
T.C. Tricas and J.F. Webb