Acoustical Behavior of Myripristis berndti — Salmon 
367 
sea water circulated through the chamber at all 
times. Two Pflueger Fish Finders (Enterprise 
Manufacturing Co.), placed 130° apart and po- 
sitioned to face toward the center of the cham- 
ber, were used to detect the movement of the 
fish. The fish finder emits an 800-kc signal as 
a narrow beam across the chamber. The re- 
flected signal is identical to the emitted one 
when no moving object is present and, when 
the signals are compared (heterodyned) in the 
receiver,, they cancel out. Movement of a fish 
past the fish finder shifted the frequency of the 
reflected signal and caused a deflection on the 
chart of a Rustrak event recorder (Model 92). 
The chamber was placed in a small room within 
1 m of a large window, so that the fish was ex- 
posed to normal changes in the daily light cycle. 
For further details concerning the apparatus, 
see Muir et al. (1965). 
A series of ablation experiments was carried 
out to determine the sound-producing mecha- 
nism. All fish produced grunts when hand-held 
by the caudal peduncle. Sounds of normal hand- 
held fish were recorded, followed by recordings 
of the same fish (record level on tape recorder 
left constant) after removal of the following: 
one or both sound-producing muscles; other 
associated muscles and bones; the gas from the 
swim bladder. All fish were held about 7.5 cm 
from the hydrophone. Operated fish were anes- 
thetized with MS-222. A few muscle potentials 
were recorded from the sound producing mus- 
cle of two fish with a Tektronix Low Level 
Amplifier (Type RM-122) and oscilloscope 
(RM-504) and were photographed with a Grass 
camera (Model C-4). The sound duration, 
number of pulses, and interpulse intervals were 
measured by photographing the recorded sounds 
from a Fairchild oscilloscope (Model 701) with 
the Grass camera, at film speeds of 100-500 
mm/sec. 
The effect of operations on the intensity of 
sounds was determined. A General Radio Co. 
Impact-Noise Analyzer (Type 1556-B) was 
connected to the output of the tape recorder 
and a peak sound pressure value was deter- 
mined for a normal fish. The peak sound pres- 
sure of the same fish after the operation was 
also obtained. The peak value for the normal 
sound was considered as 0 decibel, while the 
value for the operated fish was considered as 
positive db (if the value exceeded that of the 
normal fish) or negative db (if the value was 
less). Relative sound pressures at various octave 
band frequences were also measured. The out- 
put of a General Radio Co. Octave Band Noise 
Analyzer (Type 15 58- A) was connected to the 
imput of the impact analyzer. A sine wave of 
400 cps was applied to the imput of the octave 
band analyzer when set in the "all pass” posi- 
tion, and with the preamplifier in the 20 Kcs 
weighting (essentially flat response from 20 cps 
to 40 Kcs). The impact analyzer was then cali- 
brated to give a peak value 3 db higher than 
the root mean square value shown by the octave 
band analyzer for the sine wave. After cali- 
bration, the fish sounds from the tape recorder 
were applied to the imput of the octave band 
analyzer and readings were determined from the 
impact analyzer. The loudest of the first five 
sounds produced by a normal fish was measured 
and considered as 0 db. All sound pressures in 
various octave band frequencies of the first five 
sounds produced before and after operations on 
this fish were compared with the 0 db value. 
The sound pressures of all filtered signals were 
always less than the 0 db value. The reduction 
was measured and expressed in decibels. All 
sound pressures obtained from the impact ana- 
lyzer were relative to 0.0002 microbar. 
RESULTS 
The Sound-Producing Mechanism 
Sounds produced by hand-held specimens 
were accompanied by vibrations which could 
be felt along an area extending from the dorso- 
lateral region of the skull to the side of the 
body just lateral to the air bladder. The most 
intense contractions were in the dorsal region 
behind the eye. Removal of some of the super- 
ficial muscles, opercula, and part of the supra- 
scapular bone revealed a band of muscle slightly 
yellow in appearance, which could be observed 
to contract synchronously with the production 
of sound. The muscle was attached to the pos- 
terior part of the skull, just above the eye, and 
passed over the anterior lobes of the air bladder 
to its insertion point above the area where the 
main body of the air bladder gives rise to the 
lobes (Fig. 2). At its insertion, the muscle was 
attached medially to the first two dorsal ribs and 
