18 
PACIFIC SCIENCE, VoL XXII, January 1968 
vetula near coral ledges in Bimini were observed 
to chase approaching conspecifics from the area. 
The aggressive fish changed in body coloration 
from a light to a dark brown, while the oper- 
cular area became bright yellow. Pectoral fin 
drumming sounds were produced by one of the 
aggressive fish during chasing. 
Many pectoral fin sounds were produced by 
B. vetula and M. piceus in the fish pen at Bim- 
ini. In three hours of recordings, sounds were 
produced at least once a minute. However, the 
large number of fish present made impossible 
the identification of the individuals involved. 
On four occasions, loud sounds were correlated 
with rapid chasing of one M. piceus by another 
across the length of the pen. In most recordings, 
presumably of sounds from both species, several 
sounds were produced in a series for up to a 
9-second period. In a few cases, the sounds 
consisted of a single, loud burst. 
Individuals of M. piceus in the circular con- 
crete tank remained in the shelters for several 
days and only occasionally ventured into the 
open central area. This behavior was not com- 
parable with that of conspecific fish in the pen 
or the field, and so only B. vetula was used for 
further observations. These fish adapted to the 
tank very quickly, i.e., they accepted food (fresh 
conch), produced sounds, and swam about the 
whole area within one hour after populations 
were established. 
The most intense aggressive behavior occurred 
within the first 2-3 hours after the fish were 
placed in the tank. One fish in each of the two 
groups was dominant over all others. Chasing, 
sometimes accompanied by pectoral fin sounds, 
characterized the behavior of all aggressive fish. 
Submissive fish, usually smaller individuals, 
were dark or light brown in color. Most aggres- 
sive fish usually had bright yellow opercula. 
A total of 134 aggressive interactions were 
observed, consisting of chasing of a conspecific 
by an aggressive fish. In 44 observations, the 
aggressive fish produced sounds by pectoral fin 
drumming just before or during the chase. 
Sounds were also produced during other less 
frequent interactions, i.e., when two fish circled 
slowly around one another for a few seconds 
(sounds produced by both fish in three observa- 
tions) ; when one fish, after producing sounds, 
displaced a second fish from a shelter (12 ob- 
servations) ; and when resident fish inside their 
shelters produced sounds in response to intrud- 
ing fish (12 observations). 
DISCUSSION 
Pectoral fin movements against the drumming 
membrane appear to be responsible for produc- 
tion of sounds in three species of trigger fishes, 
representing three different genera. The data 
indicate that movements of the stiff pectoral fin 
spine across the drumming membrane contribute 
to most of the resultant sound pressure. The 
system must be extremely efficient, as removal 
of one spine or one pectoral fin lowered sound 
pressures no more than 2 db in any fish. The 
sounds of most normal fish peaked 22-25 db 
above background levels in the recording tanks, 
but we did not measure the absolute level of 
sound pressure in the water. Removal of both 
spines or most of the gas from the air bladder 
reduced intensities about 15-20 db. A very low 
level of sound remained when only the basal 
lobe of the fins was left intact. The acoustic 
energy released may represent muscle contraction 
sounds, or the sound of the base striking the 
side of the body. 
Removing the gas from the air bladder re- 
sulted in a reduction in sound intensities, but 
it was noted that the operation had little effect 
on the frequencies contained in the sounds. In 
all species, the air bladder did not appear to 
emphasize any particular frequencies, and hence 
it probably does not function as a resonator. It 
may be that the air bladder acts chiefly as an 
efficient sound coupler to the water medium. 
When the fleshy muscular lobe was held 
against the side of the body, or was removed, 
no detectable sounds were produced. Although 
no control was carried out for this specific opera- 
tion, the data indicate that an intrinsic mech- 
anism is not involved in sound production when 
fish of these species are hand-held underwater. 
It is possible, of course, that sounds may be 
produced by means of intrinsic mechanisms 
under other conditions. 
Unilateral removal of the spine or rays did 
not affect duration of sounds in M. piceus or 
B. vetula. The results support the conclusion 
that the pectoral fins move synchronously on 
either side of the body during sound production. 
