Locascio and Burton: A passive acoustic survey of fish sound production at Riley's Hump 
113 
Bullock, 1998). Another possible explanation is that 
sound production is associated with other forms of be- 
havior besides courtship and spawning. For example, 
red grouper excavate and maintain pits in the sedi- 
ment that are used by other species, and red grouper 
are believed to have strong site fidelity to these engi- 
neered features (Coleman and Koenig, 2010; Wall et. 
al, 2011). Given these circumstances, sound production 
may be used in other social contexts, such as territorial 
or agonistic interactions. 
The calls recorded in our study that were attributed 
to black grouper are consistent with the stereotypical 
characteristics of calls by groupers (low frequency, mod- 
ulated, and long duration) and agree with descriptions 
of sound production of black grouper from recordings 
made in Puerto Rico (Scharer, et al., 2013). Although 
apparent courtship or territorial behavior was followed 
closely by a BGV2 call in only one video segment, the 
black grouper was the only grouper species appearing 
in the video recorded at or near the time that a call 
of either variation was produced, strongly indicating 
that this species was the source. Additionally, for black 
grouper, the relatively high received RMS SPLs of the 
call variations indicate that the source was close to 
the hydrophone, and, in these cases, black grouper ap- 
peared in the video within a few seconds of these call 
types. A source level (i.e., decibels of SPL at 1 m from 
source) can be roughly estimated with a spreading loss 
model (Urick, 1983) and a received SPL of a call. In our 
study, a spherical model that estimates a 6-dB loss per 
distance doubling was used with the highest received 
SPL of 149.9 dB RMS SPL (re: 1 pPa), which was re- 
corded when black grouper appeared to be only a few 
meters from the A/V system. Adding 6 dB to the high- 
est received level is equivalent to 1 distance doubling, 
placing the source 2 m away from the A/V system and 
resulting in an estimated source level of 155.9 dB RMS 
SPL (re: 1 pPa). Adding 12 dB to the highest received 
level would be equivalent to 2 distance doublings and 
would place the source 4 m away from the A/V sys- 
tem with an estimated source level of 161.9 dB RMS 
SPL (re: 1 pPa), and so on. For reference, estimates of 
source levels produced by black drum ( Pogonias cromis) 
are 165 dB RMS SPL (re: 1 pPa) (Locascio and Mann, 
2011b). Extrapolated estimates of source levels for 
black grouper that were calculated with the spherical 
spreading loss model seem reasonable in the context of 
source levels reported for black drum. 
Calls of black grouper contain a frequency-modulat- 
ed feature, a mechanism associated with sound produc- 
tion that speeds up and slows down over the duration 
of a call, but the structure of calls of black grouper are 
unique from the calls of red grouper, yellowfin grou- 
per, and red hind in that they do not feature a long 
frequency down-sweep. The BGV2 call was differenti- 
ated from the BGV1 call by a series of initial pulses. 
This difference also exists between call types of the red 
grouper (Nelson et al., 2011), but the significance of 
those differences is not understood for either species. 
Sounds of most grouper species documented thus far, 
with the exception of the call of the Atlantic goliath 
grouper, give evidence of a complex structure relative 
to the more common pulsatile structure of many fish 
calls. The mechanisms associated with the sound pro- 
duction of groupers have been reported only in general 
terms as bilateral muscles that work in conjunction 
with the swim bladder (Hazlett and Winn, 1962). A 
more detailed analysis of the mechanisms and process- 
es responsible for sound production of groupers and the 
associated behavioral context is warranted given the 
unusual modulated tones. 
Estimates of acoustic communication ranges for fish 
require data on source levels and hearing thresholds, 
along with site-specific information on loss of signal 
transmission and on background levels. Locascio and 
Mann (2011b) estimated that the acoustic communica- 
tion range of black drum was 33-108 m on the basis of 
direct measurements of each of these parameters and 
found that the range for this species was limited by 
background levels rather than by hearing thresholds. 
Data for the complete suite of these parameters do 
not exist for any grouper species; however, based on 
the highest received levels of 142.0 dB SPL (re: IpPa) 
reported by Nelson et al. (2011) for red grouper and 
of 149.9 dB SPL (re: IpPa) recorded in our study for 
black grouper, a reasonable estimate of communication 
ranges of groupers in a noisy reef habitat would be on 
the order of tens of meters. 
Spawning sites of black grouper are not well known 
in the United States. Only 2 probable spawning sites 
have been documented in the literature, one in the 
Florida Keys Marine Sanctuary (Eklund et al., 2000) 
and the other at Mona Island, Puerto Rico (Scharer 
et al., 2013). In addition to demonstrating that Riley’s 
Hump is also a likely spawning site for black grouper, 
the information generated in this study can be used to 
help document other spawning aggregation sites and 
provide opportunities to learn more about the role of 
sound production by this species. 
The source of sounds produced in the range of 
300-400 Hz was verified as longspine squirrelfish by 
comparing descriptions of the call of this species made 
by Winn et al. (1964) and that made in our study 
with a handheld A/V system. Recordings were made 
as longspine squirrelfish emitted sounds just before 
retreating into their den after they were approached 
by a diver. This agonistic behavior is associated with 
sound production by this species. This behavioral con- 
text, together with this species’ strong site fidelity to 
dens (Menard, et. al, 2008) and its nocturnal behav- 
ior, may explain the crepuscular pattern observed in 
sound production by this species in our study. Other 
less commonly observed holocentrid species that oc- 
cupy Riley’s Hump s include the longjaw squirrelfish 
( Neoniphon marianus) and blackbar soldierfish ( Myri - 
pristis jacobus). Sound production by these species has 
not yet been documented in the field, but on the basis 
of their taxonomy it is likely that these species do pro- 
duce sounds. 
Acoustic signals generated in the range of 500-800 
