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Fishery Bulletin 108(1) 
a TCD-D8 digital audio tape (DAT) recorder (Sony 
Corp., Tokyo, Japan), and a personal computer. The 
hydrophone was omnidirectional and had a frequency 
range of 2 Hz to 30 kHz and a sensitivity of -164 dB 
re lV/pPa (pPa=micropascal). The DAT recorder oper- 
ated at a sampling rate of 44.1 kHz, with a frequency 
range of 20 Hz to 22 kHz (±1 dB). Data from the DAT 
recorder were downloaded to a personal computer run- 
ning CoolEdit 2000 (Syntrillium Software, Inc., Scott- 
sdale, AZ). The recording levels for the DAT recorder 
and computer were standardized for all recordings. The 
entire recording system (hydrophone, DAT recorder, 
and computer) was calibrated by recording a series 
of reference tones and using linear regression to de- 
termine how relative root mean squared (RMS) levels 
related to absolute RMS levels (r 2 = 0.99; Gannon et 
al., 2005). 
To minimize movement of the hydrophone through 
the water (thus minimizing hydrodynamic noise), the 
hydrophone was suspended approximately halfway be- 
tween the surface and seafloor from a 4-kg mushroom 
anchor that was suspended 50 cm below a 30-cm diam- 
eter buoy by an elastic shock cord. Immediately after 
making each two-minute recording, we deployed the 
trawl net. We used a 3.4-meter otter trawl with 3.8-cm 
mesh and a 3-mm mesh liner in the codend. We trawled 
in a straight line, parallel to the bottom contour, for 
four minutes at a speed of 1.3 m/sec, which resulted in 
trawl distances of approximately 300 m. The locations 
of each recording site and of the beginning and ending 
of each trawl were recorded with a Garmin GPS120 and 
a Garmin GBR21 differential beacon receiver (Garmin, 
George Town, Cayman Islands). The exact length of 
each trawl was calculated from the beginning and end- 
ing positions of the trawls. 
Three hundred meters was chosen as the trawl length 
based on estimates of the maximum range over which 
we would likely be able to detect calling Atlantic croak- 
er (-150 m). Maximum detection range depends upon 
the source level of the sound, the level of background 
noise at the same frequency, and characteristics of the 
transmitting medium (Pierce, 1989; Richardson et al., 
1995). We lacked information on these parameters; 
therefore we used the theoretical range at which a sig- 
nal would be diminished by 30 dB re 1 pPa because of 
spreading loss as an estimate of the range over which 
a sound could be detected (Gannon, 2003). 
All fish and invertebrates captured were identified, 
counted, and measured (standard length). Catch per 
unit of effort (CPUE) was calculated as the number of 
Atlantic croaker >45 mm standard length that were 
caught per 100 meters of linear trawling distance. 
CPUE was calculated for Atlantic croaker >45 mm 
because this is the size at which they develop sonic 
muscles and thus become capable of producing sound 
(Hill et al., 1987; Gannon, 2007). At each sampling site 
we also recorded temperature, salinity, and dissolved 
oxygen concentration at 0.2 m above the bottom, using 
a Minisonde multiprobe and Surveyor 4 data logger 
(Hydrolab, Hach Environmental, Loveland, TX). 
Acoustical analyses 
Acoustical analyses were performed with CoolEdit 2000 
and Raven 1.1 (Cornell Univ., Ithaca, NY). We discrimi- 
nated Atlantic croaker sounds from other sounds in our 
field recordings by the methods outlined in Gannon 
(2007). Briefly, we compared spectrographs from our field 
recordings to spectrographs of recordings from Atlantic 
croaker made in captivity. We also compared our field 
recordings to published descriptions of calls of all other 
known soniferous species encountered in the estuary 
(e.g., Fish and Mowbray, 1970; Mok and Gilmore, 1983; 
Sprague and Luczkovich, 2001). We excluded record- 
ings containing sounds of boats or rain, those in which 
the Beaufort sea state was >3, and those made when 
bottlenose dolphins (Tursiops truncatus ) were present, 
because these factors may have affected rates of sound 
production by Atlantic croaker (see Luczkovich et al., 
2000) or our ability to detect sounds. 
We investigated how trawl CPUE and environmental 
conditions related to three acoustic parameters: 1) re- 
ceived sound level (in dB re 1 pPa), 2) rate of detection 
of Atlantic croaker calls by our hydrophone (expressed 
as a calling index), and 3) peak acoustic frequency of 
Atlantic croaker calls (the loudest frequency recorded). 
We hypothesized that received sound level and calling 
index should be influenced by the number of fish calling. 
Environmental factors were included in the analysis 
because of their potential to also affect the acoustic 
parameters. For example, temperature is known to af- 
fect the frequency characteristics of sciaenid calls (Con- 
naughton et al., 2000), and the characteristics of the 
transmission medium can cause different frequency 
components of a call to differ in their propagation ef- 
ficiencies (Richardson et al., 1995, p. 27-30). 
The Atlantic croaker sounds that we recorded exhib- 
ited energy peaks between 600 and 1200 Hz (Gannon, 
2007). Besides Atlantic croaker calls, there were few 
background sounds in this frequency range. But back- 
ground noise was common at other frequencies. There- 
fore, we calculated the received sound level attributable 
to Atlantic croaker as the level within the 600 to 1200 
Hz band, using an FFT (Fast Fourier Transform) size 
of 2048. The received sound level was calculated over 
the entire two-minute recording period from each sam- 
pling station. 
Calling rate reached a saturation point at approxi- 
mately 100 calls/min, above which the exact number of 
fish calls per minute could not be determined. There- 
fore, we used a calling index to quantify the occurrence 
of Atlantic croaker calls, following Heyer et al. (1994), 
Luczkovich et al. (1999), and Gannon (2007). The call- 
ing index was on a scale from zero to three (0=no calls, 
1=1-10 calls/min, 2=10-100 calls/min, and 3=greater 
than 100 calls/min). 
Statistical analyses 
Before we performed statistical analyses, we transformed 
CPUE values by ln(x+l), which gave a better fit to a 
