Gannon and Gannon: Passive acoustic assessment of soniferous fish density 
107 
Figure 1 
Neuse River study area showing stations (•) where concurrent otter 
trawl and passive acoustic sampling for Atlantic croaker ( Micropogo - 
nias undulatus) took place from June to October of 2000. 
mately 600 to 1,600 Hz (Gannon, 2007). 
Juvenile and mature Atlantic croaker 
are known to produce knock calls in 
the estuarine waters of North Carolina 
from June to November (Gannon, 2007). 
Knock calling occurs throughout the day 
and night, but production of knock calls 
by Atlantic croaker stocked in a research 
pond peaked at night at an average rate 
of 1.0 calls per fish per minute (Gannon, 
2007). Their extensive calls make Atlan- 
tic croaker ideal for a study by passive 
acoustic means. 
We conducted trawl surveys concur- 
rently with passive acoustic surveys for 
young-of-the-year Atlantic croaker in the 
Neuse River estuary in North Carolina 
to assess the utility of passive acoustics 
to quantify temporal and spatial trends 
in the density, habitat selection, and as- 
sociations with environmental variables 
of this species. We used multivariate 
analysis of covariance (MANCOVA) to 
investigate the relationships among call 
parameters, fish density, and environ- 
mental variables; Williamson’s index to 
measure the degree of spatial overlap 
between Atlantic croaker catches and the occurrence 
of their calls; and classification and regression tree 
analysis (CART) to identify the best set of acoustic and 
environmental variables for predicting Atlantic croaker 
distribution. 
Materials and methods 
Field sampling 
From June through October of 2000, we performed 
paired trawl and passive acoustic surveys from a 7-m 
outboard-powered vessel. Estimates of Atlantic croaker 
density derived from the trawl data were used as a 
benchmark against which acoustically derived estimates 
could be compared. The study area was a 300-km 2 region 
of the Neuse River estuary, centered at 35.0°N, 76.6°W. 
The survey design consisted of 16 sampling stations 
arranged along three transects (Fig. 1). In an effort to 
sample the full range of depths and habitat types within 
the study area, we distributed the transects along the 
length of the estuary, and each transect completely 
crossed the river. 
Because sound propagation is affected by environ- 
mental conditions, each sampling station was character- 
ized with regard to its habitat type. The three habitat 
types considered were 1) the tributary creeks and bays 
(“creeks”); 2) the main stem of the Neuse River >3.5 
m in depth (“mid-river”); and 3) the main stem of the 
Neuse River <3.5 m in depth (“river edge”). These cat- 
egories were used because the Neuse River estuary has 
relatively low habitat diversity: there is little submerged 
vegetation; oyster reefs are sparse (Lenihan and Peter- 
son, 1998); and bottom substrate consists of sand, silt, 
and clay. Therefore, habitat diversity depends primarily 
on depth and sediment grain size (which is negatively 
correlated with depth). 
Knock calling by Atlantic croaker peaks at night, but 
the fish produce sound throughout the day (Gannon, 
2007). Pilot studies in the Neuse River estuary showed 
that the diversity of fish species producing calls was 
highest at night, making it difficult to quantify calling 
activity of any one species. Therefore, we sampled dur- 
ing daylight hours (from two hours after sunrise to two 
hours before sunset) to minimize any potential biases 
associated with diel variation in sound production or 
in vulnerability to our sampling net. Because few other 
species in the area call during the day, Atlantic croaker 
sounds dominated the sound field during the day. 
At each sampling station, an acoustic recording was 
made before each trawl, thus each acoustic recording 
was paired with a trawl sample. The recording location 
was at the geographical midpoint of the trawl path so 
that the acoustic recording and trawling would sample 
fish from the same geographic area. The recording site 
was approached on a heading that was perpendicular to 
the trawl path to minimize any potential disturbance 
to the fish. Upon arrival at the recording site, the en- 
gine of the boat was turned off. Recording commenced 
approximately two minutes after the boat arrived on 
station and then a two-minute recording was made. 
Choice of recording length followed the reasoning and 
methods of Mok and Gilmore (1983) and Luczkovich 
et al. (1999). The recording system consisted of an 
HTI 96 hydrophone (High Tech, Inc., Gulfport, MS), 
