106 
Abstract — Using data collected 
simultaneously from a trawl and a 
hydrophone, we found that temporal 
and spatial trends in densities of juve- 
nile Atlantic croaker ( Micropogonias 
undulatus) in the Neuse River estuary 
in North Carolina can be identified 
by monitoring their sound production. 
Multivariate analysis of covariance 
(MANCOVA) revealed that catch 
per unit of effort (CPUE) of Atlantic 
croaker had a significant relationship 
with the dependent variables of sound 
level and peak frequency of Atlantic 
croaker calls. Tests of between-subject 
correspondence failed to detect rela- 
tionships between CPUE and either 
of the call parameters, but statistical 
power was low. Williamson’s index 
of spatial overlap indicated that 
call detection rate (expressed by a 
0-3 calling index) was correlated in 
time and space with Atlantic croaker 
CPUE. The correspondence between 
acoustic parameters and trawl catch 
rates varied by month and by habitat. 
In general, the calling index had a 
higher degree of overlap with this spe- 
cies’ density than did the received 
sound level of their calls. Classifica- 
tion and regression tree analysis iden- 
tified calling index as the strongest 
correlate of CPUE. Passive acoustics 
has the potential to be an inexpensive 
means of identifying spatial and tem- 
poral trends in abundance for sonifer- 
ous fish species. 
Manuscript submitted 8 February 2009. 
Manuscript accepted 13 November 2009. 
Fish. Bull. 108:106-116 (2009). 
The views and opinions expressed 
or implied in this article are those 
of the author (or authors) and do not 
necessarily reflect the position of the 
National Marine Fisheries Service, 
NOAA. 
Assessing trends in the density of Atlantic croaker 
{Micropogonias undulatus ): a comparison 
of passive acoustic and trawl methods 
Damon P. Gannon (contact author ) 1 
Janet G. Gannon 2 
Email address for contact author: dgannon@bowdoin.edu 
1 Bowdoin Scientific Station 
c/o Biology Department 
Bowdoin College 
6500 College Station 
Brunswick, Maine 04011 
2 Biology Department 
Bowdoin College 
6500 College Station 
Brunswick, Maine 04011 
Many fish species communicate acous- 
tically, producing sounds under a 
variety of conditions, such as when 
engaging in reproductive activities, 
defending territory, competing for 
food, or responding to threats (Ladich, 
1997, 2004; Myrberg, 1997). Fish- 
ery scientists can take advantage of 
sounds produced by fishes to study 
certain aspects of fish biology. Pas- 
sive acoustic techniques (i.e., the use 
of hydrophones to receive and record 
sounds made by fishes) have been 
used by fishery scientists primarily to 
characterize temporal and spatial pat- 
terns in spawning activity (reviewed 
by Gannon, 2008). For example, some 
investigators have coupled passive 
acoustic surveys with collection of 
adult fish or newly fertilized oocytes 
to help confirm the species identifica- 
tions of the callers and to correlate 
certain behaviors (i.e., spawning) with 
specific sound types (e.g., Saucier and 
Baltz, 1993). However, the objective of 
many fishery assessments is to obtain 
some measure of fish abundance. Pas- 
sive acoustics may provide data that 
would allow indices of abundance to 
be calculated. But to our knowledge, 
passive acoustic methods have not 
yet been used in this fashion and no 
studies have formally evaluated their 
potential for assessing relative abun- 
dance. 
Atlantic croaker (Sciaenidae: Micro- 
pogonias undulatus) are an abundant, 
estuary-dependent sciaenid found in 
the coastal waters of the mid-Atlantic 
and southern United States. Off North 
Carolina, Atlantic croaker spawn in 
the ocean from September to Febru- 
ary, and juveniles are found in meso- 
haline estuaries throughout summer 
(see Eby, 2001). Sciaenids, including 
Atlantic croaker, are well known for 
their noise-making abilities. Mem- 
bers of the family produce sound by 
the movement of bilaterally paired 
muscles that surround the swim blad- 
der, which cause the swim bladder to 
vibrate (reviewed by Ramcharitar et 
al., 2006). In Atlantic croaker, both 
males and females possess well-de- 
veloped sonic muscles. Their sonic 
muscles develop when they are ap- 
proximately 45 mm standard length 
(long before sexual maturation com- 
mences), and their sonic muscles do 
not undergo atrophy after spawning 
to the extent seen in other sciaenids 
(Hill et al., 1987; Vance et al., 2002). 
Atlantic croaker produce three types 
of acoustic calls: reproductive calls, 
disturbance calls, and “knock calls” 
(Connaughton et al., 2003; Fine et 
al., 2004; Gannon, 2007). Knock calls 
consist of 1-6 brief pulses (88-106 
msec), or “knocks,” with a mode of 
2 pulses and repetition rate of 16.1 
pulses per second (Gannon, 2007). 
Knock calls are broadband with domi- 
nant frequencies varying inversely 
with fish size, ranging from approxi- 
