114 
Fishery Bulletin 108(1) 
Diagram of the classification and regression tree (CART) showing the relationship between the dependent 
variable “trawl CPUE of Atlantic croaker” (In (x+1) transformed) and eight predictor variables: day of year 
(DOY); dissolved oxygen concentration; temperature; salinity; depth; distance to creek (m) (dist_creek); 
received sound level; and calling index (Cl) in the Neuse River estuary during summer 2000. Only the 
significant predictor variables are shown. 
sion environment. This study illustrates the importance 
of planning passive acoustic surveys carefully to control 
for the confounding effects of environmental variables. 
One major assumption used in this article is that 
the trawl data reflect actual Atlantic croaker densities 
in a consistent manner over time and across habitats. 
All fish sampling techniques have biases. For example, 
the catch efficiency of our trawl gear may have varied 
among the three habitats, by fish size, or between call- 
ing and noncalling fish. Therefore, some of the varia- 
tion in the relationship between catch rates and sound 
production may have been due to variation in the catch 
efficiency of our trawl net, as well as in the variation 
in acoustic parameters. Because trawling is the most 
common method for assessing the abundance of de- 
mersal marine fishes, it is important to investigate 
the relationship between passive acoustic measures of 
abundance and those obtained from trawling. 
Either the physical presence of, or the sound produced 
by, our boat could have caused a temporary disturbance 
of Atlantic croaker. For example, it may have caused 
the fish to flee from the immediate area or to change 
their acoustic behavior, which could have biased our 
results. Any disturbance would likely have been more 
pronounced in shallow water. We did not specifically 
investigate whether our boat may have affected our 
results. But there are a few points to consider. First, 
playback experiments of boat sounds in tanks elicited 
no significant change in swimming speed, turning rate, 
or depth in the water column of Atlantic croaker (D. 
Gannon, unpubl. data). Second, field recordings from 
the Neuse River that included the sounds of passing 
boats did not indicate an acoustic response from Atlan- 
tic croaker. The croaker sounds seemed to continue un- 
interrupted as boats passed by (but these boat sounds 
often masked the croaker sounds, and therefore such 
recordings were eliminated from our analysis). Third, 
the Neuse River estuary is highly turbid. Secchi depths 
during the summer are usually less than 1 m, which 
would limit visual detection of the boat. Finally, call- 
ing indices, received sound levels, and trawl catches 
were generally higher at shallow sampling stations 
(creek and river edge) than at deep ones (mid-river). 
In future studies an investigation of potential biases 
caused by disturbance from the sampling vessel would 
be worthwhile. 
