Gannon and Gannon: Passive acoustic assessment of soniferous fish density 
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CPUE (F 2 125 = 3.130, P = 0.047), dissolved oxygen 
(F 2 125 = 4.21, P=0.017), and temperature (F 2 125 - 7.63, 
P=0.001) had significant overall effects in the MAN- 
COVA (Table 1). Between-subject tests that provided 
separate results for each dependent variable showed 
that dissolved oxygen concentration (^=7.69, P=0.006), 
temperature ( F l -6.3 , P=0.013), and habitat (F 2 - 4.55, 
P=0.012) were significantly associated with received 
sound level, and partial correlations indicated that 
each of these parameters accounted for 5-7% of the 
variance in sound level (Table 2). The interaction of 
monthxhabitat was associated with peak frequency of 
Atlantic croaker calls (F 6 = 2.31, P=0.038). For every 
1-mg/L increase in dissolved oxygen concentration, the 
sound level of Atlantic croaker calls increased by 0.87 
dB, and for every 1-degree increase in temperature 
there was an increase in sound level of 1.01 dB (Table 
3). Although CPUE had significant overall effects in the 
MANCOVA (Table 1), the between-subject analysis did 
not detect significant relationships with either of the 
two dependent variables (Tables 2 and 3). This finding 
may have been due to the low statistical power associ- 
ated with CPUE. 
Williamson’s indices demonstrated significant spa- 
tial overlap in the distributions of Atlantic croaker 
derived independently from acoustic and trawl data. 
Williamson’s indices were between 0.959 and 1.488 for 
all combinations of month and habitat (Table 4). Call- 
ing indices corresponded better with CPUE than did 
received sound levels of Atlantic croaker calls. Spatial 
overlap between calling index and the actual density 
of Atlantic croaker (CPUE) was higher in July and 
August than in June, September, or October, and cor- 
respondence was stronger in the mid-river and river 
edge habitats than in the creeks. 
With CART analysis we identified calling index as 
the predictor variable with the strongest correlation 
with CPUE (Fig. 3). Calling indices of 0 and 1 were 
associated with significantly lower densities of Atlantic 
croaker than were calling indices >1. Day of year and 
