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Fishery Bulletin 108(1 ) 
the rate at which Atlantic croaker knock calls were 
detected, were high and relatively consistent in the 
creek and river edge habitats throughout the study 
period (Fig. 2C). In contrast, the mean calling index in 
the mid-river fell to a minimum of 0.91 in August and 
then increased sharply to its maximum value of 3.0 in 
October. Received sound levels peaked during August 
in the creek and river edge habitats (116 and 113 dB, 
respectively), and then fell sharply (Fig. 2D). Received 
levels in the mid-river peaked in September (109 dB). 
Overall, there was a declining trend in peak acoustic 
frequency of the calls throughout the five-month study 
period (Fig 2E), after an initial increase in July in the 
mid-river and river edge habitats. The creek habitat 
was the warmest habitat in June and the coolest in Oc- 
tober (Fig. 2F). The mid-river was the coolest habitat 
in June and the warmest in October. Dissolved oxygen 
concentration at the bottom of the water column was 
lowest during summer and was always lower in the 
mid-river habitat (the deepest portion of the study 
area) and highest in the river edge habitat (Fig 2G). 
During August, the mean dissolved oxygen concentra- 
tion in the mid-river was 1.1 (standard deviation=1.6) 
mg/L, which was well below the 2.3 mg/L avoidance 
threshold that Eby and Crowder (2002) found for At- 
lantic croaker in the Neuse River. Salinity was highest 
in July and August and was higher in the mid-river 
habitat than in the other two habitats (Fig. 2H). 
