KENDALL and NAPLIN: DIEL-DEPTH DISTRIBUTION OF ICHTHYOPLANKTON 



40 50 60 



Developmental time (percent) 



100 



Figure lO. — Percentage of Gulf Stream flounder eggs at each depth plotted by age and stage based on collections made during the 

 vertical distribution study of ichthyoplankton in the Middle Atlantic Bight, July 1974. 



sampled, while Atlantic whiting eggs were col- 

 lected from depths near where they were spawned. 



An overall mean of 124 Gulf Stream flounder 

 larvae/100 m^ was taken during the cruise (Table 

 1). More larvae were taken at night than during 

 the day and more were caught at 15 m than at the 

 other depths sampled (Figure 3). The diel-depth 

 interaction was significant in that Gulf Stream 

 flounder, like fourspot flounder, were relatively 

 more abundant at and 4 m at night and at 15 and 

 30 m during the day, indicating a vertical migra- 

 tion upward at night (Table 2). Gulf Stream 

 flounder larvae are concentrated in the thermo- 

 cline during the day, but at least some move 

 toward the surface at night. 



From the second experiment, it appears that the 

 movement toward the surface may occur in early 

 evening because catches in the 2 m tow were high 

 at 1800 h, while the 15 m tow took more larvae at 

 other times (0000, 0600, and 1200 h). There was no 



significant difference in catches over the 3 d of the 

 experiment, nor in the mean standard lengths, 

 which ranged from 4.33 to 4.57 mm. 



Smaller larvae were taken from to 4 m during 

 the day, while larger larvae were taken from 6 to 

 30 m (Table 10). At night, it seems that some of the 

 larger larvae spread upward and were caught at 



Table lO. — Mean standard lengths (millimeters) of Gulf 

 Stream flounder larvae at six depths during day and night from 

 the vertical distribution study of ichthyoplankton in the Middle 

 Atlantic Bight, July 1974. 



719 



