FISHERY BULLETIN: VOL. 79, NO. 4 



the relative proportions of larvae of the two 

 species taken during the cruise, eggs with charac- 

 teristics of smallmouth flounder and Gulf Stream 

 flounder were assumed to be of the latter species. 



An overall mean of 9.1 smallmouth flounder 

 larvae/100 m^ was taken during the cruise. In 

 general, the 15 m tow caught most of the larvae; 

 however, more larvae were taken at 15 and 30 m in 

 the daytime, while more were caught at and 4 m 

 at night (Figure 3). The larvae moved toward the 

 surface at night and were also more abundant at 

 all depths at night. More larvae were taken on 

 each succeeding day of the experiment (Table 2). 



We found little difference in mean length dur- 

 ing day tows at all depths, and although the larvae 

 caught at night at 30 m were larger than those 

 caught at other depths, only two were taken 

 (Table 13). The mean lengths of the larvae taken 

 each day remained virtually constant from 3.58 to 

 3.61 mm. Evidently the drogue gradually drifted 

 into an area of higher larval concentration, result- 

 ing in the increasing number of larvae taken each 

 succeeding day. 



Table 13. — Mean standard lengths ( millimeters) of smallmouth 

 flounder larvae at six depths during day and night from the 

 vertical distribution study of ichthyoplankton in the Middle 

 Atlantic Bight, Julv 1974. 



DISCUSSION 



The two major techniques applied to the egg 

 data, i.e., analyses of variance of egg numbers 

 based on a factorial experimental design and 

 staging the eggs, tend to complement each other. 

 The former technique is used to point out signifi- 

 cant variation in the data that may be clarified 

 and elaborated by staging selected egg samples. 

 For example, significant differences in egg num- 

 ber at various times of day revealed by factorial 

 analysis can be accounted for by the daily spawn- 

 ing and hatching cycles revealed by staging the 

 eggs. In our experiments, analysis of variance 

 demonstrated significant changes in egg numbers 

 over four sampling depths, over 3 d, and in the 



daytime and at night. Egg staging provided us 

 with developmental timetables, and time of day 

 and depth of spawning and hatching. Spawning 

 and developmental characteristics of the three 

 species whose eggs were staged are summarized in 

 Table 14. Atlantic whiting and snake eel eggs 

 were distributed homogeneously by stage over all 

 depths sampled, whereas Gulf Stream flounder 

 eggs were stratified by stage to some extent. Stage 

 stratification of Gulf Stream flounder eggs indi- 

 cated that spawning probably occurred on the 

 bottom, and rising velocity calculations for these 

 eggs support that conclusion. The rising velocity 

 for Atlantic whiting eggs narrowed the possible 

 spawning depth to the upper 10 m of the water. 

 Fluctuations in egg numbers that appeared to 

 be tide induced could indicate that the bottom- 

 dwelling snake eel spawns on the bottom during a 

 particular time of the tidal current cycle. 



In a study of haddock eggs, Walford (1938) found 

 that the eggs were spawned on the bottom and had 

 a tendency to rise. Because egg stages were 

 homogeneously distributed with respect to depth, 

 he concluded that the eggs could adjust their 

 specific gravity within limits to match that of the 

 ambient water. Most planktonic eggs probably 

 possess some capacity to adjust their densities 

 (Walford 1938). However, because a definite age 

 stratification with depth appeared for the Gulf 

 Stream flounder eggs, we can conclude that in this 

 case the eggs rise in the water column more 

 rapidly than they adjust their density to the 

 ambient water. 



Spawning and hatching times for the three 

 species were staggered throughout the day, with 

 no two species having similar schedules. Atlantic 

 whiting and snake eel spawned in the afternoon 

 and evening, and while Atlantic whiting eggs 

 hatched in the morning, snake eel eggs hatched at 

 about the same time of day they were spawned. 



Table 14. — Summary of local characteristics of distribution of 

 spawning and development determined by staging eggs of 

 Atlantic whiting, Gulf Stream flounder, and snake eel, based 

 on collections made during the vertical distribution study of 

 ichthyoplankton in the Middle Atlantic Bight, July 1974. 



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