KENDALL and NAPLIN: DIEL-DEPTH DISTRIBUTION OF ICHTHYOPLANKTON 



about 1800 h; however, we found egg density to 

 increase until 0300 h. The downward slope on the 

 right side of the curves primarily reflects embry- 

 onic mortality, although hatching accounts for the 

 decrease during the latest developmental stages. 

 The fact that the egg batches reached their peak 



Table 7. — Percentage of abnormal Atlantic whiting embryos in 

 each spawning batch from the vertical distribution study of 

 ichthyoplankton in the Middle Atlantic Bight, July 1974. 



abundance later than expected probably reflects 

 the influence of local currents on distribution of 

 spawning adults and eggs. 



Determining the time and depth at which eggs 

 were spawned provides information about adult 

 spawning behavior. We have shown that newly 

 spawned eggs appear in the afternoon and early 

 evening. The depth at which spawning occurs can 

 be estimated from knowing the depth at which the 

 very early stage eggs were collected. Atlantic 

 whiting eggs are planktonic, tending to rise 

 toward the surface at a rate which, aside from 

 turbulence of the water, depends on their specific 

 gravity and that of the surrounding water. Esti- 

 mating the rate of rise of the eggs in the water 

 column enables us to calculate the depth at which 



Sampling times (EST) 



1200 



1000 



O 



o 





O) 



E 



3 



800 



600 



400 



200 



1200 

 — I — 



I 500 1 800 



2100 



0000 0300 



-r 



T 



0600 



— I — 



0900 1200 1500 1800 2100 0000 0300 0600 0900 

 — I 1 1 1 1 1 1 1 1 



:ii 



A 



/ "^. IV 



/ \ 



^^^ / ^• 



• ^v^ 1 1 ^. 



\ I 



y 





s 



Developmental stage 



Figure 6. — Numbers of Atlantic whiting eggs in the five batches taken during the vertical distribution study of 

 ichthyoplankton in the Middle Atlantic Bight, July 1974, plotted by developmental stage and time of day 



715 



