KENDALL and NAPLIN: DIEL-DEPTH DISTRIBUTION OF ICHTHYOPLANKTON 



10 



Figure 9. — Mean standard lengths 

 (millimetersi of Atlantic whiting larvae 

 taken at 15 and 30 m by time of day 

 from the vertical distribution study of 

 ichthyoplankton in the Middle Atlantic 

 Bight. July 1974. 



0900 



1200 1500 1800 2100 



SAMPLING TIMES (EST) 



0300 0«00 



We caught very few hatchlings (<3.0 mm 

 [Kuntz and Radcliffe 1917]), as they appear to 

 have been extruded from the 0.505 mm mesh net. 

 Therefore, we could not detect a pulse in the 

 numbers of very small larvae corresponding to 

 the daily time of hatching or shortly thereafter. 

 Significantly fewer larvae of all sizes were taken 

 on day 3. 



Gulf Stream Flounder 



Gulf Stream flounder eggs have not been fully 

 described in the literature, although Richardson 

 and Joseph (1973) described eggs stripped from 

 Gulf Stream flounder that were approaching 

 spawning condition. The very small larvae are so 

 similar to those of smallmouth flounder, Etropus 

 microstomus , that the eggs also may be virtually 

 indistinguishable from that species. In fact, eggs 

 from our samples are identical to those described 

 as smallmouth flounder eggs by Scherer and 

 Bourne (1979), who did not provide adequate 

 means of distinguishing the two species. 



We have identified eggs from the plankton that 

 we consider to be Gulf Stream and/or smallmouth 

 flounder eggs based on egg and oil globule diam- 

 eter and late embryonic characteristics. We as- 

 sume that such eggs taken during this cruise are 

 Gulf Stream flounder eggs for three reasons. First, 

 Gulf Stream flounder occurs mainly at depths 

 >46 m while smallmouth flounder is most com- 

 mon in water <27 m (Richardson and Joseph 

 1973); our samples were from an area where the 



water was 57 m deep. Second, smallmouth floun- 

 der larvae occur farther south than Gulf Stream 

 flounder; our sampling location was in an area of 

 peak Gulf Stream flounder abundance, but north 

 of the area of maximum smallmouth flounder 

 concentration (Smith et al. 1975). Finally, Gulf 

 Stream flounder larvae occurred in markedly 

 greater numbers in our samples than did small- 

 mouth flounder larvae (19.7% as opposed to 1.5% 

 of the total catch). 



Gulf Stream flounder averaged 158 eggs/100 m"^ 

 during the cruise. The catches were significantly 

 different for all three primary factors (depth, time 

 of day, and day) (Table 3). Fewer eggs were taken 

 with increasing depth (Figure 5), though com- 

 parable numbers were taken at 15 and 30 m; more 

 eggs were taken at night; and the total number of 

 eggs increased with each day of sampling. More 

 eggs were taken on each succeeding day at all 

 depths except 30 m, where the number decreased 

 with time. 



Gulf Stream flounder eggs showed patterns of 

 developmental stages that changed with both 

 time and depth. Table 8 shows stages of eggs 

 summed over all depths at each sampling time. 

 The eggs were more difficult to separate into 

 batches than Atlantic whiting eggs, but batches 

 were defined in which eggs could be traced from 

 spawning through hatching in succeeding sam- 

 ples. Usually three distinct groups of develop- 

 mental stages were apparent at each sampling 

 time. Precell stages were taken in the afternoon 

 from 1500 to 2100 h. These eggs required about 3 



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