KENDALL and NAPLIN: DIEL-DEPTH DISTRIBUTION OF ICHTHYOPLANKTON 



nets fishing at 2 and 6 m (second experiment). The 

 depth stratum from 30 m to the bottom ( 57 m) was 

 not sampled, and some larvae probably occurred 

 in this area. Bendix bathykymographs on the wire 

 near the subsurface nets checked the actual sam- 

 pling depths on each haul. During each haul we 

 placed the nets on the wire as it was paid out; the 

 vessel maintained minimal headway to keep the 

 nets from tangling around the wire. Once the nets 

 were on the wire, the vessel speed was increased to 

 5 kn ( 9.3 km/h). Hauls lasted 15 min from the time 

 the surface net started to fish to when it was 

 brought out of the water. At the end of the 15 min, 

 the ship was set adrift and the nets were hauled 

 in as quickly as possible. Since the nets had 

 no opening-closing devices, this procedure mini- 

 mized contamination of the deeper nets by or- 

 ganisms in shallower water. Such contamination 

 was considered inconsequential since the hori- 

 zontal towing distance was a nominal 2,325 m, 

 and maximum towing distance in shallower layers 

 for the 30 m net was judged to be 75 m (3.2% of the 

 haul); however, it may have accounted for some of 

 the predominantly shallow-caught larvae found 

 in the deeper nets. Samples were preserved in 5% 

 seawater buffered Formalin. The bongo nets pro- 

 vided paired samples that we designated port and 

 starboard. All starboard net samples and one of 

 the four or six port net samples from each sam- 

 pling time were brought ashore. All fish eggs and 

 larvae were removed from the plankton samples, 

 identified, and counted. 



Selected samples of Atlantic whiting,'* Merluc- 

 cius bilinearis; Gulf Stream flounder, Citharich- 

 thys arctifrons; and snake eel, Pisodonophis 

 cruentifer, eggs were staged (categorized accord- 

 ing to developmental stage), based on divisions 

 of the embryonic period used by Naplin and 

 Obenchain (1980). Although many of the early 

 stage eggs had ruptured yolks, these were con- 

 sidered intact prior to sampling. As has been noted 

 in other species (Leis 1977), until the blastoderm 

 completely covers the yolk sac, yolk breakage 

 occurs easily and is likely to happen during 

 sampling. Some middle stages, too, were ruptured, 

 but we cannot determine whether they ruptured 

 during sampling or whether they were already 



^The common name Atlantic whiting is used in this paper for 

 Merluccius bilinearis to avoid confusion with hakes i Urophycis 

 spp.). Recently the common name of M. productus has been 

 changed from Pacific hake to Pacific whiting. We suggest that 

 fish of the genus Merluccius be recognized as whitings, a name in 

 common use on the east coast already. 



dead when collected. As most of the embryos 

 looked normal and undeteriorated other than 

 having a ruptured yolk, these also were con- 

 sidered to have been alive when sampled. 



Atlantic whiting eggs were initially staged at 

 all depths for the first day of sampling (39 samples 

 at eight time periods). Predominant stages at a 

 particular time of day were the same regardless of 

 depth, indicating that stages were not stratified 

 with depth. As 95% of the eggs were taken in the 

 surface and 4 m samples, only the surface samples 

 were staged at each time period for the remaining 

 2 d of sampling. Gulf Stream flounder eggs were 

 staged at all depths for the first day of sampling, 

 and snake eel eggs, which occurred in fewer 

 numbers, were staged at all depths for all 3 d 

 of sampling. 



Stokes' law for determining the settling velocity 

 of a particle has been used to estimate the rising 

 velocity of planktonic eggs (English 1961). Stokes' 

 law, applied to this problem states that 



2 [di - d2] 2 



V = -g r^ 



9 /u. 



where V - velocity 



di = density of egg 

 d2 = density of liquid 



g = acceleration of gravity 



r = radius of egg 



fjL = dynamic viscosity of liquid. 



All values are expressed in the centimeter-gram- 

 second system. Although no measurements of 

 specific gravity of eggs for species discussed here 

 are available, values for other planktonic eggs 

 have ranged from 1.021 for pleuronectid eggs 

 (English 1961) and for eggs of the gadid Theragra 

 chalcogramma (Kanoh 1954) to 1.0287 for Argen- 

 tina silus eggs (Schmidt 1906, quoted by Breder 

 and Rosen 1966). We used the 1.021 value in our 

 calculations because a value > 1.022 would not 

 permit eggs to float in the upper 10 m where the 

 eggs we took were abundant. 



Bluefish larvae from all samples were measured 

 and those from a subset of 28 samples were used 

 for gut content analysis. From each of these 

 samples, 10 fish representing the sample size 

 distribution were examined for gut contents. The 

 number and types of food organisms in the fore- 

 gut, midgut, and hindgut were noted. Atlantic 

 whiting and Gulf Stream flounder larvae were 

 also measured. 



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