KENDALL and NAPLIN: DIEL-DEPTH DISTRIBUTION OF ICHTHYOPLANKTON 



Table L — Numbers and relative alnindaiue offish eggs and larvae at the drogue stations during the vertical 

 distribution study ofichthyophinkton in the Middle Atlantic Bight, July 1974. 



Taxon 



Hakes. Urophycis spp, 

 Bluefish. Pomatomus saltatrix 

 Gulf Stream flounder, Citharichthys arctifrons 

 Frigate mackerel. Auxis sp. 

 Butterfish, Peprilus tnacanthus 

 Fourspot flounder. Hippoglossina oblonga 

 Atlantic whiting. Merluccius bilineans 

 Smallmoutln flounder, Etropus microstomus 

 Atlantic Donita. Sarda sards 

 Searobins. Pnonotus spp 

 Cdsk eels. Ophidiidae 

 Gunner. Tautogolabrus adspersus 

 Yellowtail flounder. Umanda ferruginea 

 Eels. Anguilliformes 

 Goosefisfi. Lophius amencanus 

 Witcfi flounder. Glyptocephalus cynoglossus 

 Snake eel. Pisodonophis cruentifer 

 Miscellaneous' 

 Total 



Total 

 number 



2,988 



17.311 



4.497 



3.898 



28.243 



114 

 2.397 



2.058 

 28 



61,534 



Eggs 



Mean no., 

 100 m3 



27 



158 



41 



36 



258 



1 



22 



19 



562 



% of 

 total 



49 



28 1 



7.3 



6.3 



459 



3.9 



33 

 99 9 



Total 

 number 



1.508 

 68.840 



Larvae 



Mean no./ 

 100 m^ 



13.8 

 629 



%of 

 total 



2.2 



100 1 



' Animals tfiat were too mutilated to be identified or too sparse for meaningful analysis. 



species, were taken during the experiments (Table 

 1). Hakes, bluefish, and Gulf Stream flounder 

 were most abundant, and frigate mackerel (Auxis 

 sp.), butterfish, fourspot flounder, Atlantic whit- 

 ing, and smallmouth flounder, Etropus micro- 

 stomus, were taken in substantial numbers. 



Bluefish 



An overall mean catch of 139 larvae/100 m'^ was 

 made placing bluefish second to hakes in abun- 

 dance. A highly significant difference occurred 

 in the catches among the 3 d, with more larvae 

 caught on day 2 than on the other 2 d (Table 2). 

 More larvae were taken in the surface and 4 m 

 nets than at other depths, with more taken at 

 the surface at night than during the day (Figures 

 3, 4). Both day and night catches at 15 and 30 m 

 were so small that they may have been a result of 

 contamination of the nets as they passed through 

 shallower water. During the second experiment, 

 the larvae were concentrated at 2 m at night and 

 6 m at other times (Table 2). 



The vertical-diel migration of these larvae is 

 clearly seen by comparing the proportions of 

 larvae in the and 4 m tows at each time of day 

 sampled (Figure 4). The proportion in the surface 

 tow was lowest at midday (1200 h) when it was 4% 

 of the total catch. It increased steadily to 49*^ 

 of the catch at midnight (0000 h) when it started 

 to decline again, reaching 17% by midmorning 

 (0900 h). Thus the larvae appear to be varying 

 their depth distribution continuously on a diel 



cycle, concentrating near 4 m during midday and 

 at the surface at night. 



The percentage of the larvae with food in their 

 guts also showed a marked diel pattern (Figure 4). 

 The maximum proportions of larvae with food in 

 their guts were taken from 0600 to 1200 h when 

 86-907f of the guts contained food. At 1500 and 

 1800 h, im of the larvae contained food. By 2100 h 

 the proportion had dropped to 22'7f and during the 

 night (0000 and 0300 h) none of the larvae had 

 food in their guts. 



Most of the food consisted of various life stages 

 of copepods, including nauplii, copepodites, and 

 adults. Cladocerans and invertebrate eggs were 

 also present in small numbers. It also appeared 

 that smaller larvae had higher proportions of 

 nauplii while larger larvae had higher propor- 

 tions of adult copepods and cladocerans although 

 too few fish were examined for detailed analysis. 



Several factors indicate that food passes through 

 the gut fairly rapidly. Few fish had any food 

 particles in the foregut. At 0600 h about twice as 

 many larvae had food in the midgut as had food in 

 the hindgut. Later in the day about equal numbers 

 of larvae contained food in the midgut and in 

 the hindgut. At no time did more hindguts than 

 midguts contain food. 



The mean lengths of bluefish larvae were com- 

 pared among the tows. The mean length for all 

 tows was 4.33 mm (Table 3). We found no signifi- 

 cant difference in mean lengths among the 3 d of 

 sampling, between day and night sampling, or 

 among the six sampling depths. Larger larvae 



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