FISHERY BULLETIN: VOL. 72, NO. 4 



into the Chukchi Sea (Zenkevitch, 1963:262). 

 The near absence of high-arctic zooplankton in the 

 eastern Chukchi Sea (Zenkevitch, 1963:268), 

 verified during WEBSEC-70 by Wing (in press), 

 is only weak evidence that the cod did not 

 originate in the East Siberian Sea: the cod 

 probably spawned in January and February 

 (Rass, 1968:136), approximately 8 mo before 

 WEBSEC-70, and it seems possible that the 

 juveniles could have lost their arctic ecological 

 associates over that period of time. In any event, 

 the apparent short time that cod spent in some 

 sections of the Chukchi Sea — about 10 days in 

 the southeastern part according to the oceano- 

 graphic data of Fleming and Heggarty (1966: 

 724) — and the time elapsed between spawning 

 and capture at our IKMT stations is evidence 

 that the juvenile cod either originated at consider- 

 able distances from the Chukchi Sea or had been 

 cycled in the Chukchi Sea. Since there is reason 

 to believe that a portion of the Chukchi Sea 

 circulates in a counterclockwise gyre (Zenkevitch, 

 1963:262), similar cycling of eggs and larvae 

 seems possible. 



SUMMARY 



1. Twenty stations in the eastern Chukchi 

 Sea between Icy Cape and Cape Lisburne were 

 sampled with a 1.8-m (6-foot) Isaacs-Kidd 

 mid-water trawl (IKMT) during September 

 and October 1970. 



2. Only two species of fish occurred in any 

 abundance in the middle and upper water column 

 at night, juvenile Pacific sand lance and juvenile 

 Arctic cod (mostly age 0). Because the Arctic 

 cod were most abundant and were distributed 

 through most of the water column, their occur- 

 rence was given a detailed analysis. 



3. Two types of IKMT stations were occupied; 

 one in which hauls were replicated at the same 

 depth, and another in which hauls were made at 

 several depths. All stations but one (a replicate 

 station) were occupied starting at late dusk or at 

 night. 



4. The standard deviation in frequency of 

 juvenile cod occurrence at the replicate stations 

 seemed to be proportional to the means. The 

 variance was stabilized by log^j (A^ + 1) trans- 

 formation of the data. 



5. Analysis of variance of the transformed 

 data from the replicate stations disclosed signifi- 



cant between-station variation in number of cod 

 occurring at 11-12 m. 



6. Number of juvenile cod per unit volume of 

 water increased with depth at all multidepth 

 stations. 



7. Analysis of covariance on regressions of cod 

 abundance (logxo transformed) vs. depth disclosed 

 no significant differences in slope of the regres- 

 sions between stations but significant differences 

 in level. 



8. The characteristics of slope and level in the 

 regressions for multidepth stations described a 

 depth region of logarithmic increase in con- 

 centrations of juvenile cod. This region was at a 

 different depth at some stations than at others 

 and was called a density structure. 



9. It appeared that the density structure 

 resulted from a graded rather than a threshold 

 response to subsurface illumination. Such a 

 response seems to be a suitable behavioral 

 strategy for lessening predation by piscivorous 

 birds. 



10. The hypothesis is presented that the 

 density structure was formed during daylight 

 hours and that differences in its vertical dis- 

 placement between stations at night were caused 

 by wind-induced upwelling or downwelling. 



11. Average number of juvenile cod calculated 

 for the stations was 28/1,000 m^ or approximately 

 0.7 metric ton/km^ of ocean surface. 



12. The origin of juvenile cod could have been 

 in the northeastern Bering Sea, the East Siberian 

 Sea, the Chukchi Sea, or all three. Regardless 

 of origin, a cycling of eggs and larvae within the 

 Chukchi Sea seems likely. 



ACKNOWLEDGMENTS 



Merton C. Ingham, Atlantic Environmental 

 Group, National Marine Fisheries Service, 

 NOAA, directed the cruise and coordinated 

 investigations aboard the vessel. Bruce L. Wing, 

 Auke Bay Fisheries Laboratory, National Marine 

 Fisheries Service, NOAA, assisted in the sam- 

 pling; and James C. Olsen and Jerome J. Pella, 

 of the same laboratory, advised on methods of 

 mathematical computation. I am indebted to those 

 who read and commented on the manuscript, in 

 particular to Ingham, Wing, Olsen, and Pella, 

 mentioned above, and to Paul E. Smith, South- 

 west Fisheries Center, National Marine Fisheries 

 Service, NOAA, and George E. Watson, Smith- 



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