QUAST: DISTRIBUTION OF ARCTIC COD 



DISCUSSION 



Possible Causes of the Density 



Structure and Its Vertical 



Displacement 



The steady increase of number of cod in the 

 density structure with depth indicated a graded 

 rather than a threshold response to some environ- 

 mental factor, because if the response depended 

 on a threshold, one would expect a sudden 

 increase in density of juvenile cod when that 

 threshold was reached. Salinity relationships in 

 the water column did not appear to be a cause of 

 the density structure because the water column 

 was usually nearly isohaline through the region 

 of greatest change in concentration of juvenile 

 cod (Figure 2) — the density structure evidently 

 persisted despite forces that contributed to oceanic 

 mixing. Predation by aquatic predators did not 

 seem to be a reasonable cause because the only 

 evident potentially effective fish or invertebrate 

 predator on the cod was large jellyfish, which 

 occurred in approximately equal numbers 

 throughout the water column. Reaction of the 

 juvenile cod to a pressure gradient also was 

 dismissed, for reasons discussed below. 



A graded negative phototaxis either in the 

 juvenile cod or, possibly, in the prey they were 

 following seemed to offer the best hypothesis for 

 the cause of the density structure and its 

 variations. A gradient of increasing darkness 

 with depth could coincide closely with the 

 gradient for increase in number of cod. This type 

 of gradient is illustrated in Figure 3 (shown here 

 for the English Channel — the slope in regard to 

 perception by the cod should be dependent on 

 turbidity as well as the spectral sensitivity of the 

 juvenile cod). Under this hypothesis the density 

 structure of juvenile cod found at night was a 

 relict of earlier daylight hours, during which the 

 density structure was maintained despite 

 turbulence, upwelling, and downwelling. With 

 onset of darkness the means of orientation by the 

 fish was removed and the density structure was 

 elevated or depressed, depending on whether it 

 occurred in cells of upwelled or downwelled water. 

 The density structure presumably persisted into 

 the early night when most hauls were made 

 because of the short period of time elapsing before 

 trawling began and also because the cod were 

 relatively inactive at the low sea temperatures 



3.0 



20 30 



DEPTH (M) 



50 



Figure 3. — Conformity of the variables — density of juvenile 

 Arctic cod, loss of light (blue-green, English Channel), and 

 pressure at increasing depths. Regression of fish with depth 

 (D), I^f = 0.0699D, where N = most likely number of fish in a 

 trawl swath (see text); index of light loss based on I = log,o 

 (1/p X 100), where p = percent of illumination at surface, 

 from data of Nicol (1960:22); and pressure (atmosphere) based 

 on an increase of 1 atm/lO-m depth, after Nicol (1960:22). 



(-1.5° to 3.5°C). The freshly trawled specimens 

 were markedly inactive. The validity of the 

 gradient hypothesis possibly could have been 

 evaluated further had trawl data been available 

 for daylight hours. Data on vertical distribution of 

 zooplankton were not available because sampling 

 was entirely by vertical tows (Wing, in press). 



Orientation by the cod to a pressure gradient 

 was dismissed as an explanation of the density 

 structure for two reasons: the shape of the curve 

 of pressure on depth differs from that of the 

 density structure (Figure 3), and orientation to a 

 pressure gradient should be the same during the 

 day as at night and should not allow the 

 density structure to be elevated or depressed. 



If the density structure were a result of behavior 

 in the juvenile cod, it may have been evolved in 

 response to predation by birds. Undoubtedly, such 

 predation is a factor of tremendous ecological 

 importance to juvenile cod, primarily during 

 summer when bird populations are at their 

 peak. Arctic piscivorous birds form a spectrum of 

 depth capabilities, and because their feeding is 

 based at the surface, intensity of predation should 

 decline with depth, i.e., complement the density 

 structure. Watson and Divoky (1972) give an 

 extensive list of bird species observed in the 

 Chukchi Sea during WEBSEC-70, the majority of 

 which are either recorded as predators or are 

 assumed to have, and to make use of, the potential 

 for predation on juvenile Arctic cod. Included are 

 loons, slender-billed shearwater, pelagic 



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