FISHERY BULLETIN: VOL. 72, NO. 4 



Hognestad (1968:130) reported that adults 

 trawled from the eastern Barents Sea in Sep- 

 tember 1966 fed principally on the copepod 

 Calanus finmarchicus. In turn, Arctic cod are 

 important as forage for higher level consumers. 

 Andriyashev (1954:194-198) cites literature 

 records for predation on Arctic cod by a long 

 list of species, including char, saffron cod, 

 flounders, sculpins, seals, walrus, beluga, sea 

 gulls, alcids, and skuas. Tuck (1960:166) stresses 

 the importance of Arctic cod in the diet of common 

 and thick-billed murres of the polar basin, whose 

 total populations contain at least 15 million 

 birds (p. 51). 



METHODS 



Studies with the 1.8-m (6-foot) Isaacs-Kidd mid- 

 water trawl (IKMT) involved 81 30-min tows (at 

 depth) at 20 stations (Table 1). All stations were 

 occupied for periods of about 2 h or more, and 

 all but one started at late dusk or dark. The 

 IKMT was similar in dimensions to the trawl 

 tested and figured by Friedl ( 1971 ); it had a section 

 of coarse mesh (3.8-cm bar) preceding 0.6-cm bar 

 mesh. The cross-sectional area at the mouth was 

 calculated as 2.87 m'-^ and at the beginning of the 

 0.6-cm mesh as 1.55 m^. To compensate for the 

 possibility that fish would be herded into the 

 small-mesh section by the coarse anterior mesh, a 

 middle value between the two cross-sectional 

 areas, 2.21 m^, was arbitrarily used as the 

 effective cross-sectional area of that portion of the 

 net that captured juvenile cod. Calculation of the 

 horizontal and vertical dimensions of the net 

 swath was simplified by treating the effective 

 cross-sectional area as a square — 1.49 m on a side. 

 Tows were standardized at 2 nautical miles 

 (3,704 m) at depth by maintaining a vessel 

 speed of 4 knots (estimated by engine revolutions) 

 over 30 min. The resulting horizontal swath was 

 calculated as 5,519 m^ and the volume filtered 

 as 8,223 m^. At least four tows were made at 

 each station, either at the same depth or at four 

 different depths (Table 1) — -two on one bearing 

 and the remainder on its reciprocal. 



Sampling followed a zigzag pattern (Figure 1) 

 before a southward-advancing ice front. Because 

 blocks of ice are sometimes difficult to see or 

 become too abundant to avoid, mid- water trawling 

 at night even in the presence of light pack ice 

 is hazardous. The trawl can be seriously damaged 



or lost outright if a large block of ice becomes 

 trapped beneath a trawl warp and causes the 

 trawl to be lifted to the surface where the ice 

 would be fed into its mouth at trawling speed. 

 Consequently, trawling was usually done in the 

 relatively open water of broad leads or ahead of 

 the ice pack. 



Two types of IKMT station were occupied. In 

 one type (eight stations), nektonic organisms at a 

 single depth within stations were sought by 

 conducting four "replicate" hauls at 11 or 12 m 

 (Table 1). In the other type (12 stations), the 

 vertical distribution of nektonic organisms was 

 sought by making four or five hauls at different 

 depths (Table 1); the depths, which were usually 

 verified with a bathykymograph, were selected 

 after study of a Simrad^ echogram (Model ER2 — 

 38 kHz). No reliable association was detected 

 between presence or absence of bands on echo- 

 grams and catch at those depths. 



Data obtained on juvenile Arctic cod in the 

 IKMT hauls included counts, range of standard 

 lengths, and volumes. When large numbers of cod 

 were captured, usually the total volume was 

 measured and the number of cod was estimated 

 from the average volume per individual in a 

 subsample. When there appeared to be negligible 

 differences in size of cod between hauls, the 

 average volume per individual in one haul was 

 used to estimate the number of juveniles in a 

 volume taken in another haul at the same station. 



RESULTS 



The juvenile Arctic cod appeared to be prin- 

 cipally of age-class (young-of-the-year), based on 

 a comparison between length frequencies of the 

 Chukchi specimens and age-length data reported 

 in the literature. For the comparison, the Chukchi 

 data were converted from standard to total length 

 by a regression based on data from the Chukchi 

 Sea specimens (Table 2), because the measure- 

 ment employed for data in the literature was not 

 specific and therefore was assumed to be total 

 length (the measurement usually used in fishery 

 studies). Modal size of the Chukchi Sea specimens 

 was 44 mm, slightly higher than the average size 

 for age cod, 35 mm, from the Barents Sea and 

 Spitzbergen cited by Hognestad (1968:130). The 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



1096 



