MBL-months, the two sets of ADIs were similar and 2) 

 the frequency and magnitute of observed differences 

 were fewer and lower in the most recent years. We found 

 that differences occurred most frequently in the south- 

 eastern Bering Sea (MBLs 21 and 22) during March- 

 June for pair trawlers and in the central and northern 

 Bering Sea (MBLs 32, 33, 43, and 44) during December- 

 March for factory trawlers. 



These differences occurred in times and areas where 

 species other than pollock were caught in large quan- 

 tities. As the proportion of species other than pollock in- 

 creased in the catch, differences became greater. The 

 differences, however, were not significant although ex- 

 clusion of data from poor pollock catches by one criterion 

 resulted in ADIs that were on the high side and inclusion 

 of all data by the other resulted in lower ADIs. Since 

 we cannot distinguish which ADI was truly indicative 

 of pollock abundance and not biased by changed in 

 species composition, it was necessary to define index 



area-time cells which more accurately reflect trends in 

 abundance. 



INDEX AREA-TIME CELLS 



Based on distributional pattern of pollock and its life 

 history features (Pereyra et al. 1976 see footnote 3) we 

 identified four area-time cells (Fig. 2) which consistently 

 accounted for high proportions (75% or more) of pollock 

 in the catch for ADI computations. 



Area-time cell I — MBLs 21 and 22, March-May, 

 which represents the main 

 spawning area during the spawn- 

 ing period. 



Area-time cell II — MBLs 21 and 22, July-Septem- 

 ber, which represents the main 

 spawning area during a feeding 

 period. 



AREA-TIME CELLS 



BL 21 and 22, March-May 

 MBL 21 and 22, July-September 



BL 32 and 33, April-July 

 MBL 43 and 44, June— September 



major statistical block) 



60° N 



55° N 



180° ' 



175° W 



170° W 



165° W 



160° W 



Figure 2.— Four area-time cells used for calculating average density indices for walleye pollock in the 



eastern Bering Sea. 



