26 



Fishery Bulletin 94(1), 1996 



ommastrephids, then the actual age of squids in this 

 study should be corrected by adding the number of 

 increments between the nucleus and "natal" ring ( 10- 

 12 rings). However, in relation to the longevity of B. 

 magister, this bias appears negligible. 



Seasonal dynamics in distribution, length- 

 frequency composition, and stock structure 



We found three main seasonal groups of B. magister 

 in the western Bering Sea: winter-hatched, summer- 

 hatched, and fall-hatched groups. Winter-hatched 

 squids were caught in great numbers during the sum- 

 mer as juveniles ranging from 40 to 100 mm ML. 

 From June to September, these squids increased in 

 size over the slope of the Navarin-Olyutorsky region, 

 growing up to 110-150 mm ML. Later these squids 

 disappeared from catches. 



The summer-hatched group predominated catches 

 in Olyutorsky Bay and the Navarin-Olyutorsky re- 

 gion from June to August. About 80% of females and 

 85% of males from this group were mature in June; 

 their modal length was 250-260 mm and 200-210 

 mm, respectively. Spawning for this group began in 

 June and was encountered along the slope (400-600 

 m) between 165°E and 177°E. The average age of 

 spawning squid in June was 330-370 d for females 

 and 320-340 d for males. Spawning intensified in 

 August and finished by the middle of September. 

 Summer spawning of B. magister in the western 

 Bering Sea was identified by the pronounced pre- 

 dominance of mature males and mature females that 

 had mated and by relatively large numbers of spent 

 squid in August. Average age of spent males and fe- 

 males was 11-12 months. Aggregations of summer- 

 hatched squid were not dense; catches on their 

 spawning shoals never exceeded 300-400 kgh -1 . By 

 October these squid vanished from catches. 



Squid of the fall-hatched group occurred in the 

 western Bering Sea during the entire period of study, 

 representing the bulk of catches from August until 

 November. In June, these squid were immature (ma- 

 turity stages 1-2) and of modal size 170-180 mm ML 

 and were not abundant in catches (ca. 20% of total 

 catch) in all three regions. The abundance of fall- 

 hatched squid increased in the Navarin-Olyutorsky 

 region during summer, apparently because of their 

 continuing immigration along the slope from the east- 

 ern Bering Sea shelf. Several factors indicated the 

 presence of a westward migration. First, fall-hatched 

 juveniles appeared in Olyutorsky Bay only in July, 

 whereas in the Navarin-Olyutorsky region they oc- 

 curred from June onwards. Second, size-frequency 

 composition of squid in the Navarin-St. Matthew 

 region was rather stable from June to September; 



juveniles and immature squid predominated in 

 catches. The latter observation, together with the 

 observed growth of squid, strongly indicated that this 

 region was transient for shoals of juveniles and im- 

 mature adults migrating from the east to the 

 Navarin-Olyutorsky region. 



Growth and maturation of the fall-hatched squid 

 were observed on the slope of the Navarin- 

 Olyutorsky region and within Olyutorsky Bay from 

 June to August. By September, 90% of the fall- 

 hatched males achieved full maturity at modal length 

 200-210 mm, whereas a majority of females (about 

 60%) were still maturing (stage 3), attaining 250- 

 260 mm ML. Maturation of the fall-hatched females 

 began in October at modal length 270 mm ML. By 

 November, about 75% of females were mature and 

 ca. 70% of them had mated. Maturation of the fall- 

 hatched squid in September-October was accompa- 

 nied by their aggregation into dense shoals at depths 

 ranging from 350 to 450 m along the slope of the 

 Navarin-Olyutorsky region and in Olyutorsky Bay 

 with average catches >1 th" 1 . Thus, fall-hatched 

 squid represented the bulk of B. magister stock in 

 the western Bering Sea, with the highest fishery 

 potential of all seasonal groups. 



The population of fall-hatched squid in the 

 Navarin-Olyutorsky region successively changed in 

 structure during September and October. This shift 

 demonstrated the ongoing emigration of mature 

 squid from the Navarin-Olyutorsky region. Squid 

 possibly moved in a southwest direction along the 

 slope during October-November. Dense shoals of B. 

 magister disappeared, first in the Navarin-Olyu- 

 torsky region, and only later in Olyutorsky Bay. 



During summer and fall, seasonal groups of B. 

 magister used the continental slope of the western 

 Bering Sea in somewhat different ways. Summer- 

 hatched squid used the slope during June-August 

 as spawning grounds, mainly at depths of 350-450 

 m. For the fall-hatched squid, the slope was a feed- 

 ing ground and, at least for a number of them, a 

 spawning ground. During summer, the fall-hatched 

 squid increased in size rapidly at depths of 350-500 m. 

 In September-October, they formed dense aggrega- 

 tions and after spawning, rather quickly disappeared 

 from the region. As for the winter-hatched group, its 

 migratory behavior remained unclear, and we can- 

 not say whether or not this group used the slope as a 

 feeding ground only or as a spawning area as well. 

 With respect to spawning seasons of different groups, 

 it is obvious that the time lag between summer 

 spawning and fall spawning is much greater than 

 that between fall- and winter-spawning groups. Un- 

 fortunately, we do not have enough data on the win- 

 ter season, and it is possible that there are not three 



