Arkhlpkin: Intrapopulation structure of ///ex argentinus during its feeding penod over the Patagonian Shelf 



11 



erable predominance of males (2:1) in catches within 

 the EEZA in February-March. Similar sex ratios have 

 also been noted in the fishery region north of the Falk- 

 land Islands (Koronkiewicz, 1995). Another explana- 

 tion for the predominance of males in shelf catches 

 may be the earlier migrations of mature males (in con- 

 trast to females ) from the southern part of their feed- 

 ing area through the region of 47^9°S. This migratory 

 pattern occurs during prespawning migrations of /. 

 argentinus ft-om the southern part of the Patagonian 

 Shelf (Arkhipkin, 1993). 



In April large mature males and females, which had 

 been aggregated over the shelf edge, began to shift 

 to the continental slope and migrate to great depths 

 (>600 m), where they mixed with the already migrat- 

 ing schools of July- and August-hatched squid that 

 had fed in the southern part of the Patagonian Shelf 

 and around the Falkland Islands (Arkhipkin, 1993). 

 Such a redistribution of /. argentinus aggregations 

 caused a rather sharp decrease in the CPUE of jig- 

 ging vessels on the shelf in April-May and a simulta- 

 neous regi'ouping of trawlers — a shift from the shelf to 

 one over the continental slope (Hatanaka, 1988; Nig- 

 matullin, 1989b). Medium-size squid remained on the 

 shelf and probably made their prespawning migra- 

 tions along the shelf edge. 



Stock structure of winter-hatched ///ex argentinus 



From the complex of biological characteristics (stato- 

 lith microstructure, modal length in different months, 

 sizes at maturity, types of feeding, and prespawning 

 migrations), it is possible to consider the Patagonian 

 Shelf south of 45°S as a feeding ground for two intra- 

 specific groups of winter-hatched /. argentinus: the 

 "shelf group that matures at medium sizes" (ShG) and 

 the "slope group that matures at large sizes" (SIG). 

 These two groups correspond well with the bonaeren- 

 sis north Patagonian stock (BNPS) and south Pata- 

 gonian stock (SPS) distinguished by Brunetti (1988) 

 by using length-frequency analysis. Later, Brunetti 

 et al. (1998) postulated that the spawning of both 

 groups takes place near the shelf edge and over the 

 continental slope; the BNPS squid spawn north of 

 43°S in winter, whereas the SPS squid spawn south of 

 43°S in autumn. It was shown however that the SPS 

 squid definitely migrated from the southern part of 

 the Patagonian and Falkland shelves along the con- 

 tinental slope farther north at 41^2°S (Arkhipkin, 

 1993), but location of their spawning grounds is still 

 unknown (Haimovici et al., 1998). 



The shelf group also corresponds to the winter shelf 

 group (WSG), and the slope group corresponds well 

 to the winter oceanic group (WOG), both (WSG and 

 WOG) of which were distinguished by different loca- 



tions of juvenile feeding and by type of life cycle 

 (Parfeniuketal., 1992; NigmatuUin and Laptikhovsky, 

 1996). 



The shelf group of /. argentinus has a neritic life 

 cycle, characterized by the following features: spawn- 

 ing in warm waters of the northern part of the spe- 

 cies range (27-36°S); southward feeding migrations of 

 juveniles < 100-150 mm ML over the Patagonian Shelf; 

 a "shelf type dark zone within the statolith micro- 

 structure (Arkhipkin, 1993); fast juvenile growth but 

 rather slow growth of immature squid; medium sizes 

 at maturation (males at 160-220 mm ML, females at 

 180—240 mm ML); medium maximum sizes for mature 

 squid (males of 180-260 mm ML, females of 220-320 

 mm ML); and northward prespawning migrations over 

 the shelf The slope group of /. argentinus has an 

 oceanic-slope life cycle characterized by the following 

 features: slope spawning in the northern part of the 

 species area (27-36°S); southward feeding migrations 

 of juveniles < 100-150 mm ML in the open part of the 

 Argentine Basin; an "oceanic" type dark zone within 

 the statolith microstructure (Arkhipkin, 1993); slow 

 juvenile growth but rather fast growth of immature 

 squid; large sizes at maturation (males at 180-240 

 mm ML, females at 240—340 mm ML); large maximum 

 sizes for mature squid (240-340 mm ML, females up 

 to 280—400 mm ML); and northward prespawning 

 migrations over the continental slope. The taxonomic 

 status of the two groups of winter-spawned /. argenti- 

 nus remains unclear (Arkhipkin and Scherbich, 1991; 

 Parfeniuk et al., 1992; NigmatuUin and Laptikhovsky, 

 1996; Santos and Haimovici, 1997). 



Interannual changes in stock structure 



It has been shown that growth rates of/, argentinus 

 from the same hatching month vary to a lesser 

 extent between different years from those of the dif- 

 ferent months of hatching within one year (Arkhip- 

 kin and Laptikhovsky, 1994). Thus it is possible to 

 make comparisons of modal lengths of squid from 

 the same month of hatching but in different years. 

 The results of this study (based on data collected by 

 the trawl fishery in 1991) are somewhat different 

 from those obtained from the Japanese jigging fish- 

 ery in 1989-1990 (Uozumi and Shiba, 1993). Gener- 

 ally, during the same month and in the same region 

 of sampling, a majority of males and females caught 

 by jigs in 1989 were about a month younger and 

 correspondingly 20-30 mm smaller than those sam- 

 pled by the trawl fishery in 1991 (Figs. 8 and 9 in 

 Uozumi and Shiba, 1993; and Figs. 4 and 5 of the pres- 

 ent study). Unfortunately, there are no data on the 

 length-frequency composition of trawl-caught /. argen- 

 tinus in 1989 (Arkhipkin and Laptikhovsky, 1994), and 



