Abstract.— Stock structure dynamics of 

 the important commercial squid Illcx 

 argentinus were studied by using bio- 

 logical data from about 25 thousand 

 squid caught January-April 1991 by Rus- 

 sian trawlers in three fishery regions: 

 51-52°S; 47-49°S within the exclusive 

 economic zone of Argentina (EEZA); and 

 45-47°S outside the EEZA. A total of 

 2664 statoliths were read to prepare 

 age-length keys for each 10-day interval 

 of the period studied. It was found that 

 between January and April, the Patago- 

 nian shelf south of 45°S was a feeding 

 ground of two intraspecific groups of 

 winter-hatched /. argentinus: a shelf 

 group that matured at medium sizes 

 (ShG) and a slope group that matured 

 at large sizes (SIG). After massive immi- 

 gration of /. argentinus from the north 

 in January-February into the two fish- 

 ery regions within 45-49°S, the stock 

 structure remained rather stable until 

 April, composed predominantly of June- 

 and July-hatched squid. Squid grow and 

 mature rapidly, and males mature at 

 younger ages (from one to two months) 

 than do females. During feeding, some 

 redistribution of the stock was observed: 

 maturing and mature SIG squid (mainly 

 females) tended to shift ft-om the shelf 

 (130-150 m depth) in a northeast direc- 

 tion and concentrate over the shelf edge 

 (160-170 m depth). In April, mature SIG 

 squid began to shift to the continental 

 slope around 45-47°S and migrated to 

 depths >600 m where they then mixed 

 with schools of SIG squid that had fed 

 in the region 51-52°S and that were 

 already migrating northwards along the 

 slope. ShG squid remained on the shelf 

 and made their prespawning northward 

 migrations along the shelf edge. 



Intrapopulation structure of winter-spawned 

 Argentine shortfin squid, ///ex argentinus 

 (Cephalopoda, Ommastrephidae), during its 

 feeding period over the Patagonian Shelf 



Alexander I. Arkhipkin 



Atlantic Research Institute ol Marine Rshenes and Oceanography (AtlantNIRO), 



5 Dm. Donskoy street Kaliningrad, 236000 Russia 



Present address, Fishenes Department, Falkland Islands Government 



PO Box 598, Stanley, Falkland Islands 

 E-mail address fish figiaihonzon.co.fk 



Manuscript accepted 13 Julv 1999. 

 Fish. Bull. 98:1-13(2000). 



The Argentine shortfin squid, Illex 

 argentinus (de Castellanos, 1960), 

 is a common neritic species occurring 

 in waters ofFBrazil, Uruguay, Argen- 

 tina, and the Falkland Islands in 

 the Southwest Atlantic (Nesis, 1987). 

 This squid is an important world fish- 

 ery resource. According to the FAO 

 (1997), since 1978, its total annual 

 catch has varied firom 180 to 250 

 thousand metric tons (t), achieving 

 300-330 thousand tons in 1993-95. 

 However, actual total annual catch 

 of/, argentinus could reach up to 700 

 thousand tons (Uozumi and Shiba, 

 1993). Illex argentinus is captured 

 by the international fleet consisting 

 of both jigging light vessels (mainly 

 fi-om Asian countries) and trawlers 

 (mostly fi-om European countries: 

 Poland, Spain, and Russia (former 

 USSR) in two fishery regions off 

 the Argentine Exclusive Economic 

 Zone (EEZA): at 42°S and 45-47°S. 

 In the 1970s and 1990s, /. argen- 

 tinus was also caught in consider- 

 able numbers within the EEZA and 

 Falkland Islands Interim Conserva- 

 tion Zone (FICZ) (Csirke, 1987; FAO, 

 1997 ). Such an extensive fishery has 

 induced detailed studies of different 

 biological aspects of /. argentinus 

 in order to monitor and forecast its 

 stock structure dynamics. 



Originally, /. argentinus was con- 

 sidered to be a single stock (Sato 

 and Hatanaka, 1983; Csirke, 1987). 

 Then it was found that the species 



consisted of two populations differ- 

 ing both by season and place of 

 their spawning: an abundant winter- 

 spawning population (more than 95% 

 of the total stock) and a small sum- 

 mer-spawning population (Hatanaka 

 et al., 1985; Hatanaka, 1988). Bru- 

 netti ( 1988) divided winter-spawning 

 squid into two stocks: the bonaerensis 

 north Patagonian stock (BNPS) and 

 the south Patagonian stock (SPS), dif- 

 fering both by feeding groimds and size 

 of adults (medium and large, respec- 

 tively). On the basis of occurrence of 

 mature females in different seasons, 

 Nigmatullin (1989a) revealed that 

 /. argentinus spawn throughout the 

 year, and proposed to subdivide the 

 total stock into four seasonal spawning 

 groups. Although Tsygankov (1987) 

 found qualitative differences in three 

 loci of esterases extracted from the 

 buccal muscles of various intrapopu- 

 lational groups of 7. argentinus, the 

 taxonomic status of these groups, 

 however, still remained imclear. Anal- 

 ysis of dynamics in length-frequency 

 compositions showed that comple- 

 tion of the life cycle of/, argentinus 

 populations took one year (Hatanaka 

 et al., 1985; Hatanaka, 1986) and 

 this length of time was then con- 

 firmed by statolith aging investiga- 

 tions (Arkhipkin, 1990; Rodhouse 

 and Hatfield, 1990). 



The life cycle of the most abundant 

 winter-spawned group can be subdi- 

 vided into five stages: a postlarval 



