Arkhipkin: Intrapopulation structure of ///ex argentinus during its feeding period over the Patagonian Shelf 



and from 190 to 660 m in April. The duration of trawls 

 ranged from 4 to 8 hours, and the average towing 

 speed was 6-8 km/h. 



Every ten days, average daily catch per unit of effort 

 (CPUE) was calculated as a mean of daily CPUEs 

 of the Soviet fishing trawlers that caught squid in 

 each fishery region. Mean CPUEs were calculated 

 separately for the fishery region of 45-47°S outside 

 the EEZA (where a majority of fishing vessels are 

 2700-GRT trawlers) and for the fisheiy regions within 

 the EEZA (where all fishing vessels were 4000-GRT 

 trawlers). To obtain an objective picture of the squid 

 fisheiy, CPUEs of the 2700 GRT trawlers may be 

 adjusted to those of 4000-GRT trawlers by a coeffi- 

 cient of 0.7 (Arkhipkin, 1993). 



Length-frequency sampling 



A random sample of one hundred squid was taken 

 by scientific observers from each of two catches (at 

 night and day) everyday on board each of the four ves- 

 sels. Dorsal mantle length (ML) was measured to the 

 nearest 1 mm, total body weight (BW) was weighed 

 to the nearest 1 g. Sex and maturity stages were iden- 

 tified according to the maturity scale elaborated for 

 Illex argentinus (Nigmatullin, 1989a). Sex ratio was 

 determined. A total of 16,436 squid were analyzed in 

 the fishery region of 45^7°S outside the EEZA and 

 9893 squid were analyzed within the EEZA. Every 

 ten days, three length-frequency curves of males and 

 females were constructed for three maturity periods; 

 immature ( maturity stages 1-2 ), maturing ( maturity 

 stages 3-5 J ) and mature (maturity stages 5.,-5g). 



Age sampling and statolith processing 



Eveiy ten days, from January to April, statoliths were 

 dissected from 100-150 individuals of Illex argenti- 

 nus from two successful catches on board each of 

 the four vessels. The length-frequency distribution 

 of the 10-day age sample was proportional to the 

 length-frequency distribution of squid caught during 

 these ten days. Statoliths were washed in distilled 

 water and stored in oil-paper envelopes in 969f eth- 

 anol. A total of 1700 statoliths were sampled in the 

 fishery region of 45^7°S outside the EEZA, and 1150 

 statoliths were collected within the EEZA. 



All statoliths sampled were processed by statolith 

 aging techniques in the Laboratory of Commercial 

 Invertebrates ofAtlantNIRO( Arkhipkin, 1991). Stato- 

 hth terminology follows Clarke (1978) and Lipinski 

 et al. (1991). Statoliths were attached to the micro- 

 scopic slides with Pro-texx mounting medium and 

 were ground on both sides on a wet waterproof sand- 

 paper of 1000-grit gi-ade. During grinding, the stato- 



lith rostrum was completely removed, so that growth 

 increments could be easily distinguished from the 

 nucleus to the edge of the dorsal dome. Ground stato- 

 liths were embedded in Canada balsam and covered 

 with glass covers. Ready preparations were placed in 

 an oven at 90-100°C for one hour to dry the balsam 

 and improve the readability of growth increments. 

 Statoliths were read under a Biolam Rl light micro- 

 scope at 450-500X magnification. To avoid possible 

 counting errors, each statolith was counted twice by 

 two observers using the gradation of an eye-piece 

 micrometer The total number of growth increments 

 for each specimen was obtained as a mean of these rep- 

 licate counts if the deviation between the two counts 

 was less than 57(. If deviation exceeded 5%, the stato- 

 lith was recounted by the two observers once more. If 

 such deviation did not decrease after the recounting, 

 the statolith was rejected from further analysis. From 

 the whole sample, 1597 statoliths fi-om the region outside 

 the EEZA (93.97^) and 1067 statoliths from the region 

 within the EEZA (92.7% ) were prepared and read. 



Length-at-age data analysis 



Deposition of putative growth increments within 

 /. argentinus statoliths has not yet been validated. 

 However, incorporation of either tetracycline or stron- 

 tium marks into statoliths of the congeneric species I. 

 illecebrosus kept in captivity has shown that growth 

 increments are formed daily ( Dawe et al., 1985). Stato- 

 lith microstioicture in both species is similar; therefore 

 growth increments within statoliths of/, argentinus 

 are considered to form daily in the present paper. 

 Hence, their total number was considered to repre- 

 sent squid age in days. Hatching dates were backcal- 

 culated. Month classes of hatching were defined by- 

 pooling squid into each month of hatching ( Ai'khipkin, 

 1990; Rodhouse and Hatfield, 1990). Length-at-age 

 data were analyzed separately for both sexes. The 

 10-day age stnacture was determined by construction 

 of age-length keys. Age-length keys were constructed 

 by using numbers of squid for each month class sepa- 

 rately for each sex and maturity period (Arkhipkin et 

 al., 1996). 



Results 



CPUE dynamics 



Region 45-47°S Illex argentinus were caught in all 

 trawls of the research vessels (Fig. 2). In January, 

 schools of squid aggregated mainly north of the region. 

 Trawl catches were variable ( from 1 to 20 t per vessel 

 day, \Jd ), and mean January CPUEs were low (8-9 t/d ). 



