Environmental associations of shortfin squid Illex argentinus (Cephalopoda: Ommastrephidae) in the...

Fisheries Research 76 (2005) 401–416

Environmental associations of shortfin squidIllex argentinus (Cephalopoda: Ommastrephidae)

in the Northern Patagonian Shelf

Gaston Bazzinoa,∗, Renato A. Quinonesb, Walter Norbisa

a Direccion Nacional de Recursos Acuaticos (DINARA), Constituyente 1497, Montevideo, Uruguayb Centro de Investigacion Oceanografica en el Pacifıco Sur Oriental (COPAS) & Departamento de Oceanografıa,

Universidad de Concepcion, Casilla 160-C, Concepcion, Chile

Received 3 March 2005; received in revised form 19 July 2005; accepted 26 July 2005

Abstract

We identified environmental associations (depth, bottom temperature and bottom salinity) of the shortfin squidIllex argentinusin the Northern Patagonian Shelf. Data are from 15 research bottom trawl surveys conducted in the Argentine–UruguayanCommon Fishing Zone (AUCFZ) between 1985 and 1998. Squid distribution in the surveyed area showed significant associations(p < 0.05) mostly with depth and bottom temperature. Bathymetric associations of squid detected in March–April, May–June andJuly–August cruises revealed a preference of squid for deep depths (80–280 m, intermediate and external shelf) and cool bottomtemperatures (4–10◦C, subantarctic water) within the survey area. In contrast, the results observed during November–Decemberc ity of two-w etween thee nd bottomt porting thee ront.©

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ruises showed a preference of squid for shallower depths (70–90 m) and warmer bottom temperatures. The majoray environmental analyses showed significant associations, suggesting the existence of a strong correlation bnvironmental factors used in our analyses. The identification of significant associations between squid distribution a

emperature seem to be related to the presence of strong thermal gradients within the study area. This could be supxistence of an association betweenI. argentinus with the Brazil–Malvinas confluence front and also with the shelf break f2005 Elsevier B.V. All rights reserved.

eywords: Shortfin squid;Illex argentinus; Environmental associations; Northern Patagonian Shelf

∗ Corresponding author at: Centro de Investigaciones Biologicasel Noroeste (CIBNOR), Programa de Doctorado – Ecologia Pes-uera, Mar Bermejo 195, Colonia Playa Palo de Santa Rita, C.P.3090, La Paz, Baja California Sur (BCS), Mexico.el.: +52 612 1198998.

E-mail address: [email protected] (G. Bazzino).

1. Introduction

The shortfin squidIllex argentinus (Castellanos1960) is a neritic-oceanic species distributed althe continental shelf and slope of SouthwesAtlantic Ocean (SWAO) between latitudes 20◦S and55◦S, occurring more abundantly in the area 35–5◦S(Brunetti, 1988; Haimovici et al., 1998). It is the

165-7836/$ – see front matter © 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.fishres.2005.07.005

402 G. Bazzino et al. / Fisheries Research 76 (2005) 401–416

most important cephalopod species in the SWAO,not only because of its wide distribution and largeabundance (annual catches attained as much as500,000–750,000 ton;FAO, 1997), but for its ecolog-ical role in the ecosystem due to trophic interactions(Santos and Haimovici, 2000).

I. argentinus is an opportunistic species (strategyr; Brunetti, 1988) having rapid growth, a short lifecycle (annual approx.;Arkhipkin, 1990), opportunis-tic feeding (Ivanovic and Brunetti, 1994; Bazzinoand Quinones, 2001), and a high degree of intra-population differentiation (Brunetti, 1988; Nigmat-ullin, 1989; Arkhipkin, 1993, 2000). Furthermore, it isa species with seasonal migrations (Arkhipkin, 1993)from cold-water feeding areas (Malvinas/FalklandCurrent) to warmer spawning areas (Brazil Cur-rent). Some of these characteristics are relevantfor the management ofI. argentinus’ fishery inthe SWAO (Caddy, 1983; Bazzino and Quinones,1999).

Neritic-oceanic squid species (ommastrephids; e.g.,I. argentinus) tend to distribute themselves in relation tolarge ocean current systems (O’Dor and Coelho, 1993)and in relation to the fronts that are generated wherewater masses with different hydrographic characteris-tics converge (Caddy, 1983). In fact, the life cyclesof several squid species are carefully synchronizedwith the movements of the water masses they inhabit(Hatanaka et al., 1985; Rodhouse et al., 1992; Bakunand Csirke, 1998; Dawe et al., 2000; Anderson andR aleo nver-g ttie 8;W

O( ug-g bun-da s,dn cef al.,1 -i t ism ent( slo,2

The Northern Patagonian Shelf is a highly com-plex region from an oceanographic point of view.The circulation in the upper layers (0–2000 m) isdominated by the opposing flow of the Brazil Cur-rent and the Malvinas/Falkland Current (Peterson,1992). The Brazil Current (warm) moves southwardalong the edge of South America, whereas the Malv-inas/Falkland Current (cold) flows northward along thecontinental shelf, originating as a branch of the Antarc-tic Circumpolar Current (Peterson and Stramma,1991).

The Brazil and Malvinas/Falkland Currents meetaround the 33–39◦S area, causing an important ther-mohaline front that separates the subtropical watersfrom the subantarctic waters (Brazil–Malvinas Con-vergence Front;Legeckis and Gordon, 1982; Gordon,1989; Olson et al., 1988). Strong thermal and salinegradients can be observed in this convergence area,due to the interaction between both currents, but alsoto the influence of the Patagonian Current (also knownas the western branch of the Malvinas/Falkland Cur-rent) and the discharge of the Rıo de La Plata, whichgenerates high levels of primary production (Hubold,1980a,b; Haimovici et al., 1998). Productivity is fur-ther favored by another front, located over the shelfbreak that indicates the border between shelf waters(Patagonian Current and Rıo de La Plata discharge)and waters of Malvinas/Falkland Current (Bakun andParrish, 1991).

Considering the ecological and commercial impor-t plor-ih ae slo,2 per-s thed ec tionso

enso ianS ingg1 r-t otht ingfl ter( wn-

odhouse, 2001) and associated with the mesoscceanographic processes that are found in these coence zones or fronts (Rodhouse et al., 1996; Bertolot al., 1996; Gonzalez et al., 1997; Brunetti et al., 199aluda et al., 2001a).The variability of main current systems in SWA

Brazil and Malvinas/Falkland Current) has been sested as one of determining factors in the aance and distribution ofI. argentinus (Andersonnd Rodhouse, 2001). According to some authoruring spawning and first life stages,I. argenti-us is associated with Brazil–Malvinas confluenront (Hatanaka, 1988; Leta, 1992; Rodhouse et995; Haimovici et al., 1998), whereas dur

ng maturation, growth, and feeding stages, iostly associated with Malvinas/Falkland Curr

Rodhouse et al., 1995; Laptikhovsky and Reme001).

ance of this squid species in the SWAO, studies exng the environmental associations ofI. argentinusave been intensified (Brunetti et al., 1998; Waludt al., 1999, 2001a,b; Laptikhovsky and Reme001). Nevertheless, a high degree of uncertaintyists as to which environmental variables influenceistribution ofI. argentinus in certain stages of its lifycle, such as during the reproductive concentraver the Northern Patagonian Shelf.

According to the existing records, the specimf I. argentinus captured in the Northern Patagonhelf correspond mostly to the winter shelf spawnroup (Bonaerensis–North Patagonic stock;Brunetti,988; Leta, 1989). This stock is the most impo

ant from a fishing perspective, being caught by bhe Uruguayan and Argentine jigging and trawleets, principally during austral autumn and winApril–September). The presence of the spring spa

G. Bazzino et al. / Fisheries Research 76 (2005) 401–416 403

ing group and the summer spawning group in theNorthern Patagonian Shelf is only detected duringaustral spring (October–December) (Brunetti, 1981;Hatanaka, 1986).

In this context, the objective of this study is toidentify the environmental associations (depth, bottomtemperature and bottom salinity) of squidI. argenti-nus in the Northern Patagonian Shelf, specificallywithin the Argentine–Uruguayan Common FishingZone (AUCFZ).

2. Materials and methods

Squid catch and hydrographic data were obtainedfrom 15 research bottom trawl surveys carried outbetween 1985 and 1998 (Table 1) in the Northern Patag-onian Shelf within the Argentine–Uruguayan Com-mon Fishing Zone (AUCFZ, 34◦00′–39◦30′S; Fig. 1).The surveys were based on a stratified random design(Erhardt et al., 1977) with 24 strata defined by latitudeand depths intervals (Fig. 1).

Standard bottom trawls were performed in allcruises with an Engel trawl (100 mm mesh in the wingsand 60 mm mesh in the cod ends, 4 m vertical openingand 15 m horizontal aperture). Each tow lasted 30 min

Table 1R tago-n onF

C

MMM

MMM

JJJ

SSS

NND

of effective bottom trawling time at an average speed of3 knots. All tows were performed during daytime whenshortfin squid is located near ocean bottom. The catchof I. argentinus was recorded after each tow and thefollowing oceanographic parameters were registered:tow depth (m), bottom temperature (◦C) and bottomsalinity.

The depth (m) was estimated by averaging the start-ing and final tow depths. Bottom temperature and salin-ity were obtained from water samples collected withNansen bottles in those surveys performed within theperiod 1985–1989; and directly using a CTD in theperiod 1994–1998.

2.1. Spatial distribution of squid abundance

I. argentinus biomass estimates per strata were cal-culated in each research cruise through swept areamethod (Alverson and Pereyra, 1969). We classifiedstrata with high and low squid abundance in orderto analyze the spatial distribution within the AUCFZ.The strata with high squid abundance were thosewhose biomass was equal or greater than the aver-age biomass obtained with all the strata during eachcruise.

2.2. Identification of habitat associations

The method ofPerry and Smith (1994)was appliedto determine the existence of significant environmentala nS asso-c UE( dy,t idc -t thea -t

em ns:( on-m edw if-f eduref en-t h(

esearch bottom trawl surveys performed in the Northern Paian Shelf, specifically within the Argentine–Uruguayan Commishing Zone (AUCFZ), during the period 1985–1998

ruise Year

arch–April 1987arch 1988arch–April 1998

ay–June 1985ay–June 1986ay–June 1995

une–July 1988une–July 1994uly–August 1989

eptember 1986eptember–October 1988eptember–October 1994

ovember–December 1985ovember 1986ecember 1987

ssociations inI. argentinus over Northern Patagoniahelf. These authors developed a non-parametriciation test for an environmental variable and the CPcatch per unit of effort) of a given species. In our stuhe CPUE ofI. argentinus was represented by squatch per fishing tow (CPUEsquid= catch/30 min botom trawling). The environmental variables used innalyses were depth, bottom temperature (◦C) and bot

om salinity.The method ofPerry and Smith (1994)calculates th

aximum absolute difference between two functioi) the cumulative frequency distribution of the envirental variablef(t), and (ii) the same function weightith the squid CPUEg(t). The maximum absolute d

erence is used as a statistical test in a random procor evaluating the existence of significant environmal associations. For further details seePerry and Smit1994).


Fig. 1. Argentine–Uruguayan Common Fishing Zone (AUCFZ, 34◦00′–39◦30′S) with the 50, 80, 100, 200 and 400 m isobath contours. Numbersindicate the 24 strata considered in the sample design applied during research cruises.

Analyses of each environmental variable were car-ried out separately for all individual cruises and withthe pooled data for each season. Significant associa-tions (p < 0.05) provide an indicator of environmentalpreferences ofI. argentinus.

It is important to mention that some of the envi-ronmental variables are often correlated. For example,temperature decreases when depth increases. Thus,an association with a particular depth range couldbe reflecting an association with a certain temper-ature range. In such cases, two-way analyses wereperformed (two environmental variables at the sametime) with those environmental variables that weresignificantly associated (p < 0.05) during one-wayanalyses.

2.3. Oceanographic conditions in the study area

Bottom temperature–salinity scatter diagrams wereconstructed for each cruise in order to observethe influence of water masses in the study area:subtropical water (STW, temperature = 18◦C, salin-ity = 36; Boltovskoy, 1970); subantarctic water (SAW,temperature = 5◦C, salinity = 33.5;Boltovskoy, 1970);coastal water (CW, temperature = 17.5, salinity = 31.5;Severov, 1991). Tows with and without squid catchwere distinguished in each diagram to discern any rela-tionship between species availability with any of thewater masses.

Isothermal maps of the ocean bottom were madefor each cruise in order to examine horizontal distribu-


tion of bottom temperature (presence of strong thermalgradients).

3. Results

3.1. Spatial distribution of squid abundance

The spatial distribution of squid abundance withinthe AUCFZ revealed a considerable seasonal variabil-ity between the research cruises analyzed (Fig. 2).The strata showing high squid abundance duringMarch–April cruises were mainly located in the south-ern sector of the AUCFZ and at greater depths(100–400 m). In May–June cruises the strata withhigh squid abundance were located more northern inthe AUCFZ and at intermediate depths (80–200 m).This pattern became more evident in June–July andSeptember–October cruises. On the other hand, dur-ing November–December cruises those strata with highsquid abundance were found in the southern sector ofthe AUCFZ and at shallower depths (50–100 m).

3.2. Identification of habitat associations

March–April: significant associations (p < 0.05)were found for I. argentinus with depth and bot-tom temperature in all the analyses done with indi-

vidual March–April cruises and also with pooledMarch–April data (Table 2). On the other hand, theonly cruise that reflected a significant association withbottom salinity was March–April 1998.

Squid were strongly associated (p < 0.01) with: (1)deep depths, as 98% of the population occurred wherebottom depths were deeper than 80 m (mostly withinthe range 80–280 m,Fig. 3), which represented thedeepest 50% of the survey area; (2) cooler bottom tem-peratures, as 95% of the population occurred within therange 4.0–8.0◦C, at the lower 40% of available bottomtemperatures (Fig. 4).

The results obtained with the two-way analysesshowed a significant association between squid withthe simultaneous distribution of depth and bottom tem-perature in the study area (p < 0.05;Table 3).

May–June: analyses of May–June pooled datarevealed significant associations (p < 0.05) for I.argentinus with depth and bottom temperature(Table 2). However, these associations were not con-sistent in the individual cruises analyses. Only thecruise of May–June 1986 reflected significant associ-ations with all the environmental variables, includedbottom salinity. No significant environmental associ-ations were observed in the other May–June cruises(Table 2).

In those analyses that reflected significant environ-mental associations (May–June 1986 and May–June

F CFZ. B ndance.W

ig. 2. Spatial distribution ofI. argentinus biomass within the AUhite strata: no squid. For further details see Section2.

lack strata: high squid abundance. Grey strata: low squid abu


Fig. 3. Cumulative frequency distributions of observed depth (solid line) and squid CPUE weighted data (dashed line) from (A) March–April,(B) May–June, (C) June–August, (D) September–October and (E) November–December cruises.


Fig. 4. Cumulative frequency distributions of observed bottom temperature (solid line) and squid CPUE weighted data (dashed line) from (A)March–April, (B) May–June, (C) June–August, (D) September–October and (E) November–December cruises.


Table 2Test results ofPerry and Smith (1994)one-way analyses

Table entries are probability (p) values for having a test statistic, from the Monte Carlo simulation, equal or greater than the test statistic observed.* Significant environmental associations (p < 0.05).

pooled data), squid showed preferences for: (1) deepdepths, as 74% of the population occurred deeper than80 m (mostly within the ranges 80–200 and 360–400 m,Fig. 3), which represented the deepest 50% of the avail-able depths; (2) cooler bottom temperatures, as 80% ofthe squid occurred within the range 5–10.5◦C, at thecoldest 50% of available bottom temperatures (Fig. 4).

No significant associations were found betweensquid and the simultaneous distribution of depthand bottom temperature in the two-way analyses ofMay–June pooled data (Table 3). For this reason wecompared the results of one and two-way analyses.

During one-way analyses, the maximum absolute dif-ference between the functionsf(t) andg(t) was observedat 140 m in the case of depth and at 9.5◦C in the caseof bottom temperature. In two-way analyses, the max-imum absolute difference between both functions wasobserved at 140 m for depth, which coincided withthe one-way results, whereas the bottom temperaturechanged to 22.0◦C.

June–August: the analyses of June–August pooleddata revealed significant associations (p < 0.05) of I.argentinus with depth, bottom temperature and bottomsalinity (Table 2). However, these significant associ-


Table 3Test results ofPerry and Smith (1994)two-way analyses

Bottom temperature Bottom salinity

March–AprilDepth <0.01* –Bottom temperature –

May–JuneDepth 0.16 –Bottom temperature –

June–AugustDepth 0.07 <0.01*

Bottom temperature <0.01*

September–OctoberDepth – –Bottom temperature –

November–DecemberDepth <0.01* –Bottom temperature –

The analyses were performed with the pooled data and only withthose environmental variables that reflected significant associationsduring one-way analyses. Table entries are probability (p) values forhaving a test statistic, from the Monte Carlo simulation, equal orgreater than the test statistic observed.

* Significant environmental associations (p < 0.05).

ations were not consistent during the analyses withindividual cruises.

Squid were significantly associated with: (1) deepdepths, as 78% of the population occurred where bot-tom depths were deeper than 80 m (mostly within therange 80–150 m,Fig. 3), which represented the deepest50% of the survey area; (2) cooler bottom temperatures,as 87% of the squid population occurred within therange 5.5–9.5◦C, which represented the lower 45% ofavailable bottom temperatures (Fig. 4); (3) lower bot-tom salinities, as 91% of squid population occurredwithin the range 32.9–33.7 at the lower 50% of avail-able bottom salinities in study area (Fig. 5).

All two-way analyses performed with June–Augustpooled data registered strong associations (Table 3).Squid showed an effective association with the simul-taneous distribution of depth, bottom temperature andbottom salinity.

September–October: individual and pooled dataanalyses for September–October cruises did not regis-tered significant environmental associations (Table 2).

November–December: analyses of November–December pooled data revealed significant associations(p < 0.05) forI. argentinus with depth and bottom tem-

perature (Table 2). However, these significant associa-tions were not consistent with the individual analysesof November–December cruises.

Squid showed a strong association (p < 0.01) with:(1) shallow depths, as 97% of the population occurredwithin the range 70–90 m which represented the shal-lowest portion of the surveyed area (Fig. 3); (2)warmer bottom temperatures, as 80% of the populationoccurred within the range 9.5–12◦C which representedthe warmer 50% of available bottom temperatures inthe surveyed area (Fig. 4).

Two-way analyses performed with the November–December pooled data reflected significant associa-tions (p < 0.05) betweenI. argentinus with the simul-taneous distribution of depth and bottom temperature(Table 3). Both environmental variables were effec-tively associated with November–December squid dis-tribution in the study area.

Annual variability: environmental significant asso-ciations (p < 0.05) were found forI. argentinus withdepth and bottom temperature all across the yearexcept during September–October cruises (Table 2).The associations detected in March–April, May–Juneand July–August cruises revealed a preference of squidfor deep depths and cool bottom temperatures withinthe survey area. In contrast, the results obtained duringNovember–December cruises showed a preference ofsquid for shallower depths and warmer bottom temper-atures.

The majority of two-way analyses reflected signifi-c eousds ntala ses(

3

cteds them era-t easeo picalw andN sr towsw ass( nt).

ant associations between squid with the simultanistribution of environmental factors (Table 3). Thetronger (p < 0.01) and more consistent environmessociations were found during March–April cruiTable 2).

.3. Oceanographic conditions in the study area

Oceanographic conditions in the study area refleeasonal changes which were associated withovements of different water masses. Bottom temp

ure–salinity scatter diagrams reflected an incrf subantarctic water and a decrease of subtroater in the study area between March–Aprilovember–December cruises (Fig. 6). These diagram

evealed a close association between those fishingith positive squid catch and subantarctic water m

Malvinas/Falkland Current and Patagonian Curre


Fig. 5. Cumulative frequency distributions of observed bottom salinity (solid line) and squid CPUE weighted data (dashed line) from (A)March–April, (B) May–June, (C) June–August, (D) September–October and (E) November–December cruises.


Fig. 6. Bottom temperature–salinity scatter diagrams from (A) March–April, (B) May–June, (C) June–August, (D) September–October and (E)November–December cruises. SAW: subantarctic water; STW: subtropical water; CW: coastal water. Black symbols: fishing tows with squidcatch. Cross symbols: fishing tows without squid catch.


Fig. 7. Spatial distribution of bottom temperature (isotherms;◦C) in the surveyed area during the following cruises: (A) March 1988; (B)May–June 1986; (C) June–July 1988; (D) July–August 1989; (E) September–October 1988; (F) December 1987.


Bottom isothermal maps of some monthly cruisesshowed strong thermal gradients in the study area(March–April cruises,Fig. 7), reflecting the presenceof Brazil–Malvinas confluence front and shelf breakfront.

4. Discussion

The results obtained using the method ofPerry andSmith (1994)indicated significant environmental asso-ciations of I. argentinus in the Northern PatagonianShelf, principally with depth and bottom temperature.However, the majority of these associations were notconsistent across cruises carried out in the same sea-son but in different years. A possible explanation of thisinconsistency is that shortfin squid may have a broadtolerance to environmental factors within the availablerange in the study area, which correspond to a smallproportion of the overall geographic range of species.The same type of inconsistency was also observed byBrodziak and Hendrickson (1999)in the northwesternAtlantic Ocean for another squid species of the genusIllex (Illex illecebrosus).

Depth and bottom temperature were the environ-mental variables that registered the higher number(n = 11) of significant associations (p < 0.05) with squiddistribution. The influence of these environmental vari-ables was observed all around the year except inSeptember–October cruises. In contrast, the influencee mem por-t stentw losea

pthi sesr atedo m).A rvedw uidb itht ra-t tra-t lledB ;Bn er

(Brunetti, 1988; Leta, 1989). These concentrations aremade up of adult specimens that have initiated their sex-ual maturation or are sexually mature, a prerequisite foremigration toward spawning areas in deeper waters ofthe oceanic region outside the AUCFZ (Brunetti, 1988;Hatanaka, 1986, 1988).

On the other hand, bathymetric associations of squidduring November–December cruises showed prefer-ence for shallower depths (70–90 m) which coincideswith the bathymetric distribution of pre-reproductiveconcentrations of the summer spawning group andreproductive concentrations of the spring spawninggroup (Brunetti, 1981; Hatanaka, 1986). In fact, squidbiomasses during November–December cruises weremostly concentrated in those strata with depths between50 and 100 m.

The significant associations registered with bot-tom temperature in March–April, May–June andJuly–August cruises showed a preference of squidfor cooler temperatures (4–10◦C) included within therange known for the subantarctic water mass (Severov,1991). This suggests an association between squid dis-tribution and the influence of subantarctic water massin the study area. The existence of this link was alsodemonstrated with the results observed in the bottomtemperature–salinity scatter diagrams which revealeda close association between those fishing tows withpositive squid catch and the subantarctic water mass(Malvinas/Falkland Current and Patagonian Current).

The lack of significant environmental associationsd finals ingg iono

vedb ceo esses( -i t thes ents( nta-ta os-s tingt ausen oci-a ationb yses.

xerted by bottom salinity was only detected in soonths, mostly during June–July cruises. The im

ance of depth and bottom temperature is consiith demersal behavior of squid and reflects a cssociation with bottom hydrographic features.

The significant associations detected with den March–April, May–June, and July–August cruieflect a preference of squid for those regions locver the intermediate and external shelf (80–280similar depth preference range was also obse

ith the analyses of the spatial distribution of sqiomasses within the AUCFZ. This coincides w

he bathymetric distribution mentioned in the liteure for pre-reproductive and reproductive concenions of the winter shelf spawning group, also caonaerensis–North Patagonic stock (Brunetti, 1988runetti et al., 1997; Haimovici et al., 1998) observedorth of latitude 43◦S between April and Septemb

uring September–October maybe is reflecting thetages in the life cycle of the winter shelf spawnroup which is completing the reproductive migratutside the AUCFZ toward spawning areas.

Our results coincide partially with those obsery Waluda et al. (2001a), who analyzed the influenf bathymetry and mesoscale oceanographic procthermal gradients) onI. argentinus around the Malvnas/Falkland Islands. These authors found thaquid was directly associated with thermal gradimainly observed over the 200 m isobath, represeive of the shelf break front;Bertolotti et al., 1996),nd indirectly with depth. In our study, it was not pible to determine the environmental variable exerhe greatest influence on squid distribution becearly all two-way analyses showed significant asstions, suggesting the existence of a strong correletween the environmental factors used in our anal


Stronger and more consistent environmental associ-ations were detected during March–April cruises whichcoincide with the presence of intense thermal gradi-ents in the study area. These thermal gradients corre-sponded partially to the Brazil–Malvinas Confluencefront and partially to the shelf break front that indicatesthe limit between the shelf waters (Patagonian Currentand Rıo de La Plata discharge) and those of the Malv-inas/Falkland Current (Bakun and Parrish, 1991).

The association between shortfin squid with thepresence of oceanographic fronts in the study area(Brazil–Malvinas confluence front and shelf breakfront), could be related to higher food availability.According toOlson et al. (1994), frontal regions areareas of high nutrient concentration in which phy-toplankton, zooplankton, and nekton abundances canchange significantly due to greater productivity andprey availability. Feeding and spawning concentrationsof commercially important oceanic species are com-monly seen at frontal boundaries.

A better understanding of the influence of physi-cal oceanographic processes onI. argentinus’ distri-bution would be considered very useful for the man-agement of its fishery in the Southwestern AtlanticOcean (SWAO). Considering the annual nature ofI.argentinus life cycle, the high interannual abundancevariability and the weak stock–recruit relationships, itis very difficult to predict the recruitment strength ofthis species (Csirke, 1987; Basson et al., 1996; Bakunand Csirke, 1998). For an adequate management of thist t ofe ncep a,b;R en-c tionw s asi iona orthP

ncew ex-p ra iron-m oa a-tw eda n the

Malvinas/Falkland Islands and the Argentine ExclusiveZone suffered an early closure in 2004 and 2005.

Acknowledgements

We express our appreciation to the authorities of theDireccion Nacional de Recursos Acuaticos (DINARA;Montevideo, Uruguay) for providing the data used inthis study. Also, we thank the CIPO/IAI project “Con-sorcio para la Investigacion del Cambio Global delPacıfico Oriental” (EPCOR-IAI, Eastern Pacific Con-sortium of the InterAmerican Institute for Research onGlobal change) for partially financing this study.

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B astal

48,

B tments.),e6.

B Pur-ent ofst

Be-na

BmSpan-

ype of resource, it is very useful to know the effecnvironmental variables on distribution and abundaatterns (Caddy, 1983; Waluda et al., 1999, 2001odhouse, 2001). For example, Japanese fishery agies use oceanographic information in combinaith pre-recruit and recruit abundance estimate

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