Taro Ichii,2004.Differing Body Size Between the Autumn and the Winter Spring

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Differing body size between the autumn and the winterspringcohorts of neon flying squid (Ommastrephes bartramii)related to the oceanographic regime in the North Pacific: a

hypothesis

TARO ICHII,1,* KEDARNATH MAHAPATRA,2

MITSUO SAKAI,1 DENZO INAGAKE1 ANDYOSHIHIRO OKADA2,3

1National Research Institute of Far Seas Fisheries, 5-7-1 Orido,

Shimizu, Shizuoka 424-8633, Japan2Tokai University Frontier Ocean Research Center (T-FORCE),

3-20-1, Orido, Shimizu, Shizuoka 424-8610, Japan3School of Marine Science and Technology, Tokai University,

3-20-1, Orido, Shimizu, Shizuoka 424-8610, Japan

ABSTRACT

The neon flying squid (Ommastrephes bartramii), whichis the target of an important North Pacific fishery, iscomprised of an autumn and winterspring cohort.During summer, there is a clear separation of mantlelength (ML) between the autumn (ML range: 3846 cm) and the winterspring cohorts (ML range: 1628 cm) despite their apparently contiguous hatching

periods. We examined oceanic conditions associatedwith spawning/nursery and northward migration hab-itats of the two different-sized cohorts. The seasonalmeridional movement of the sea surface temperature(SST) range at which spawning is thought to occur(2125C) indicates that the spawning ground occursfarther north during autumn (2834N) than winterspring (2028N). The autumn spawning groundcoincides with the Subtropical Frontal Zone (STFZ),characterized by enhanced productivity in winterbecause of its close proximity to the Transition ZoneChlorophyll Front (TZCF), which move south to the

STFZ from the Subarctic Boundary. Hence this area isthought to become a food-rich nursery ground inwinter. The winterspring spawning ground, on theother hand, coincides with the Subtropical Domain,which is less productive throughout the year.Furthermore, as the TZCF and SST front migrate

northward in spring and summer, the autumn cohorthas the advantage of being in the SST front andproductive area north of the chlorophyll front,whereas the winterspring cohort remains to the southin a less productive area. Thus, the autumn cohort canutilize a food-rich habitat from winter through sum-mer, which, we hypothesize, causes its members togrow larger than those in the winterspring cohort insummer.

Key words: neon flying squid, North Pacific,Ommastrephes bartramii, satellite remote sensing,Subtropical Frontal Zone, Transition ZoneChlorophyll Front.

INTRODUCTION

The neon flying squid, Ommastrephes bartramii, isan oceanic squid occurring worldwide in subtropical

and temperate waters (Roper et al., 1984). In theNorth Pacific, this species plays an important role inthe pelagic ecosystem and is an international fish-eries resource with high commercial value (Seki,1993).

The neon flying squid migrates between itsspawning grounds in subtropical waters and feedinggrounds in Subarctic waters (Murata and Hayase,1993). Recently, Polovina et al. (2000, 2001) reportedthat the Transition Zone Chlorophyll Front (TZCF)migrates seasonally between about 30N in winter and40N in summer in the north Pacific. The TZCF marksthe transition from waters with a low near-surfacechlorophyll a (chl a) concentration (0.1 mg m)3) inthe south to waters with higher chl a concentrations(0.2 mg m)3) in the north and can be readily mon-itored by ocean color remote sensing. Albacore tuna(Thunnus alalunga) and loggerhead sea turtles (Carettacaretta) have been shown to exploit the TZCF as amigration pathway and foraging habitat (Polovinaet al., 2000, 2001), however, to date it is not clear if orhow the life cycle of neon flying squid is associatedwith the TZCF.

*Correspondence. e-mail: [email protected]

Received 27 March 2003

Revised version accepted 29 September 2003

FISHERIES OCEANOGRAPHY Fish. Oceanogr. 13:5, 295309, 2004

2004 Blackwell Publishing Ltd. 295


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Neon flying squid in the North Pacific comprise foursize classes, i.e. extra large (LL), large (L), small (S) andextra small (SS) groups, based on monthly mantlelength distribution and maturity stages (Fig. 1), asreported by Murata (1990); Yatsu et al. (1997) andMurakami et al. (1981). Unlike the other groups, theLL group consists only of females. During spring andsummer, there is a significant difference in body sizebetween the LL group and the others. For this reason,the LL group was initially believed to live for 2 yr,

maturing and spawning in the second year, in contrastto the 1-yr life span of the L and S groups. The SS groupwas even thought to correspond to the first year of theLL group (Murata, 1990; Sinclair, 1991). However,recent age estimations from statolith microstructurehave shown that all groups have a 1-yr life span, the LLgroup being an autumn cohort that hatches in Sep-tember to early January, and the other groups (L, S, SS)forming a winterspring cohort that hatches in Januaryto July (Yatsu et al., 1997). This poses the question asto why such a clear separation of modes in mantle-length composition occurs between the autumn andwinterspring cohort despite their contiguous hatchingperiods (Yatsu et al., 1997, 1998). The size separationcould be caused by differences in growth or simplybecause of separated hatching periods of the two co-horts. Recently Chen and Chiu (2003) demonstratedthat the autumn cohort grows faster than the wintercohort, based on intensive examination of statolithsincrements. However, there is no report to support thelatter possibility. Hence, the size separation can beattributed to the growth difference between twocohorts.

Recently, the spawning locations of the two cohortswere found to differ geographically (Mori et al., 1999).Paralarvae occur at a sea surface temperature (SST)range of about 2125C (Bower, 1994; Mori et al.,1999), suggesting that this is the temperature range atwhich this species spawns. Based on the seasonalmovement of this SST range, Mori et al. (1999) sug-gested that the autumn cohort spawns farther north(2834N) than the winterspring cohort (2028N).A seasonal shift in the presumed spawning grounds

can be seen in Fig. 2 based on the data sets used inMori et al. (1999). In the present study, we examinedthe oceanographic regime in this region to investigatepossible causes for the size difference between the twocohorts.

MATERIALS AND METHODS

Data were collected from the spawning grounds, alongthe northward migration route and at the northernfeeding grounds, to examine the influence of oceano-graphic variables on these habitats of the neon flyingsquid. Data were collected from October 1997 throughAugust 1998, when relevant research and fisheryoperations were undertaken.

Paralarvae survey

The distribution and abundance of paralarvae in thespawning ground were examined from 15 October to 7

November 1997 along five latitudinal transects (156,158, 160, 162, and 164W between c. 28 and 37N)at 90-nautical mile intervals. As paralarvae of thisspecies are distributed near the sea surface throughout

Figure 1. Neon flying squid mantle length variability by month for four putative cohorts (data from Murata, 1990). LL: extra

large size group; L: large size group; S: small size group; SS: extra small size group.

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the day (Young and Hirota, 1990; Saito and Kubodera,1993; Saito, 1994; Bower, 1996), horizontal surfacetowing was conducted using a ring net (diameter 2 m,

mesh size 526 lm) at a speed ofca. 2 knots for 10 min.Two tows were carried out at each station. Collec-ted paralarvae of the family Ommastrephidae were

Figure 2. Distribution of paralarval neon flying squid in relation to SST during October, November, February and May.

Densities of paralarvae were standardized as numbers of individuals per 20-min surface tow using a ring net (diameter 2 m). The

SST range of preferred spawning (2125C; Bower, 1994; Mori et al., 1999) is depicted by bold contours. Paralarval data were

pooled over a period of 9 yr (19932001) from the data sets used by Mori et al. (1999). The SST data source is the Reynolds

climatological SST database (ftp://ncardata.ucar.edu/datasets/ds277.0/data/oi/mnly/data/).

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identified to species. A CTD cast was made at eachtowing station.

Drift net survey dataCatch data from drift net surveys by the NationalResearch Institute of Far Seas Fisheries (NRIFSF) andHokkaido University (HU) (Anonymous, 1999a,b)were used to investigate the northward migration ofneon flying squid. The NRIFSF survey was conductedalong 17000E and 17530E between 35 and 41Nduring mid- to late-May, 1998, and the HU survey wasconducted along 17530E between 40 and 4730Nduring early August 1998. At each drift net site, 49 or50 net panels were deployed in the evening andretrieved at the following sunrise. Each panel was50 m long and 7 m deep. Research drift nets used inthe NRIFSF survey comprised 20 commercial meshnets (stretched mesh size: 115 mm) and 30 non-size-selective nets [mesh size range (MSR) 48157 mm].Drift nets used in the HU survey comprised 12 com-mercial mesh nets (MSR 112118 mm) and 37non-size-selective nets (MSR 19157 mm). Thedorsal mantle length of each neon flying squid wasmeasured to the nearest 1 cm. For those caught in the

NRIFSF survey, sex and maturity stages were alsodetermined.

Fishery data

Japanese commercial jigging vessels used in the neon

flying squid fishery ranged in size between 96 and494 tonnes, and were equipped with about 20100automatic jigging machines with double reels. Toexamine the northward migration of neon flyingsquid, monthly catches derived from commercialfishery jigging data were calculated for each 1 1grid between 30 and 50N, and between 160Eand 150W from May through August 1998. Atotal of 140 fishing vessels operated during this sea-son.

Satellite image analyses

Satellite-derived data were used to investigate SST andchl a concentrations at the spawning grounds duringSeptember 1997 to August 1998 and in the northernhabitats during May to August 1998. Monthly averageSST data were compiled from the Advanced Very HighResolution Radiometer (AVHRR) Pathfinder globaldata sets produced by the National Oceanic andAtmospheric Administration and NASA (Vazquezet al., 1998). Sea-viewing Wide Field-of-view Sensor(SeaWiFS) version 4 space-time binned (level 3)monthly average surface chl a concentration data were

Figure 3. Distribution of paralarval neon flying squid in relation to (a) SST (C) and (b) surface salinity, during 15 October to 7

November 1997. The boundaries of the Subtropical Frontal Zone (34.6 and 35.2) are shown by the bold salinity contours. For

paralarval densities see legend to Fig. 2.

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derived using the global OC4v4 algorithm (OReillyet al., 1998). These products met the 35% accuracygoal fixed for SeaWiFS-derived chl a concentrationsover a limited, but diverse, set of open ocean validationsites (Hooker and McClain, 2000).

To depict the distribution of SST during mid- tolate-May and early August 1998, when the drift netsurveys were conducted, we estimated the average SSTthat occurred during May 18 to June 2, 1998 and July29 to August 13, 1998 using the 8-day average AVHRRPathfinder global SST data sets available during theseperiods. Likewise, average chl a concentration dataduring mid- to late-May and early August were derivedusing the space-time binned, 8-day average SeaWiFSchl a data available for May 9 to June 1, 1998 and July20 to August 12, 1998, respectively.

The SST and chl a data sets were used with spatialresolutions of 0.263 of both latitude and longitude for

generating contour lines. Contour lines representingthermal and chl a densities across the study area werecomputed using a contour plot subroutine of theGeneric Mapping Tools (GMT) software package.

RESULTS

Distribution and abundance of paralarval neon flying squidin autumn

Paralarvae of neon flying squid collected in autumnwere distributed at SST of 2225C (Fig. 3a). Withinthis SST range, paralarvae tended to be abundant

( 10 squid/20-min tow) near the surface salinity front,i.e. the Subtropical Frontal Zone (STFZ) defined as thezone occurring between 34.6 and 35.2 isohalinesaccording to Roden (1991), between ca. 29 and 34N(Fig. 3b). The northsouth cross sections along 160,162 and 164W of salinity and temperature indicatedthat cooler and less saline water from the north sankbelow the warmer and higher saline water to the southin the STFZ (Fig. 4). Furthermore, surface salinityfronts extended to ca. 50-m depth. The proximity ofparalarvae abundance to the front location suggeststhat the autumn spawning ground was associated with

the STFZ.

Oceanographic conditions of spawning/nursery groundsfor the autumn and winterspring cohorts of neon flyingsquid

Monthly surface chl a and SST measured by satellitefrom September 1997 through August 1998 showseasonal changes in oceanographic conditions at thespawning ground (Fig. 5a,b). The autumn cohortspawns in September to early January, and the winter

spring cohort spawns in January to August (Yatsuet al., 1997). Assuming that spawning occurs at 2125C, the spawning area will shift southward duringSeptember to February ahead of the southward-moving TZCF and then shift northward during March

to August behind the northward-moving TZCF. Thus,at no time of the year does the spawning ground occurin the high chl a area.

During winter (January to March), the TZCF waslocated in the southernmost position of the year(Fig. 5a,b), enhancing surface chl a concentrations inthe STFZ (2934N). Hence, juveniles of the autumncohort, which are believed to be distributed in theSTFZ, will experience enhanced chl a. However,juveniles of the winterspring cohort never experienceenhanced chl a levels because they are believed to bedistributed at 2028N (i.e. the Subtropical Domain)where surface chl a was low (< 0.10.2 mg m)3)

throughout the year. Thus, the surface chl a levels inthe nursery grounds differ between the autumn andwinterspring cohorts.

Northward migration habitats of neon flying squidin spring and summer

Monthly distributions of neon flying squid catchesfrom the jigging fishery are shown in relation tovarious oceanographic fronts from May to August(Fig. 6). Jigging vessels targeted the autumn cohortduring these months, because larger individuals (theautumn cohort) have higher commercial value than

smaller ones (the winterspring cohort). Over time,the fishing areas shifted northward, reaching theSubarctic Boundary in June and the Subarctic Front inAugust. Compared with the monthly location of theTZCF, substantial catches were taken north of thechlorophyll front in areas where surface chl a con-centrations were higher than 0.20.3 mg m)3. Theseasonal northward shift of fishing areas also coincidedwith the northward shift of SST ranging from 12 to18C (the SST front).

Mantle length (ML) compositions of neon flyingsquid caught by non-size-selective drift net surveysshows that larger individuals were distributed farthernorth than smaller individuals (Fig. 7). Mature indi-viduals caught in May were considered to be southwardmigrating members of the winterspring cohort, so theywere excluded from the following analysis. Individualswere divided into the northward migrating autumn(i.e. ML 30 cm for May and 34 cm for August) andwinterspring cohorts (ML < 30 cm for May and


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Figure

4.

Densityofparalarvalneonflyingsquidinrelationtonorthsouthsectionsofsalinityandtemperature(C)alongthreelatitudinaltransects(a)1

64W,(b)162W

and(c)160W,during15Octobe

rto7November1997.TheSubtropicalFrontalZonecorrespondstotheareabetw

eentheboldsalinitycontours34.6and3

5.2atthesurface.

HorizontalbarsatthetopindicatetheSubtropicalFrontalZone..,

m:

stationlocations.ForparalarvaldensitiesseeFig.2.

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(a)

Figure 5. (a) Monthly satellite images of SeaWiFS chl a (mg m)3) (left panels) and AVHRR-derived SST (C) (right panels),

during September 1997 to February 1998. Bold line in left panels indicates the Transition Zone Chlorophyll Front. SST of

spawning preference (2125C) is depicted by bold contours. (b) Monthly satellite images of SeaWiFS chl a (mg m)3) (left

panels) and Advanced Very High Resolution Radiometer (AVHRR)-derived SST ( C) (right panels), during March to August

1998. Bold line in left panels indicates the Transition Zone Chlorophyll Front. SST of spawning preference (2125 C) is

depicted by bold contours.




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caught in May suggests that a substantial part of thewinterspring cohort was distributed south of the sur-vey area at that time. Catch compositions of the driftnet surveys indicated that the autumn and winter

spring cohorts were separated by the TZCF during bothMay and August (Fig. 8); the winterspring cohortoccurred in low surface chl a ( 0.2 mg m)3) areas andthe autumn cohort occurred in high chl a areas

(b)

Figure 5. Continued.

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Figure

6.

Monthlydistribution

ofneonflyingsquidcatches(tonnes)fromtheJapanesejiggingfisheryforeach1latitude

1longitudeinrelationt

oSeaWiFSchla

(mgm)

3)(leftpanels),satellite-

derivedSST(C)(middlepanels)andw

atertemperatureat120mdepth(C)(rightpanels),duringMaytoAugust1998

.Theboldlinein

theleftpanelsindicatestheTra

nsitionZoneChlorophyllFront.The4and8Cisothermsat120mdepthinth

erightpanelsindicatetheSubarcticFrontandSubarctic

Boundary,respectively(Uehara

,1992).Temperaturedataat120marefromtheJointEnvironmentalDataAnalysisCenter(http://jedac.ucsd.edu/D

ATA_I

MAGES/

index.html).




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(> 0.2 mg m)3) during their northward migration. Thetwo cohorts were also separated by a zone with an SST

range of 1618C. While the autumn cohort occurredin association with the SST front, the winterspringcohort occurred mainly south of it.

DISCUSSION

Oceanographic factors related to the formation of neonflying squid spawning grounds in autumn

Neon flying squid paralarvae were abundant in theSTFZ within the SST range of spawning preference

between 21 and 25C in autumn (Figs 3 and 4). In asurvey undertaken by the first author in autumn 2001

in the same area, paralarvae were abundant in theSTFZ within the same SST range. Ishino (1975) sug-gested that oceanic fronts play an important role in theformation of fish concentrations in three ways: (i) byforming a barrier to fish movement, (ii) through theaccumulation of surface drifting food, and (iii) bymaintaining high productivity. Strong shear betweenthe STFZ and Subtropical Domain (Seki et al., 2002)may have acted as a barrier to movement, facilitatingthe accumulation of spawning squid.

Figure 7. Mantle length composition of neon flying squid caught with non-selective drift nets at each station along (a)

17000E and 17530E in mid- to late-May and (b) 17530E in early August, 1998. Combined frequencies from all stations

during the respective periods are shown at the bottom. MI: immature males; MM: mature males; FNC; non-copulated females;

FC: copulated females.

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Spawned egg masses of ommastrephid squids arepresumed to be suspended around the depth of thepycnocline (thermo- and/or haloclines) and requirewater temperatures 14C for successful embryonicdevelopment (ODor et al., 1982; ODor andBalch,1985; Sakurai et al., 1996; Sakurai, 2002).Oceanographic conditions in the STFZ metthe temperature requirement for embryonic survival,as temperatures in the pycnocline were well above14C (Fig. 4).

The STFZ occurs in a region between the westerly(to the south) and the easterly (to the north) tradewinds, so it is a zone of Ekman transport convergencein the central north Pacific (Roden, 1975). Driftersreleased in the STFZ were reported to remain nearthe region (Roden, 1991), suggesting paralarvae of the

autumn cohort that hatch here will remain in thezone, which becomes productive but colder duringwinter.

Contrasts in productivity of the spawning/nursery groundsfor the autumn and winterspring cohorts of neon flying

squid

The STFZ spawning ground of the autumn cohort wascharacterized by enhanced surface chl a concentra-tions in winter because of the proximity of the TZCF,which moved southward to the STFZ in winter fromits summer location at the Subarctic Boundary(Fig. 5a,b). Recent works have identified two prom-inent winter features of the STFZ, the SubtropicalFront (STF) and South Subtropical Front (SSTF)(Polovina et al., 2000; Leonard et al., 2001; Seki et al.,

Figure 8. Distribution of neon flying squid CPUE (number of individuals per 30 non-size-selective drift nets) in relation to

SeaWiFS chl a (mg m)3) (left panels) and AVHRR-derived SST (C) (right panels) for mid- to late-May and early August

1998. Autumn and winterspring cohorts shown by black and white circles, respectively. Definitions of the cohorts are indicated

in text. * in upper panels indicates the jigging station with no squid catch. Bold line in left panels indicates the Transition Zone

Chlorophyll Front.




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2002). The STF occurs near the 17C SST isothermand coincides with the TZCF, and the SSTF occursnear the 21C SST and coincides with an increase insubsurface chl a concentration. Thus, integrated chl a

concentration in the water column is enhanced in theSTFZ during winter (Leonard et al., 2001; Seki et al.,2002).

The STFZ becomes a good fishing ground forswordfish (Xiphias gladius) during winter and spring(Seki et al., 2002). Research surveys suggest that itsmain prey, i.e. neon flying squid, is most abundant inthe STFZ, although the spawning ground of the squidat this time of year occurs farther south (Young andHirota, 1999; Seki et al., 2002). Furthermore, stomachcontent data for neon flying squid suggest that thenumber of fish eaten (estimated from otoliths and eyelenses) increases near the STFZ (Young and Hirota,1999). Therefore, the increased biological productivityassociated with the STFZ in winter may provide theframework for a food-rich nursery ground for juvenilesof the autumn cohort.

On the contrary, the spawning ground of the winterspring cohort, i.e. the Subtropical Domain, is less pro-ductive throughout the year (Fig. 5a,b). The stablesubtropical surface layer that generally occurs in theupper 100 m (Roden, 1991) is the principal forceinhibitingthe vertical fluxof nutrients into theeuphotic

zone (Mann andLazier, 1991).In fact, fluxes of nutrientsinto the euphotic zone in the Subtropical Domain areprobably the lowest of any oceanic environment (Cul-len, 1982). Thus, feeding conditions may be poor in the

nursery ground of the winterspring cohort.Forsythe (1993) suggested the importance of water

temperature for field growth of young squid; a small(12C) increase in water temperature experiencedby exponentially growing squid during the first24 months after hatching can produce significantlyfaster growth. In the case of neon flying squid in the

North Pacific, the autumn cohort experiences coldertemperatures (1620C) than the winterspringcohort (2125C) during the nursery period becausethe former occurs farther north than the latter. Con-sidering that the autumn cohort grows faster than thewinterspring cohort during the juvenile and subadultstages, food conditions may have relatively more effecton the growth of neon flying squid than water tem-perature does, as in the case of the California marketsquid Loligo opalescens (Jackson and Domeier, 2003).

Contrasts in northward migration habitatsof neon flying squid in spring and summer

As the TZCF shifted northward in spring and summer,the autumn cohort had the advantage of being in theSST front and high surface chl a concentrations

(a) (b)

Figure 9. CPUE (number of individuals per 30 non-size-selective drift nets) for fishes and squids collected along 175 30E in (a)

mid- to late-May and (b) early August 1998. Black and white parts of the circles represent the autumn and the winterspring

cohorts, respectively. (The small mesh sizes of 1938 mm of drift net were not used during May, so small fish species, such as

Japanese anchovy and Pacific saury, were not collected.)

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(> 0.2 mg m)3) north of the TZCF (Figs 6 and 8),whereas the winterspring cohort remained in a lowsurface chl a area (0.2 mg m)3) to the south (Fig. 8).Watanabe et al. (2004) showed that stomach fullnessof the autumn cohort tended to be greater than in the

winterspring cohort during their northward migra-tion. Furthermore, according to the drift net surveys ofthis study, many epipelagic nekton, such as Pacificpomfret (Brama japonica), Pacific saury (Cololabissaira), Japanese anchovy (Engraulis japonicus), blueshark (Prionace glauca) and boreal clubhook squid(Onychoteuthis borealijaponicus) also forage north of theTZCF, as in the case of the autumn cohort (Fig. 9).Only skipjack tuna (Katsuwonus pelamis) does so southof the front. Hence, from a trophic standpoint, thehigher chl a area north of the TZCF may set theframework for an enhanced feeding regime for manyepipelagic nekton, compared with the low chl a area to

the south.The diet of the autumn cohort includes fishes

(mainly myctophids, followed by anchovy and Pacificsaury) and squids (such as the boreal clubhook squid,Onychoteuthis borealijaponica, and the minimalarmhook squid, Berryteuthis anonychus) in spring andsummer (Naito et al., 1977; Sinclair, 1991; Watanabeet al., 2004). Many of these prey species, such asanchovy, Pacific saury, boreal clubhook squid andsome myctophid fishes are migratory (Ogawa, 1961;Kubodera, 1986; Shimazaki, 1986). Hence, it appearsthe autumn cohort follows these migratory fishes and

squids in their northward migration. The winterspring cohort, on the other hand, feeds on euphausiids,amphipods and small fish species, such as Maurolicusimperatorius (Watanabe et al., 2004), which are con-sidered to be endemic to the Transition Zone. Thus,there appears to be a marked contrast in the food websbetween the high chl a area north of the TZCF andlow chl a area to the south.

Overall food availability for neon flying squid

The habitat of juveniles and subadults of the autumncohort appears to be more food rich than those of thewinterspring cohort. This could be one reason whythere is a clear separation, during summer (Fig. 7b), inML between the autumn cohort (ML range: 3846 cm; mode: 43 cm) and the winterspring cohort(ML range: 1628 cm; modes: 18, 22, and 26 cm) inspite of no apparent separation of hatching periodsbetween them. Females of the winterspring cohort,however, compensate for their slow growth by pene-trating the Subarctic Boundary and moving northwardinto the Transitional Domain, feeding there duringsummer and fall. Consequently, by the time of

spawning their final body size equals that of the au-tumn cohort (Fig. 1; Murata and Hayase, 1993).

Whereabouts of males of the autumn cohort areunknown but speculated to be in the southern Trans-ition Zone during spring and summer, when females

occur in the northern Transition Zone and Trans-itional Domain (Yatsu et al., 1997). In the case of thewinterspring cohort, immature males and femalesoccur in equal numbers until they migrate to theSubarctic Boundary (Kubodera et al., 1983; Murataand Nakamura, 1998). At this point, only femalespenetrate into the northern area of the TransitionalDomain, most males remaining around the SubarcticBoundary. This suggests that, whether or not they use afood-rich area, e.g. the Transitional Domain, duringfeeding, migration may lead to an extraordinary dif-ference in size between the sexes (Fig. 1).

ACKNOWLEDGEMENTS

We thank J. Mori for sampling and identification ofparalarval neon flying squid. We are also grateful to

J. R. Bower and H. Watanabe for their critical readingsof the manuscript. Our sincere thanks are due to thecaptains, officers and crew of the R.V. Wakatori Marufor their assistance during the cruise.

The authors would like to thank the SeaWiFSProject (Code 970.2) and the Goddard Earth SciencesData and Information Services Center/DistributedActive Archive Center (Code 902) at the Goddard

Space Flight Center, Greenbelt, MD 20771, for theproduction and distribution of SeaWiFS data,respectively. These activities are sponsored by

NASAs Earth Science Enterprise.

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