ICES C. M. 1996

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Theme Session SICES CM 1996/S:25

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The effectof currents and hydrography on the distribution of blue whiting~ . . ' ~

eggs and larv~e on Porcupine Bank

by

. MATTHIAs KLOPPMANN*, CHRISTIAN MOHN** AND JOACIIIM BARTSCH*

. * Biologische Anstalt Helgoland, Notkestniße 31, D-22607 Hamburg, Gcrmany** Institut für Mecreskunde der Universität Hamburg, Troplowitzstraßc 7, 22529 Hamburg;

Germany."\i '

Abstract

.. In March!April 1994 the Porcupine Bank area west of Ireland was surveyed to dctcrmine tlic .distribution of cggs und larvac of blue whiting (Micromesislius pOlilassoll) as weIl as toinvestigate the corresponding hydrography. BIuc whiting eggs wcre found in 'n ratherrcstricted area above the \vcstern slope ofthc bank and wcre abundant in watcrs cooler than 10 .oc,' cither to the cast of a thermal front on thc Porcupinc Bank or indeepcr watcrs (400 - 600m) off the bank In contrast to thc'cgg distribution, larvae of blue whiting wcre not restrictedby the frontal structure and \vere found in a widcr area, both iri the cool waters on PorcupineBank and to the west of the bank in water warmer than 10°C. The distribution of eggs andlarvac is discusscd in relation to hydrography .and currcnts. Larvae hatching at the westcrn 'slope of Porcupine Bank are transported to the nOI1hern parts of Porcupinc Bank. Here thcymay bc rctained above the bank by a Taylor column circulation associated with cold and lowsalinc water. Larvac hatching 'off the slope are entrained by warm and saline waters of theShclf Edge Current and may. b~ transported northwards into the Rockall Trough. These twodivcrgcnt circulation and drift pattcrns suggest that the early life history stages of two distinctspawning populations were caught.' . '

Introduction

The major spawning region of the European blue whiting (Micromesislills poulassoll) stock,lies to thewest of the British Isles' (BAILEY 1982)., During the spawning season betweenMarch and April large quantities of eggs and larvae can be found in an area stretching fromPorcupine Bankto an area northwest of the Hebricles (see e. g. BELIKOV et ci!. 1986;MONSTAD ct al. 1995). According to Bailey (op. cit.) two stocks spa\\n in this area: Thenorthern stock in 'the Hebrides area and the southern stock in the Porcupine Bank area.Howevcr, in recent ycars a discussion on the population structure of these stocks has becnraised (see e. g. ISAEV and SELIVERSTOV 1991; ANON. 1991) and thus the separation intodifferent stocks has been postponed until the question on thc identity of blue whitingpopulations has been c1arified (ANON. 1995). Marine populations often constitute themselvesaccording to physical oceanographic features affecting the drift or retention of their early lifehistory stages (SINCLAIR, 1988). Within the, Shelf Edge Fisheries and Oceanography Studies(SEFOS), a EU/AIR funded project, the recruitment ofblue whiting (one ofthe target speCies)

is studied in relation to the oceanography of the European shelf edge. Results from thesestudies should also clucidate the population structurc discussion. Among aseries of survcysconducted within SEFOS, German RV Hein'cke carried out a eruise to Poreupine Bank inMarch!April 1994 to' study the horizontal and vertieal distribution of blue whiting eggs andlarvae in relation to their physical and biological environment. The results of this cruise arepresented in this paper. . .

Material and l\1ethods '

Astation grid abovc Porcupinc Bank was sampled' from 24 March to 10 April 1994 fromboard RV Heincke. The station gridand chronological course of sampling is shown in figure1. Prevailing bad weather eonditions with wind forces up to 12. Bft. eaused sevcralinterruptions of the sampling proeedure and ''las the main reason for the somewhat "strange"

. course. Thus thc quasi-synoptic presentations ofthe results have to be treated with some eare.

At each station one CTD profile to 1000 m depth or 5 - 10m abovc' thc sea floor ''lascollected. Thc ME-CTD was attached to aGeneral Oceanics Rosette water sampler. In orderto calibrate eonductivity, up to 3 water sampIes were taken during each CTD cast. Stratifiedplankton sampling was done by one or two (depending on water depth) eonsecutive multiple-

. opening-closing-net (MCN, see e. g. KLOPPMANN 1994) hauls down to amaximum depth of650 in or 5 to.lO m above the sea bed. The water column was divided into 9 discrete depthstrata according to table 1. .

NETNO. SAMPLED DEPTHS SAMPLED DEPTHS, (SHALLOW) (DEEP)

1· 200-150m .. 650 - 500 m2 150-100m 300-400m3 IOO-50m 400 - 300 m4 50 - 25 m 300 - 200 m5 25 -Om 200 - 0 m

Tablc 1: The sampled deptll strata of thc MCN tows during Heinckecruise to Porcupine Bank. .

The MCN was equipped with a buHt in CTD to record depth, in-situ temperature and .conductivity and flowmeters to measure the amount of water filtercd. The net gauze had amesh width of 150 f-lm. After each haul the eontent of each net was thoroughly washed intothc eod end buckets. All plankton sampIes were' preserved and stored in 4 % bufferedformaldehyde seawater solution.

In the laboratory the sampIes were sorted for fish eggs and larvae. Eggs and 'Iarvae of bluewhiting were then identified and the larvae measured to thc nearest 0.5 mm helow standardlength (SL). No eorrections have bcen made for shrinkagc of the larvae. For the horizontaldistribution the larvac were grouped into '4 length classes: < 3 mm; 3 - < 4 mm; 4 - < 5 mmand ~ 5 mm. Numhers of blue whiting eggs and Iarvae '\Tere standardised to nurnbers per 100

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, m3 for thc prcsentation of thc vertical distribution and in numbers under 1 m2 sea surface forthc horizontal distribution. "

Results '

, 'Veather and hydrography',

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This cruise was characterised by prevailing bad weather conditions during thc.cntire course.Prevailing strong winds from thc southwest to northwest with an average force of abo'ut, 8 Bft.,caused \vaves up to 10m high. Only during very short periods moderate winds below 6 Bft. "were encountered.

The surface temperature '~md salinity distributions are shown in Jigures 2a and 2b. Bothisothcnnals and isohalincs generally follow the bottom topography. Above the dceper areas (>

, ,1000 m) to thc wcst ofPorcupinc Bank and in thc PorcupincScabight \varmcr and more salinesurfacc waters with valucs abovc 10 oe and S > 35.43 respcctively werc observcd. In contrast,surfacc waters abovc thc bank and on thc shclfare cooler and less ~alinc. Strong temperatureand salinity gradicnts along the' Irish shelf break c1eady show the course of the Irish shelffront. ' ' ,"

Thc vertical temperature and salinity distributions fo~ transects B to F are given in figures 3and 4. These show a warm corc of high saline water from thc surface down to 300 m (T >10.5 oe, S = 34.49 at 51 ° 3D' N, transect F) west of Porcupine Bank, which cools and freshensalong its northward path. ,The slight decreaseof thc corc tem"peraturc north of 52° 3D' N and

, the position of tlie eore on transects D and e suggests a ~vestward deflection ofthis warm,saline subsurface eore south of 52° 3D' N and a northeastward movement farther north. '

Between 500 and 800 in an intrusion of relatively cold and less saline ~vater at thewestemn"iost easts between 51°,30' N and 52° 3D' N ean be seen on transccts D~ E and F. At'approximately 900 m a clear signal of Oibraltar Water (OW) is present in the vfcinity of thecontinental slope west of Porcupine Bank at 51 ° 3D' N (transect F). This ~ater freshens andmoves westward at 52° 00' N (transect E), but to the north these" characteristics are no longer

, visible. ,," .

, .Abovc the western slope ofPorcupine Bank a frontis evident and it separates the warm, saline\vate'rs off the bank from the cooler and fresher waters on the bank. Above Poreupine Bank a "dome of cold, less saline water is present between 51 ° 30' N (transect F) and 52° 3D' N '(transect D). The S-shape of the isolines, at 53° N and 53° 3D' N (transects e and B) isprobably due to the mixing and stirring effects of two stOmlS that passed over this area only afew days before sampling took place at these stations. A rapid eooling and freshening ofwaters ofthe entire water eolumn ean be seen to the east ofthe lrish shelfbreak. '

Horizontal distribution of bluc whiting cggs and larvae

Eggs of blue whiting occurred in high abundances 6nly in a rather restricted area abovc thcwestern sl~pe of Poreupine Bank (figure 5). Duc to a large number of stations with zero orlow abundancc only a relatively small mean abundance of 64.3 eggs per 1 m2 (s = 191.8;median = 2.5) was estimated. However, in the area of high abundanee density values weIl

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above 500 eggs per 1 m2 \~ith, a peak value of 1113.5 ;cggs/m2 wcrc found. Highcstabundanccs occurred at or eastward ofthe thermal front and in waters le~s than 1000 m depth.

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The verticaI distribution of bIue whHing cggs and Ian'ac

The vertical distribution of all blue whiting eggs on each transect is shown in figure 8. Blue "•whiting eggs occurred in all depth strata sampled and modes were discovered in both, shallowand deep layers. It is evident that high densities of eggs were only found in waters of"temperatures less than 10 oe: Westward of the, thermal front where the 10 oe isothermal runsdeep we find a mode of the egg distribution below 400 m depth and only few eggs in the top100 m. In thc cooler \vaters abovc thc bank eggs occur throughout the whole water column incomparativ~ly high densities and modes in t~e top 100 mare by no means thcexception".

Figure 9 shows the vertical distribution'of all blue whiting larvae arid the mean larvallengthson each transect. Like the eggs, larvae were found in all depth strata sampled. Modes oc~urred

in the deep and shallow laycrs. Thc mcan length distribution indicatcs that thc "dcep modesprincipally corrcspond to small mean lengths while thc shallow modes correspond to thelarger mean Icngths. Blue whiting larvae found at 'depth Ure thus almost exclusively recentlyhatched larvac. ..

Transects B and e show that blue' whiting larvae become largcr from the deeper to theshallowcr layers and from' the west towards the shallow parts of Porcupine Bank. From

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Porcupine Bank to the lrish shelf the mean length decreases, fi~st and increases ~gain when, reaching the shelf break. On transect B larger larvae in the shallower layers of the twowesternmost plankton stations were obscrved. It should be notcd that the deep disperscddistribution of the Iarge larvae on station C6 was found right aftcr a 48-hour storm of force 10Bft. Thus this .distribution can be intcrp~eted as a result of deep mixing. A shallowerdistribution of these larvae is usually more likely (COOMßS ~t al. 1981).

Distributions on transect D generally rcsemble the distributions on transccts B· and C but.ccntrcs of high densities are scattered over a wider arca than in the t\VO northem transccts.~rom transcct E on the distribution changes somewhat. The h}ghcst densities were observcd

, on thc two wcstcrnmost stations but the largest mean lengths are still evident in thc shallower,layers above the bank. On transect F higher larval blue whiting dcnsities occurred on thc'outcrmost stations on both the castcm und the western side. These larvae wcre larger than .'those encountercd in betwccn. . i

Discussion

The uppcrwatcr masscs in the Europeun Basin are part of the Subpolar Mode \Vater(SPMW),.which is characteriscd by a dccrcase oftempcrature and saHnity ofthe original North AtlunticCcntral Watcr (NACW)~ as it movcs castward across the Atlantic (MCCARTNEY und TALLEY, .1982). In thc' vicinity of thc Rockall Trough two components of thc SPMW can bedistinguished:

1. Eastcrn North Atlantic \Vater (ENAW), which enters thc Rockall Trough from thc south, ischaractcrised by typical temperatures between 8 and 12 oe (HARVEY, 1982; ELLEIT ct al.1986).

2. Modified NorthAtlantic Water (tv1NAW), which enters thc northeastern boundaries of theRockall Troughwcst ofHatton Bank (ELLEIT et al. 1986).

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Mid-depth waters are dominated by Subarctic Intermediate Water (SAIW),which.is entraincd.near the polar front (HARVEY, 1982) with core characteristics of T = 5.7 °C and S =;= 34.85

. (ELLEIT ct al. 1986). Another important water mass observed in this area is Gibraltar Water(GW), formed within the Medi.terranean outflow. . .

Figure 10 shows TS diagrams for each transect of the RV Heinckc cruise CTD casts. Along53° 30' N (transect B) the upper water masses at the westernmost stations BIO to BI i aredominated by ENAW (above 600 m). Below this thc decreasc' in temperature arid saHnitysuggcsts un increasing influence of SAIW. In the shelf arcas aboveand east of Porcupine

. Bank the TS characteristics show homogenous water, probably influenced by strong verticalmixing due to the strong winds that prevailed during thc cruise (station B4).

A similar situation is cvidentalong 53° N and 52° 30' N (transectsC and D). However, along52° N und 51'? 30' N (transect E' und .F) thc influence of SAIW' is still present, but less'pronounced (westernmost stations E9 to EIOund F12). Near 900m GW is clcarly evident westof Porcupine Bank (stations EIO and FIO to Fll) and in the Porcupine Seabight (stations E4.' "

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and F5). The isolated TS feature of station EI is typical for the water masses of the Irish shelf ­front during that season (HUANG et al., 1991):

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The observed domi~g of cool and' low saline waters above Porcupine Bank is a prominentfeature typical for the area (see e. g: TITOV et al. 1993; McMAIION et aI. 1995). Thedisturbance of the dome like appearance on the two nörthernmost transects of thisinvestigation can be attributed to thc stirring and mixing effect of hvo strong storms passingover the study area shortly before sampling. Following thc investigations ofMEINCKE(1971),HUPPERT Und BRYAN(1975) and VASTANO and \VARREN (1976) on the theory and naturaloccurrence of Taylor column circulation effects, the temperature. and salinity data suggestssuch a phenomenon over Porcupine Bank: cold and low salinity water is trapped over the bank

- causing a quasi-persistent, anii-cyclonic eddy like circulation feature. The cxisicnce of such aTaylor column may explaill: the obscrved drift pattern of the blue whiting larvae onto theshallow parts of the bank. The significance of Taylor colunins for the recruitment of fishpopulations inhabiting suhmarine rises has often been addressed in recent publications (see e.g. HEATH 1992 and ROGERS 1994 for review) but evidence. for retention of fish larvae byTaylor column circulation is still sparse (BOEIILERT and MUNDY, 1993).' Howevcr, the •existence of such a quasi-persistent eirculation pattern above Poreupine Bank may generate asmall retention area supporting the existence of a local, self sustaining spawning population ofblue whiting. The eggs which are laid above, the .western slope of Porcupine Bank areentrained into the Taylor eolumn circulation and after hatching the larvae are retained above

. the bank. Analysis ofthe feeding environment and ofthe feeding success ofthe larvae showedthat the area may support successful recruitment (HILLGRUßER et al. 1995, 1996).

Ouring their rise to the surfaee layers larvae that hatched off the slope of Porcupine Bankbecome entrained in thc warm and saline subsurfacc waters. The temperature and salinity datasuggest that this water forms a northward flow that is deflectcd from the bank just south of52° 3D' N while it moves northeastwards and towards the bank again further north (transectB). This northward flow is in essence the shelf edge eurrent. Larvae entrained into this currentmay thus be transported northwards into the Rockall TrOligh. This assumption)s corroboratedby the larger larvae observed in the shallower layers of thc westernmost station of transect B.It seems likely that these larvae derive from the large Porcupine population spawning above •the deeper waters west ofPorcupine Bank (lsAEv and SELIVERSTOV, 1991; ISAEV et aL.1992)'ofwhi~h only the castem margin oftheir occurrence was sampled. .

The identity of these· two populations remain~ uncIear.· High abundanccs ~f blue whitinglarvae above Porcupine Bank have not always been observed' as earlier studies have shown(HALßEI~ENI982; BELIKOV et al. 1986). The spawning characteristics'ofboth populations arcidentical: As thc results from this study have. shown both populations spawn in waters 'ofsimilar teinperature and salinity characteristics. It is thus possible that. the larvae .retainedabove Porcupine Bank derive from expatriates of the so called Porcupine Population thatmoved up the bank and spawned ihere. . . .,.

Acknowledgerilents

This study was funded as partof the EU/AIR grant no.AIR2-CT93-1105. We are verygrateful to the Captain and erew and the scientific eo-workers on board RV Heinckc who allworked to their limits during this very strenuous eruise.

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References

ANONYMUS 1991: Report of the blue whiting assessment working group. ICES C.M.1992/Assess:3

ANONYMUS 1995: Report of the blue whiting assessment working group. ICES C. M.1995/Assess:7

. DAILEY, R. S. 1982: Thepopulation biology ofblue whiting in the North Atlantie. Adv. mur.Diol., 19,257 -'355.

BELIKOV, S. V.; ISAEV, N. A.; SHEVCHENKO, A. V. 1986: Some results of iehthyoplanktonand hydrographie observations west and northwest of the Dritish Isles in 1983 - 1985.ICES C. M. 1986/C:l1.

BOEHLERT, G. W.; MUNDY, D. C. 1993: Ichthyoplankton assemblages at seamounts andoeeanie islands. Bull. mar. sei. 53,336 - 361.

COOMBS, S. H., PIPE, R. K.; MITCHELL, C. E.1981: The vertieal distribution of eggs andlarvae of blue whiting (Micromesistills poutassou) and maekerel (Scomber scombrlls) inthe eastern North Atlantie and North Sea. Rapp. P.-v. Reun. CIEM, 178, 188 - 195.

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ELLETT, D. J.; EDWARDS, A.; BOWERS, R. 1986: The hydrographyofthe Roekall Channel- an. overview. Proc. R. Soe. Edinburgh, 88B, 61 - 81. .

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HALßEISEN, H. W. 1982: Verteilung und Häufigkeit von Fischbrut in der \Vestirischen See.Diploma Thesis, University of Kiel, 185 pp. " ..' .

HARVEY, 1. 1982: 9-S relationship and water masses in the eastern North Atlantie. Deep-Sea '• Res. 29, 1021 - 1033.

HEATH; M. R. 1992: Field investigations of the early life stages of marine fish. Adv. mar. biol.28, 1 - 174.

HILLGRUBER, N.; KLOPPMANN, M.; WESTERNHAGEN, H. v. 1995: Distribution ofblue whitingAficromesistills pOlltassoll larvae in the Poreupine Dank area, west of Ireland, in relationto hydrography and the feeding environment. ~CES C. M. 1995/H:27

HILLGRUßER, N.; KLOPPMANN, M.;WESTERNHAGEN, H. v. 1996: Food and feeding seleetivityof. blue whiting (Micromesistills pOlltassoll) larvae from Porcupine Bank, off WestIreland. ICES C. M. 1996/S:1.Poster

HUANG, W. G.; CRACKNELL, A. P.; VAUGHAN, R. A.;DAVIES, P. A. 1991: A satellite andfield view of the Irish Shelf front. Cont. Shelf Res. 11, 543 - 562.

HUPPERT, H. E.; DRYAN, K. 1976: Topographieally generated eddies. Deep-Sea Res. 23, 655 -679.' ,

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ISAEV, N. A.; SELIVERSTOV, A. S. 1991: Population structure of the hebridean-norwegianSchool ofblue whiting Micromesistius poutassou. J. Ichthyol. 31,45 - 58.

ISAEV, N. A.; SELIVERSTOV, A. S.; TRET'YAK, V. L.; KORZHEV, V. A. 1992: Populationdynamics of the Hebrides and Porcupine populations of blue whiting, .Micromesistiuspoutassou, and their rational exploitation. J. Ichthyol. 32, 32 - 42.

KLOPPMANN, M.1994: Kleinskalige Vertikalverteilungen von Fischlarven bei Helgoland.Dissertation, Universität Hamburg, 214 pp.

MCCARTNEY, M. S.; TALLEY, L. D. 1982: The subpolar water ofthe North Atlantic Ocean. 1.phys. Oeeanogr. 12, 1169 - 1188.

McMAHON, T.; RAINE, R.; TITOV, 0.; BOYCHUK, S. 1995: Some oeeanographie features ofnortheastem Atlantic waters west ofIreland. ICES J. mar. Sei. 52,221 - 232.

MEINCKE, 1. 1971: Observation of an anticyclonic vortex trapped above a seamount. 1.Geophys. Res. 76, 7432 - 7440.

MONSTAD, T.; BELIKOV, S. V.; SHAMRAI, E. A.; McFADZEN, I. R. B. 1995: Investigation onblue whiting in the area west ofthe British IsIes, spring 1995. ICES C. M. 1995/H:7.

ROGERS, A. D. 1994: The biology of seamounts. Adv. mar. biol. 30,305 - 350.

SINCLAIR, M. M. 1988: Marine Populations. An Essay on Population Regulation andSpeciation. Washington Sea Grant, Seattle, 252 pp.

TITOV, O. V.; BOYCHUK, S.; McMAHON, T. G:; RAINE, R. 1993: Results of oeeanographicinvestigations in the northeast Atlantic in spring, 1993. ICES C. M. 1993/C:54.

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VASTANO, A. C.; WARREN, B. A. 1976: Perturbations to the Gulf Stream by Atlantis IIseamount. Deep':'Sea Res. 23, 681 - 694. •

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Captions to thc figures

Figure 1: The station grid ofthe RV Heincke cruise to Porcupine Bank in March/April 1994.The arrows and thedates indicate the course of the sampling exercise. Isobaths at200m, 500m and 1000m are indicated.

Figure 2:' Thehorizontal distribution of the seasurface temperatures (a) and salinities (b)above Porcupine Bank in March/April 1994.'

Figure 3: The vertical distribution of the potential temperature along transects B (top) to F(bottom) in the Porcupine Bank area in March/April 1994.

Figure 4: The vertical distribution of salinity along transects B (top) to F (bottom) in thePorcupine Bank area in March/April 1994.

Figure 5: The horizontal distribution of blue whiting eggs above Porcupine Bank In

March/April 1994.

Figure 6: The representative length distribution of blue ~hiting larvae from waters abovePorcupine Bank in March/April 1994.

Figure 7: The horizontal distributions of the four specified length' classes of blue whitinglarvae above Porcupine Bank in March/April 1994.

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Figure 8: The vertical distribution of blue whiting eggs along transects B (top) to F (boHom)in the Porcupine Bank area ,in March/April 1994.

Figure 9: The vertical distribution of blue whiting larvae and of mean larval lengths alongtransects B (top) to F (bottom) in the Porcupine Bank area in March/April 1994.The presentation of larval mean length distribution is exclusively based on'undamaged larvae. Depth strata with exclusively damaged larvaewere excluded

• from the estimation. . .,

Figure 10: TS diagrams of selected stations from RV Heincke transects B .:. F.Stationnumbers, water depths' (* 102 m) and O't,s contoufs are indicated., Thick linesrepresent the TS characteristics of ENAWand GW. ,.'

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Figure 6

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