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A NUl\IERICAL l\!I0DEL SYSTE1\I OF THE REGION AHOUND TIIE
IUERIAN PENINSULA: 1\IODEL VALIDATION AND APPLlCATION
'1'0 HAKE LAHVAE DRIFT IN TIIE BAY OF 13ISCAY
Joachim 13artsch, Alicia LavEn and Lorenzo l\Iotos
AßSTHACT
\\'ithin the EU fuudcd SEFOS project (Shclf Edge Fisheries and Oceauography Studies) a
numerical model systcm cOllsisting of CI circulatiou <lud t.r<lnsporl 1l1Odel !JilS \)('('11 IIsed 10
model the dispersiou of hake eggs and lan'c\(' (Jltrll/ccil/.~ /l/crIILCcill.~). Tlw iln'" of 111<'
model systcm rangcs fwm Nortll\\'cst Africa. to t he 1I0rthern Ba)" of Biscay aud co\'crs 1h('
shclf edge and adjacent waters with Cl horizontal rcsolution of 10' x 10'. The circulation
model was run using tidal forcing, c1imatological density fieIds a., H'cll as realistic six-hourly
wind stress fields 1'01' 1994 amI 19%. Current mcter data collected in the Cantabrian Sea
in the mixed layer and the upper part of the North Atlantic ('entral Water is used for
model validation. Simulated currents from the circulation model H'cre found to be in good
agreemcnt with observed current data on the shdf amI in thc vicinity 01' the shc1f cdge.
Simulated currcnts are used as input to the transport model. Egg and larval data used in
thc dispersion simulation was collected on two cruiscs carried out in February (lnd ;\Iarch
1995. Horizontal distributions of stage I eggs were uscd as lohe initi<ll distribution for the
dispersion scenado. Tracers were disperscd in the depth range 50 - 150m for 39 day:;. Based
on estimated egg development times and larval growth rates, stage I eggs will have growll
into 6mm larvae in approximately 39 days. The tracer distribution after dispersion from 17
February to 27 March is compared to the distribution of Gmm lan'ae observcd betH'ccn 22 27 March 1995. Agreements amI discrepancies bet,,"cen observations and model results are
discussed.
Kcywords: NumcricaI model system, model validation, larvae dispersion
Joachim Barlsch: Biologische Anstalt He/goland, Notkestl'. ;]1, J:J6'07 Ilambll/'!J, (,'u'/wwy
(lel: .{9 40 89U'9.1177, leu: 494089693 115. e-mail: bartsch@:ifm.llni-Iwmbu/'!J.dt).
'Alicia LavEn: Ccntl'O Oceanografico de Santal/der, Apdo. ~240, 39080 Sal/lander, Spain (tel:
34 42275033, fa:t: 34 42 275072, e-mail: lavilla@ccai.r3.11llican.es).
Lorenzo 1\lolos: AZTI, Food and Fish Technological Inslitute, A vda. Satrllsteglli 8, 20008
San Sebastian, Basque Counll'y, Spain '(tel: :J.f 4:3 214124, fax: 3443212162. e-mail:
[ol'cnzo@I'p.azli.es).
1
1. INTRODUCTION
One of the specific objeet.ives of the SEFOS (Shelf Edge Fisheries ami Occanography
Studies) project is to gaiu information on the transport. patterns or the eggs a1ld lan'ae
01' thc target species in thc SEFOS area. In this invcstigation J/t'/'lllccilt8 111( rlllccilt ...
is considered. '1'0 stu<.ly thc drift 01' eggs and larvae o\'er wecks (lud months lIumcrical
model systems are nce<.led. Only thcy can providc the \'ariable circ1\lation fleld O\'cr
these time spaus with the required ::;patial and temporal resolution. CurrclIt clata
from field sun'eys and moorings are too sparsc to accomplish this. Examplt's of such
modclling drift studies are those carried out in the Gulf 01' Alaslm Shelf arca on \Valleyc
Pollock (Stabcuo et al., 1995), in thc Georges Bank area on eod and Iladdock (Werner
ct. al., 1!.JD:3) and in the German I3ight on Sprat (I3artsch, 199·la and b).
The occanographic and biological data collected during SEFOS in tlw Bay 01' ßiscay
i::; us~d to \'alidat.e t.hc circlllat.iou model and to test tlH.' prl'dicl in)lilldcilSl ability
tlll" model system.
ur •
2. THE CIHCULATION AND TIL-\NSPORT MODEL SYSTEM
A thrce <.limensionalnonlinear baroclinic lllllilericalmodel is u:,;e<.l to :,;inlldatc thc circulation in t1le region around the Ibcrian Peninsula, the shelf edgp area and adjacenl
oceanic l'egions.TI}(' cil'culationmodel is hased on 1l:\~lSO;\I (llAl\lhurg Slwlf Ocean
l\lodel) which was deve\oped at thc Institut für !\IccrcskulHk. lIamburg and W<I:'; trilll:>fel'red to thc sout1lern SEFOS area. No details of thc model will Iw givcn hen~. <I:'; tlwse
hase been pu blishcd clsewhere (13ackha us, 1985).
The model area reaches from 33°N to .!i°N aud frotll 15°\\' to lOW (fig. 1). The
horizontal grid resolution is 10'x 10', i.e. 1:3 km (east-west) by 18..) km (norl h-south)
in thc I3ay of Uiscay. The vertical resolution consists or IG layers, with layer <.lepths
ranging from 10 m 1.0 ·10 m in the upper 100 m aud increasing steadily in the lo\\'er
layers. The model is forced by the l\h-tidc, six hOlldy wind stress fields from ECl\lWF
(European Centre 1'01' l\Iedium-Range Weather Forecasting) as weil as monthly climatological density fields which are trcilted prognostically hut. ",hieh are relaxed 10\\'ards
the climatological Illonthly mean.
The time step 01' the circulation model is ·15 min. Duc to the high data storage
requirements, the current data, as weil as the variance of the current was only output
on a daily basis, i.e. the currents we1'e integrated ove1' two l\12 - tidal cycles. The <.laily
three dimensional cur1'ent anel variance fielcls serve as input to t\ie transport model.
To eluciclate the configuration of the model system, a schematic diagram i::; given in
fig. 2.
The simulations 01' the drift routes are perfol'med by mcans 01' tracers, in esscnce
"lllarkeel" water pa1'ticles which are introduced int.o the model area and are pllrsued in
the space and time dOlllain. A simple lllethod with \vhich the transport of substances
2
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(particIes) can be simulated is the r..lontc - Carlo method (Bork anel I\laier - Heimer,
1978). It is assumed that the current fieIel can be split into a large scale mean current
(rr) anel a small scale random fluctuation (-n'), i.e. u = U + u /. The small scale
fluctuations are parameterised using the current variance (Backhaus, 1~8~; Bartsch,
19!Ha). Thus, at cach timestep, the particle experiences 0. <.lirectional transport by tlle
me~n current (advection) as weIl as 0. ran<.lom transport, in magnitude anel direction,
by the small scale fluctuations (diffusion).
The boundary conditions in the transport model prescribe a. 110 - flux cOlldition at
closed boundaries, whereas the tracers may leave the model area at open boundaries.
The model area; grid size and the vertical resolution are in accordance with that of
the circulation model, while the time step of the transport model is tllrec 1I0urs.
3. I\IATERIALS AND I\lETIIODS
(a) CURHENT ~IETER DATA
•
Within tlle SEFOS projcct current data from three moorings were collected for the
duration 01' one year. Data from the upper layers and from the first deploYlllcllt
period from 19 February to 15 r..lay 1995 \\"crc uscd 1'01' nlOdel validation. DuC'
to thc fishing effort in the area and based on previous experience, moorings \\"cre
placed in the locations shown in fig. 3. The current meter on the shclf was
deployed at 75 m depth, which is below the depth of nets used to catch peIagie
fish. At the shelf break (mooring 2) the shallow eurrent meter was eleployeel to
study the mixed layer and the second at 180 m to study the upper part 01' the
North Atlantie Central \Vater.
Moorings were deployed from the BIO Francisco Na,·arro. eTD profiles were
eollected during tllc deploymcnt to observe thc hydrographie cOllditions. 01' the
area with thc aim of studying thc influcncc 01' thc wind strc.'ss on tht' stratification
of the upper layer. In March-April al\(ll\lay-Junc Hl95 two oceanographic SEFOS
cruises were earried out in the Cantabrian Sea eovcring thc area in thc vicinity
of the eun'ent meters (Porteiro et al, 1996).
The original eurrent meter time serics were obtaincd with a sampliilg interval of
30 min. 1'0 obtain a preliminary smoothing 0. Godin A2 A2 A3 filter was used,
whieh effeetively eliminated the high frequeney noise. The data was filtered again
to eliminate the effeet 01' the l\1 2-tide. This filtering was carried out. with a Godill
A24 A2·1 A25 filter with a. cut off period 01' 25 hours (Godin, 1991; Alonso anel
Diaz dei Rio, 1996). After filtering the data. was ayailahle onee an hOIlr.
3
(b) HAKE EGG AND LARVAL SURVEYS
Data on distribution and abundance of hake eggs anti larvae \\ere collected during
two consecutive plankton cruises carried out with RV Investigador in tlte time
pcriods 15-2-1 February and 22-30 1'1arch 1!J95, respectiv<,ly. Tlte cruises were
specifically clesigned to establish the horizontal and "ertical distributions of ltake
eggs and larval' togetlter with complementary hydrographical observations at two
consecutive times Tl and 1'2.
Bot.h cruises covered similar areas of the 8ay 01' Biscay (fig. ·1), i.t'. frolll t Ill'
Spanish coast in the south up to -17 0 30' N and from the 100 III isobatll in the
east to 20-30 nauticalmiles beyond the shelf edge in the \'lest. 80th areal coverage
ami timing of the cruises were dctennined according to previous work in the area
ami a lit.erature review (Casarino and l'[otos, 1!J!).l) with thc aim of cO\'ering a
main area of adult hake distribution at thc peak of thc spawning period.
•
A detailcd report on the metltods used during the cruises is gin'n in l'[otos et a!.
(l!J%). The horizontal distribut.ion of hake eggs and lan·at· was dl'tel'lllillt'd I'I'OIlI
sampies colleckd hy oblique BONGO (GO CIlI diatllt'l<-r) Iwl hauls (Stlli,,11 <lIld
nicllilrdson, J!)T7), These sampies n-ere colleckd het.n-ceu l(; - ~~ February and
22 - 27 l\Iarch respecti\'cly. Thc vertical distribution was derived frolll sömples
collcctcd at sclccted stations hy 1 m'l mouth aperture ring net hauls. This uel was
fumished with a double aperturcjclosurc mcchanism allO\ving for thc sampling of
discrcte layers.
Plankton sampies were fixcd inmcdiately after the complction of thc haul in a
buffered cOl11merci<d formaldehyde solution on tap water (10%). All fish eggs an<!
lan'<w were sorted frol11 thc plankton sal1lples in the laborator)'. Hake eggs ami
lan'~e wcre idcntificd (Porehsky, U)75; Coomhs, EH).!; Marrale ct al.. U)!)G; Husseil, lU(8), rounted and dassified by de\'(~loplll('l\tal stagt' (('()olllhs <llld \litcllt'11.
1082). Lan'ae were measureu (SL) to the lower 0.1 1Il111. No correctioll was made
to allow for shrinkage of larvae due to fixation procedures. 8ailey (1981) gave a
shrinkage figure of 9-22% for small Pacific hake larvae « <1 mm) fixed 5-10 minutes after collectioll, and he assulIled sm aller shrinkage rates for bigger .larvae.
Egg andlarval abundances per haul were transformed to densities (numbers per
10 111 2 ) using standard procedures (Smith amI nichardson, UH7).
Temperature, time ami depth profiles were recorded by a. l\IINILOG VEl\ICO
data logger in all planktoll 10ws. CTD SEABInn ~!) profiles 01' tClll(lcrature,
condllc\.ivity. tlll'bidity ami clowphyll a \\'ere ("ollt,(:\('<1 CI! se!e('(ed st.ill iOllS (fig.
'1). Unfort ullatcly, thc CTD was not deployed d uri IIg thc FebruCl ry cru i:->e d ue to
rough sea cOllditions.
•
(c) EGG DEVELOPl\lENT AND GROWT1I RATES
Data on temperature relateu developmental rates of hake eggs amI yolk sae larvae
are a\'ailable in the literature (Coombs and ~litcheJl, 1982; ~Iarrale et al., 1906).
1Iowe\'er, no data is available on growth rates of post-larvae 01' tbe Europeall hake,
ncither frolll our cruise Bor frolll thc literature. Alteruati\-el)', we used tlte onl)'
availahle larval growth data for t he gelllls ~ Icrlucci us, i.e. da ta frolll ;\ lerlucci us
productus, a hake species inhabiting waters in thc Northcastern Pacific Occan
(Bailey, 1981; Sailey ct al.,UJ82).
Aecording to Coombs and !vIitchell (1982) the time necessary 1'01' eggs to reach
thc end of stage IV is identical to the incubation time (duration from spawning
to hatching). This ranges from 6.82 to 5.27 days at 10 °C and 12°C, respectivcly.
•
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Data on growth rates of yolk-sac larval' is also available. Fig. 2 of Coombs
and l\litchell (1982) showed pictures 01' different stages 01' yolk-sac lalTa reared
at different temperatures. An early stage at 8.1°e, C\. lllid stage at 10.0 oe, a.
late stage at 15.6°C, ami a very la te one at 15.GoC. They assigll to thesl' stages
.agl's from hatching to ferti1ization 01' 200, 191, 1Ti amI 1!)1 hours, respectively.
1\larra1e et al., (1996) ineubated hake eggs and yo1k-sae larval' from an artifical
fertilization experiment. They observed that the dl'velopml'nt of yolk-sac larval'
from hatching to the cxhautioll of thc yolk lasted around 84 hours at 17°e.
In order to have some idea of the actual growth rates 01' post-larval' of 1\1. merlueeius, the a\'ailable information regarding other 1\lerlueeius speeies was reviewed.
Bai1ey (1981) presented developlllellt and growth rates of eggs, yolk-sac larvae
ami post-larval' of the Paeific hake, 1\lerluccius productus. The results are SUlllmarized in tab. 1. Egg development rates are slower in tlle European hake than
in the Pacific hake. These slower uevcloplllent rates lllay a.lso hold during the larval stages. Assuming similar growth patterns 1'01' yolk-sac larval' 01' both species,.
the end of this stage at 12°C would be reached one da)' later in M. lllerluccius,
when they are'" 12 days old.
Using the (post) larval growth rates given in tab. 1 1'01' M. productus and the
lengths at the beginning of the postlarval stage (1.72 mm for the Gompertz fit
and 2.75 mm for the lineal fit), it would take about 31 days (G0111pertz fit) 01' 32
days (Linear fit) to reach a length of 6 111111. Similarly, it \\'ould take between 34
(Gompertz fit) to 38 days (Linear fit) to reach 7 nll11. Assullling further, that \1.
merluccius larvae have slower growth rates than their paeifie counterparts, it is
probable that the stage I eggs on cruise 1 (around 17 February) dc\'clopcd into
6 mm lar\'ae by cruise 2 (around 27 1\larcl1) at a tcmpera.ture of '" 12°C, i.e.
uuring 39 days.
.5
.1. RESULTS
(a) cunnENT OBSERVATIONS
The metcorological COlHlitions over the Ba,)' of Bisca,)' during the sampled period
are eharacterised oy a. eontinuous track of high and 100v pressure eells (Instituto
Nacional de l\leteorologia., 1995). These conditions praduee NW, SW and NE
winds as weil as relaxed periods. There were two periods with a high pi'essurc ecll
remaining o\'er the area 1'01' longer periods, one in late ~Iarch anti thc othcr during
thc first. half 01' April. In these periods NE winds domillated tlw meteorologieal
eonditions o\"(~r the l3ay of Biseay.
Current meter mooring :3 lies on the shelf in water ~O m deep. The obscl'\:c<1 time
scries for this eurrcnt meter, deployed at '·lm, is given in rig.. 5a. Ovcr the shd!",
currents were oscillatory NW /SE with periods around 8 - 10 da,)'s. ;\Ieiln values
01' the cast and north compollents \\'ere 0.-12 em/s and -0..17 em/s, ;Illd maximum
values were l:l cm/s and - ·1.5 cm/s rcspeetively 1'01' the pcriod.
1\Iooring 2 was deploycd at the shclf cdge in a water depth 01' .568 nl. The top
current meter at mooring 2 was deployed at 75m and tllC' tilllC' ;,;cries is given in
fig. Ga. At the bcginning 01' the sampling period, currents "'ere mainly eastwanl,
eorresponding to the winter eirculation in the southern Bay of Biscay. :\round
the middle of 1\Iarch, the mean direction of the currellt revcrsed and now showed
westward flow, whieh is usually ooserved during spring amI summer (Pingrce and
Le Cann, 1~90; Diaz del Rio et al. , 1992; Porteiro ct al., 19~G). l\Iean values
for the cast and north eomponents were -:1.2 em/s and -:3..5 cm/s, and maximum
values were -18 cm/s and -12 em/s respectively for thc pcriod.
The seeond current meter a! mooring 2 was deployed at 180m depth alld the time
series is gi\'en in fig. 7a. Thc current was predominantly eastward throughout the
period and only a! the end of 1\larch ami at the end of April west.ward direeted
currents were ooserved. 1\Iean values for the east and lIorth componellts were 1.7
C111/S and -2.5 cm/s, ami maximum \'alues were - 12 cm/s and -~ cm/s respectively
1'01' the period.
In this area of the Bay of Biscay, dellsity gradients are very small anu winu stress
is the l11ain forcing factor of cii'clilation in the area. The x-component of the
wind stress sccms to oe corrclated to cast component of thc current in thc mixed
layer. O\"cr the continental shclf salinity stratification was ooservcd in winter
and during spring salinity ami temperature stratification was obsen'ed above the
upper current meter. Intcnsification of the currcnts in the central period could
bc duc to a lack of stratification.
At the shelf break at the beginning of the samplillg period the upper :WO m of
the water column was nearly homogeneous, out in 1\Iay watci's in thc vicinity
of 11l00rings 2 and 3 wcre stratified. \Vestward currents are forced by a high
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• pressurc cell over thc I3ay of I3iscay. If thc situation is not persistcnt, as was the
case aroulld February 28 amI !\Iarch 12, a pulse 01' wcstward current 01' 1:2 cm/s
01' 14 cm/s at 75 m depth and weaker currents at ISO 1Il depth were observed.
Ir the high pressure cell ispersistent, westward Oow persists over thc period at
75 m dcpth as was the case iil thc sccond half of ~Iarch anti April with some
Ouctuations in intensity, and reduced wcstward Oow is found at ISO m depth.
(b)
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~10DEL
VALIDATION
Thc horizontal model grid resolution is 13 x 18.5 km in the Bay of Biscay. Silllulatecl CUlTent data is taken from that model grid box which cncompasscs the area
in wh ich the current meter mooring was deployed. The appropriate layer is chosen in accordance with thc dcpth of t11e currcnt meters. r..10del data is availablc
once a day alHlwas extracted at the specific grid points and layer depths. Time
series were constructed for the period February to May 1995 amI were compared
to thc obscrved currcnt time series.
The modelled time series 1'01' mooring 3 is given in fig. 5b. It ('all Iw seen that
modelIed currcnts speeds orten underestimate observcd LUlTeHt speeds. C'urreHt
Ouctuations in thc u - componcnt (cast - west.) are reprodliceJ by tbc model.
although peaks do lt~t always coincidc. SilHulated ClI1Tcnt \'ariability in tlw v component (north - sOllth) is rl'duced comparl'd to the observations.
Thc corresponding model time series (model layer 5) 1'01' mooring 2, top current
meter is shown in fig. Gb. Again current speeds are gene rally unJerestimated by
thc model bot11 in thc u- and cspecially thc v - components. Current fluctuations are reproduced by the model aud usually peaks in current speeds between
obsen'ations and simulations coincidc.
The model timc series (model layer 7) for mooring 2, bottOlll current meter is
shown in fig. 7b. 1\lodelled and observed time series show many similarities
for this currcnt meter. r..la:ximum observe'd currcn t speeds are
12 cm/s whill'
ma.ximum modelled current speeds are
10 cm/s. Current variability is similar
for observation and simulation both for the u- and v-component and usually
current speed peaks coincide.
I"v
I"v
(c) HAKE EGG AND LARVAE DISTRIBUTIONS
Figs. Sa an<! Sb show the distribution of st.age I hake eggs Oll the first crllis(' alld
G nun larvae on thc second cruise respectively. Uoth larval <llIll egg abulldallces
were an order of Illa~nitude higher ill ~larch titan they were ill February sltowillg
that thc secoild cruise \vas eloser to the seasonal spawning peak.
In February, stage I eggs werc found in two patches (Fig. 8a). A bigger patch
extenclecl from 45° N to 4Go N having 10wer abundances on the central transect.
Eggs \vere more abundant over the shelf edge both in the northern amI in the
southern tips of the patch and extended over the outer platform (100-200 m)
7
in the northern and southern parts. Another, smalleI' patch \vas located in the
southeastern corner of thc Bay of ßiscay (eastern Cantabriall Seal.
The second cruise started on 22 1\larch, 29 days after the end of thc first one, and
was completed on 30 March. Larvae bigger than G lllm SI. appeal' in t\\'o areas
(Fig. Sb). A more extensive one in the northern part of thc sampling area, from
·16° to .l/o 15' N, with main abundances around ·17° N over the shelf edge and
20-40 miles off the shelf cdgc. This patch cxtcndcd further to the south down to
·16° N over shelf waters 100-200 m deep. A second. snwller patch was observcd
in the southeastern corner of the Ba)' of Biscay.
(d) TRACER SIJ\lULATIONS
Currents in the region around the Iberian Peninsula. were simulatcd for 19!H and
1995. The simulated eurrents as \\'ell as the daily variance of the CUlTents for
February ami 1\Iarch 1995 served as input to the t.ransport model. :\n examplc
of the eireulation field on 10. ~l. 19!)5 in model layers 5 anel G is given in figs. !Ja
amI 9b.
A simplified scheme of vertical distributions of hake eggs amI larvae in the Bay
of Biseay base<.! on the field data eollected in the February emd l'darch cruises
was used in the transport model. As most eggs and larval' were found in the
depth range of 50 - 150m (1\lotos et al., 1996), the tracers migrated randomly in
this depth range, i.e. tracers spend an equal amount of time in thc modellayers
between 60 - 150m. The tracers thus [eeI the combined effeet of thc currents in
this depth range. This random migration is not light depcndcnt as the \'crtical
distribution of hake eggs and larval' was similar during night and da,\".
The distrihution of stage I eggs mound 1i February (fig. Si\.) was illtcrpolatcd Ollto
the model grid (fig. 10a) and servcd as the initial distribution for the dispersion
simulation. Thc simulation was begun on li Februar)' CIS practically all stage I
eggs \vere caught on 17 and 18 Februar)'. The G mlll hake larvCle collectcd on
the second eruise bewteen 22 - 27 1\larch (fig. Sb) was interpolated onto the
model grid (fig. lOb) and served as the final distribution for comparison with the
dispersion simulation. The dispersion simulation was terlllinated on 27 1\Iarch
as most 6 n1111 larval' werc caught 011 26 and 27 March. except for those in thc
southeastern corner of thc ßay of Biscay which \\'ere caught on 2~ ~darell.
This cmise sun'ey design enab!cd the testillg of the trallsport llIodd hy llsing t!H'
distribution of stage I eggs at time Tl as initial distribution, sillllliating for :39
days and cOlllparing the resulting tracer distribution at timc T2 with thc 6 mm
larvae distribution actually found during the second cruise.
Tracers were introduced into thc model domain at the appropriatc points given
by the interpolated egg distribution in fig. 10a ill the depth range GO - 150m on
17 February. Low concentrations are in yellow and the highest concentrations are
in \"iolet. Three distinct areas of high stage I egg concelltrations ean be secn. The
northel'll and centra! (at '15°) arcas lie elosc to thc shelf cdgc. Thc ccntret! patch
8
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•
with the highest concentrations lies further off shelf than the northcrn patch.
Another patch \vith lower concentrations than thc other t\\'o is evident in thc
southcastern corner of thc Day of Biscay.
Thc interpolated larvae distribution of thc 6m111 length dass given in fig. lOb
shows six areas of highcr concentrations w.r.t. the surroullding areas. The highest eoneentration was found in the north at ·16 0 JO' N and :) 0 \\1 and was
situated olT-shdf in deeper water (2000 - :noo 111). :\nother high concl'ntration
patch was found in the southeastern corner of the Uay of Uiseay. Four sllwller,
lower eOllcentration pntehes werc found along thc shelf edgc, sitllated in a band
stretching from ·16 0 50' N southeast.ward to ·15 0 ·10' N Oll the shelf side in water
oetween 100 - 200m deep.
Thc tracer distribution after 3!J days dispersion is givell in fig. 10c. Five areas of
higher concentration W.r.t. the surrounding areas can be seen in thc distribution.
Thc highest concentrations ",ere found at the shelf cdgc at .15 0 20' N (md in the:'
southcasteru part of thc Day of Biscay. Three iIl'e<lS of high!..'r COIlccntratiolL cau
bc seen aligned aloug t.he shdf edge on t.lle sllelf side l"rolll ./GO :W' .\l \,0 _1;")0 ·10' N.
COlllparing the si mula ted distri bu tiolL (Iig. 1Oe) to t.lw obsen'cd la l'\'ae distri bution (fig. lOb) it is evident that there are two mailL discr<~pancies. The obsel'\'(~d
high conccntration of l,u'\'ae in the north at ·lG 0 :30' N and ;j 0 \V is not evident
in thc modelIed distribution. Secondl)' t.he high simulated tracer eOllcentration
at thc shelf cdgc (at .15 0 20' N) eanllot be secn in the observed distribution. On
thc other hand, simLilatcd and ohsen'ed distributions are in accordance in the
southeastern Bay of Discay and in the threc distinct higher coneentratioll areas
along the shelf edge from ·16 0 30' N to ·15 0 -10' N.
.
5. DISCUSSION
•
As current meters measure at a specific point and llumericalmollels give clIrrents representative for an area, i.c. a model grid box, it is commOll to comparc transport
rates through a section of a lltuuber of moorings to the appropriate scctioll of a number
of model points. This was not possiblc in this case as moorings \\'cre located such that
no transport rates throllgh a section perpendicular to the shdf edge. i.c. pcrpcndicular
to the dominant current direction could bc ealclllated. Thus vclocity time series of
cularian mcasuremcnts (currellt meter) are COlll pared to veloei ty ti mc series represcntative for an area of 240.5 km 2 (model). This is one reason for model - mcasurement
disercpalleies.
A seconcl important eause for discrcpancies between obsen"ations and si;nulations is thc
horizontal resolution of the windstress fields from ECi\lWF usecl in the simulations.
These dnta have aresolution of 1 x 10 ",hieh is subsequently interpolated onto the
model grid. This horizontal resoltition causes a spatial homogeneity of thc windstrcss
fields which is not necessarily ohserved in reality. Sma11 scale fronts, which are linked
to strongly fiuctuating momentum Buxe::; at the sea slll-facc (\"OU Storclt, 198-1) 01'
9
mesoscale vortiees whieh givc ris~ to exccptional strong winds (Paulus, 1~)8:n can
cause strong currents. These phenomcna arc not rcsoh'cd Ly thc windstress field use<!
for the foreing of the eirculation model. Thus clIl'rent peaks in the observations caused
Ly such small scale phenomcnii will bc laeking in the simul'lted time serics.
The third major reason for diserepancies is to be found in thc densilY field u"ed in
the simulations. As no quasi - synoptic density fields for tltt' lllOdel area and the
time under eonsideration are existent, lllonthly climatological dcnsity fields are used
(Levitus, 1!)S2). These da ta have aresolution of 1 x 1° which is intcrpolakd onto thc
mode! grid, Although these data give a good approximation of the 1Il0nthly density
fieltl for the area. under consideration, they wi 11 neyert heless bc different t 0 I hc real
dcnsity field for the time pcriod eonccl'lled. Different horizontal density gradi(~nts and
l1lcsoscale eddies me to he expccted in thc real dcnsity field and these will inf!uenc<'
the CUlTent obse1'\'ations.
Bcaring in mind the abo\'clllcntioncd eOlllmcnts, the agreC\llent betwe('n o1>s('1'\'('d ,md
modelIed currenls is good and thc disercpaneics are mainly caused by local, \W.'soscale
effects whieh are not resoh'ed by thc input data to thc circulation model.
The cgg/larvae dispersion exercise assumed that 6mlll lar\'(w oLsen'cd dlll'ing the second cruise deriw'd frolll the stage I eggs ob8el"\'cd dlll'ing the first cruisc, w!lieh de\'('loped at incuLation tcnlJwmtlll'es of arou!ld 12°C (~Iotos ct aL 1!J!J6). The a\'ailablc
data on hake egg dC\'e!opment amI hake laiTal growth rates was revised and rorlllS t.he
basis of this assumpt.ion (see section :k). This assulllption is also supporkd hy tlw
Smallllllll1ber of larval' I'l.l'ger than Gmm found during t.he sccolld cruisc at :~ stations,
These stations coincidcd with stations where (j nHll lan'ac were found. 11. lllUSt. be
noted hcre that the catchability 01' plankton uds depellds, aillollg olher fadors, on
the sizc of fish larvac to be caught.. Large lan'ac are more mobile (lnd are aGle to
cseapc the nds and t.hus thc abundances 01' large lan',1e obselTed during field sur\'cys
are gellerally lInderestimated (Sherlllau and 1I0ney, lU'1). IIo\\'c\"cr, differellt authors
have reportl'd catches of rather large lan'ae ",hen using BONGO ncts. For iustance,
Olivar et al. (1!JS8) reported ·10 C111 BONGO catches of C(\pe hake lar\'ae, ~Ierluecius
capensis, up to a Icugth of 16 llUll, with a. unifol'll1ely decrcasiug frequeney histogralll
from 3nll11 to 11.5m111 and discontinllous catches aftcl'wanls. Taking illto account l!Ie
use of 60 C111 nONGO nets iu the current in\'estigation, it cau Ge cxpected that lar\'al
abundances in the eateh are an cstimation 01' the field ahundances of larval' llntil thl'Y
reach about 10 m111 length. Furtherlllore, small changes in gl'owth rates (cg. 0.0·1
mm/day) resldts in a larvae length diffcrcnce of 1 111m after 25 days. These factors
need to be considered when comparing the simlllated tracer distributions with the
observed G lllm larval' distributions,
During the second cruise high llumbers of 6 mm lan'ae were found on thc two northern
sections (fig. Sb). Duc to bad wealher these two sections, exc('pt for foul' statiolls,
. were not salllpled on thc first crllise. This could imply t1wt CI sizl'abl(' portioll 01'
egg production in this area was missed, Furthermore, egg abum[anccs 01' stages ülder
than stage I peakcd in thc llorthcl'lllost station 01' thc Februar)' cruise, showiug that
there were high egg abundanees in the area. These zero stage I egg aGundanees in
10
•
•
the far north, which are used in the initial'distribution in the tracer simulation could,
at least in part, account for the cliscrcpancy bctween larvae observations and tracer
distributions in the far nm,th.
'
The high simulated tracer conccntration at the shelf edge (at ,15° ~O' N) origiuates frolll
the high egg concentration at 45°N (fig. 10a) which has been dispersed northwestwards
along the shelf edge. This high concentration is not evident in the larvae obsen'ations
and may be duc to either enhanced advection onto the shelf in reality, high mortality
which is not considered in the transport model 01' stronger advection along the shelf
edge than given by the model system.
•
•
The European hake has several known juvenile nursery areas (Anon., 1996). In the Bay
of Biscay, a main nursery area is 10cated off the northern coast (Southern ßritauny),
where hake juveniles (0 group) concentrate from 120m water depth to the coastline
from the end of thc spring to the autulllll (Bez ct al., lU95). llo\\'c\'er. bakt' eggs in tbi~
area in Februar)' aud ~darch are distribuled wcll offsbore lllaillly o\'t'r tb(' sbelf \'dg<'
(l\lotos et al., 19%). Eggs and larvae with tll('ir lilllited acti\,(, borizoulal S\\'iUllllillg
abilities, must therefore be transported towards the coast o\'er the 5helf into the llursery
areas. Prevalent residual currents in the area and diffusion processes will determine
if the eggs and larvae migrate towards the coast, remain in the spawning areas 01'
. are dispersed offshore. Data on recruitment in the Northern ßiscay area show that
settlement of juvenile hake starts in the outer part of muddy bottoms and that a
progressive displa.cement proceeds towards the inner basin (Guichet, 1988). At any
rate, larvae must appeal' over the platform to have chances 01' settling onto lllUtidy
bottoms and a general transport across the shelf would enable the dispersiou into tbe
nursery areas.
The results 01' the tracer simulation show that northern transport Was prevalent in the
area in the relevant layers where the eggs and larvae developed (Fig. 9). In addition,
it also shows drift onto the shelf (Fig. 10e). These conditions, both northern transport
and drift onto the shelf, favour the arrival of hake larvae in coastal areas. In the field
data, a patch of 6mm larvae extended over the platform between .!G 0 and ·n° 15'N at
water depths from 120m to 160m. These larvae are probably approaching the nursery
areas which start at 120m water depth (Guichet, 1988).
1I0wever, the main patch 01' Gmm hake larvae is found along <lnd off tl1(~ shelf edge at
47° N. This larval patch roughly Coillcides \Vi th a me::;o::;cale gyre ::;tructl1l'e l cyclonic,
70 km diameter) identified in the arca (fig. 11). The gyre was located off shell' and
vclocities were of the order of 10-15 clll/s (l\lotos ct al., 199G). This gyre can entrain
outer shelf and shelf edge waters and conscquently rctain biological material, c.g.
fish eggs and larvae 01' it can advect the biological material towards offshore areas.
This mechanism would act against the onshore transport of hake eggs and larvae into
nursery areas and can potentially be detrimcntal for thc success 01' the recruitment.
Anothcr known nursery area is the southcastern corner of the Bay 01' ßiscay at '" 2°W
(Sanchez, 1993). There, settlement to thc bottolll starts in water dcpllts dos<' to ~OOlll
an<! juveniles progressively Illove tO~\'<nds the 100m depth contour. Afteni:ards, they
11
disperse over the nursery area located between 100 anel 200m water depth (Farina
and Abaunza, 1991; Sanchez, 1993). In this area, a patch of stage I eggs anel another
patch of 6mm larvae w"ere found in the first and in the second cruise, respectively. This
suggests that eggs spawned in February were retained in the arca and de\"clopcd to
6mm larvae by 22 1\larch.
Alternatively, the occurrence 01' 6111111larvae cau result frolll thc (lCCUlllulation 01" larvae
coming from tbe coastal areas of thc Cantabriall Sea.. The gClleral circulation ill thc
area often shows a.n castwarcl transport elose to the coast amI a recurring cyclonic gyre
in the soutbeastern part of the Bay of Biscay. This can result in the acculllulation
of biological material at the eastern tip of thc Cantabrian Sea, where the directiol1
pattern of the currents is variable and the speeds diminish. The general castward
transport and northwestcrly wind regime may explain the entrainmcnt of hake lar\"ae
on the Cantahrian shelf during winter months (Sanchez, 1993; Cabanas, 1993). This
retention proccss in the southeastcrn part 01' the Bay 01' ßiscay was correctly silludated
by the model system.
•
ACKNOWLEDGEIVIENTS
This research is fundcd by the Europeall Union under grant NI'. AIR2 - CT93 - 1105. 1'hanks
goes to ECl\lWF (European Centre for l\Iedium-Hallge WeatlH'r Forecasting) in HC'ading anel
the DKRZ (Deutsches I~lirna Rechenzelltrum) 1'01' lIlaking c\.\'ailable th(> lllt't('orological dala
for the circulation model. We are grateful to the Spanish Instituto Nacional de l\letcorologia
for providing us with meteorological time series at Cabo Penas. Adolfo Uriarte amI Paula
Alvarez collaborated in the processing anel analysis of the hake egg allel larval cruise data.
Finally we would like to thank aH SEFOS colleagues who participated directly 01' indirectly
in this investigation for their contribution.
12
•
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analysis of tidal currcnts (submitted to Oceanologica Acta)
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BACKllAUS, J. 0., 19S9: On the atmosphcrically induced variability of the circulation of
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BEZ, N., J. RIVOIRAHD and J. Ch. POULAHD, 19!)5: Approchc transitive ct densites dc
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in a tidal river applied to the river EIbe. Advances in \Vater Resourees, 1, 161 - 1GS.
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Gonzalez-Garces, A. y Pereiro, F.J. eds. 1994. Jornadas sobre el cstado actual delos eonocimientos de las poblaciones de merluza que habitan la platafo1'1na continerital Atlantica y
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CASARINO B. and L.MOTOS, 1994: Identification and distribution 01' hake, M erluccius
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Ass. U.K., 74: 449-450.
13
COO1\lBS, S.lI., amI l\lITCHELL, C.E., 1982: The dc\"clopmcnt rate 01' eggs and larvac of
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Estudio <.1inamico en la plataforma contincntal dei norte de Galicia (Abril-junio 19!)1). Inr.
Tee. I.E.O. NI'. 13-1, 2:3 pp.
FARINA, A.C. and ABAUNZA, P., 1991: Contribueion al cstudio dc los juveniles de merluza
entre Cabo Villano y Cabo Prior (NW Galieia) mediante prospeeciones pesqueras. Bo!. Inst.
Esp. Oceanogr. 7 (2): 155-1G3.
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•
INSTITUOT NACIONAL dc 1\IETEOROLOGIA, 1995: Boletin mcteorologico diario, 1\Iadrid, February - 1\Iay 1995.
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14
•
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•
STAUENO, P. J., HERMANN, N. A., BOND, N. A. anel S. J. BOGRAD. 1D9.S: ~Iode­
ling the impact 01' climate variability Oll the advection 01' larval walleye pollock (l'/w/'ag/'a
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simple behaviour on the distribution 01' co<! amI haddock early lire stages on Geürges Bank.
Fisheries Ocea,nography, Vol. 2, Nr. 2, pp. ·13 - 6"1.
•
15
~------------
--
----
•
Fig. 1 Southern SEFOS model area and topography showing the 100m, 400m, 300m and
2000m isobaths.
.
M 2-Tide, Wiudst.ress
and Dcnsit.y Fields
Circulnt.iou Modcl
Sout.hern SEFOS Area
Fish Larvne
Dist.ributions
Sca Sm'face Elevat.ion
3-D Cmrcnt Fields
'l'ran~port Mod(!l
(Advect.iou, Diffusion nlld
Vertical Migrat.ion of Lnrvac)
Drift Rontes
Tn\cer Distributions
Fig...2 Schematic diagram or model system. Oval boxes denote input and output data.
•
..
45°
500m
200m
100m
44°
·90
•
-60
_8°
Fig. 3 Location of moorings. Mooring 1 was deployed at 1150 m depth, mooring 2 was
deployed at 568 m depth at the shelf break and mooring 3 was deployed at 90 m depth on
the continental shelf. The 100 m, 200 m and 500 m isobaths are indicated. ,
N
0:
-'-" "!1J~
•.•.•
:'
':~:;
y '\
. ".
.' 00
0;
(,orD
:
..
,fP
•
-
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o
o
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rit°
-
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00.......
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otea;
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7
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W6
4
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100....
.\
SAN sEBAmAN
5
"
Fig. 4 Sampling grid of the hake egg and larval surveys carried out on board RV INVESTIGADOR in 1995. Two consecutive cruises covered the time periods 15 . 24 February alld
22 - 30 March respectively. The same grid was to be covered in both cruises. See legend for
details on the sampling scheme. Areas surrounded by closed lines were not covered during
the February cruise.
O'1/S
FEB
HAR
""PR
HAY
FEB
HAR
""PR
HAY
("HIS
•
Fig. 5 a) Observed current components (u bottom, v top) at mooring 3 (74 m) between
February - May 1995.
Oll'
~
..
FEB
HAR
""PR
HAY
FEB
HAR
""PR
HAY
CM/S
Fig. 5.. b) Simulated current components (u bottom, v top) at mooring 3 in modellayer 5
(80 m) between February - May 1995.
.
•
•
CM/S
l-
--L
-..l.
KAR
FEB
--I-
--'
MAY
APR
('"HIS
,.
~ ,
.
,. .L--
--L
FEB
.1--
....L.APR
KAR
--'
KAY
Fig. 6 a) Observed current components (u bottom, v top) at mooring 2 (80 m) between
February - May 1995.
CHIS
,,~ .'L-~
_ _----L
_L___'__
~
__l_
FEB
KAR
APR
KAY
FEB
KAR
APR
KAY
CHIS
~
.,
.
Fig. 6 b) Simulated current components (u bottoin, v top) at mooring 2 in model layer 5
(80 m)' between February - May 1995.
CHIS
~
."
FEB
HAR
APR
HAY
CHI!';
"
.L-
FEB
-L..
-L..
HAR
APR
_
HAY
Fig. 7 a) Observed current components (u bottom, v top) at mooring 2 (180 m) between
February - May 1995.
CHIS
FEB
HAR
APR
HAY
FEB
HAR
APR
HAY
alls
~
...
Fig. 7 b) Simulated current components (u bottoin,
(175 l~) hetween February - May 1995.
V
top) at mooring 2 in modellayer 7
--------1
J
N
48
~(;v
",,
NANTES
47
..
'".-
'
••
••
".-
19~.'S
16-12 Ji'ebruary
STAGE I HAKE EGGS <111 1_1)
•
45
f-
.....
RN "INVESTIGAVOR"
+....:
44
GIJON
BILBAO
w ,
3
4
5
SAN SEBAS11AN
Fig. 8a: Hake stage I egg abundances found during the survey period 16-22 February 1995 (mid date:
19 February). Egg abundances are given in numbers per 10m2 •
N
48
. -< h)(" "'_ .
. '"
NANTES
47
...'
'
,,.;.'
RN "INVESTIGADOR"
11-17 M.rcla 1995
'mm HAKE LARVAE (IIIIOm1)
'"
.~
45
-""
44
lvI) 11'
SAN SEBASTIAN
43
w ,
5
4
3
Fig. 8b: Abundances of6 mm hake larvae found durlng the survey period 22-27 March 1995 (mid date:
25 March). Larval abundances are given in numbers per 10m2 ,
1·'"
././,/./ • • • • • • • " \ \ \ 1 \ \ 1 . , , • • • • • • •
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Fig. 9 Simulated circulation field on 10. 3. 1995 in modellayer 5 (SOm, (a)) and In model
layer 6 (125m, (b)). Smallest arrows denote speeds< 2 cm/s. As arrow size lllcreases,
current
.' speeds increase by 2 cm/s increments.
•
10 a)
44°N
•
10 b)
44°N
•
10 c)
Fig. 10 a) Observed hake egg (stage Ia and Ib) distribution in the Bay of Biscay on 17
February 1995 interpolated onto the model grid. Initial distribution of tracer simulation.
Fig. 10 b) Observed hake larvae (6 mm) distribution in the Bay of Biscay on 27 March 1995
interpolated onto the model grid.
Fig. 10 c) Simulated tracer distribution after 39 days dispersion in the Bay of Biscay on 27
March 1995.
f-==·~-·==-=~"==-=~·=··~~.--LÄ·';,~=-=oo1l=:,=::,,;=(=:-=-i-=:~=.-=-~~~-'~_-!-_~=====~'
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Fig. 11: Dynamic heights (0.2cm intervals) from 10/100 dbars, indicating the general geostrophic
circulation (arrows) during the period 22-27 March 1995. Open circles illustrate the abundance of 6mm
hake larvae (the biggest circle represents 7 individuals ofthe 6mm lenght class per 10 m2).
Table 1: Summary ofthe infonnation available on the egg development and larval growth rates ofhake.
VaJues in the table are time (in days) to reach the different developmental stages: Hatching, end ofyolksac larva and different sizes ofpost-Iarva. In the latter, G refers to the Gompertz curve fit and L to the
Iinear fit to observed larval growth figures (Bailey, 1981). This author pointed out that larval growth
was better explained by a linear increment of 0.16 nun d'l up to approximately the 32nd day of live. In
italies, time to the end ofthe yolk-sac larval stage assuming that the growth pattern of M merluccius
at this stage is similar to that of M productus. See text.
ternperature
hatching
end of yolk-sac larva
length
post-Iarva
4 rnrn
5 rnrn
6 mm
7 rnrn
8 mm
Merluccius productus - Haller ( 1981)
10
11
12
13
5.28
4.81
4.40
4.05
14.68 12.73
11.08
9.68
G
L
22
18
26
24
30
31
33
37
43
36
1~
3.49
7.50
M. rnerluccius - Coornbs & Mitchell (1982)
10
11
12
13
15
6.82
5.96
5.27
4.71
3.85
16.47 14.09
12.12
10.49
7.96
G
L
23
19
27
25
31
32
34
38
37
44
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