lCES 1996
The ShelfEdge Current and lts Effects on Fish Stocks
0 ---
C.M. 1996/5: 12
Poster Session
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INCIDENC~
OF THE POLEWARD CURRENT
AND VERTICAL MOTION IN HAKE RECRUITl\IENT
by
•
Gi!, J. and F. Sanchez
Instituto Espaiiol de Oceanpgrafia
Apdo: 240. Santander, SPAIN
ABSTRACT
•
The historical series of battom trawl surveys carried out in autumn in the waters of Galicia and the
Cantabrian sea show that the processes ofhake recruitment lead to well-defined aggregations ofjuveniles
which are found in certain areas ofthe continental shelf. These concentrations vary in density according
to the strength ofthe year class, although they remain generally stable in size and spatiallocation. The
size ofthese aggregations, estimated using geostatistical techniques, is found to be between 30 and 50
km. The mesoscale dynamic and vertical motion ofwater masses associated with it seem to have clear
consequences with respect to the distribution and size of hake recruit concentrations. The areas of
greatest spatial eoincidenee are found where there is vertieal foreing (obtained from the fonnulation of
the vector Q in the Omega equation), whieh may be caused by hake feeding behaviour. This species
makes nocturnal vertieal migrations to prey on organisms living in upper layers.
1
fNTRODUCTlON
The area covered by this study (Fig.l) is the waters of Galicia and Cantabrian sea (lCES Division VIIlc
and the northem part ofIXa). In this area. european hake (Merluccius merluccius L.) spa\vns during the
early months ofthe year and adults gather in the deep \vaters ofthe shelfbreak (Fig. 2). The larvae orthis
species are usually found over the slope at depths between 100 and 250 m. The process ofrecruitment
of juveniles to the battom originates in the shelf break and they move towards shallov.,·er areas to
. concentrate in autumn in bottoms between 90 and 150 m (Sänchez, 1995). These juveniles make vertical
migrations at night to feed on spedes which live in upper layers, rnainly anchovy arid silvery pout (Olaso,
1990).
The historieal series of battom trawl surveys earried out in auturnn show that the processes of hake
reeruitment lead to well-deflned aggregations of juveniles whieh are found in certain areas of the
eominental shelf(Fig.3). These eoneemrations vary in density aeeording to the strength ofthe year class,
although they remain generally stable in size and spatia! loeation. The size of these aggregations,
estimated using geostatistical techniques, is found to be between 30 and 50 km. The distributions of
juveniles in the area studied over the last decade (Sanchez, 1995) show the highest concentrations in three
defined areas which are repeated every year, and which are those denominated as La Coruiia, Ribadeo
and Peiias. Iri some years another two smaller concentrations appear, that of Rias Bajas and that of
Guetaria. These areas ofhigh concentration, situated between 90 and 180 m and ~ith predomiriantly
muddy bottoms, must provide the appropriate habitat and ihe availability of the necessary food for
recruits.
•
The strength 0 f annual hake recruitment has been contmually 0 bserved in these waters since 1983 by
means ofa series ofautumn battom trawl surveys carried out by the RN CORNIDE DE SAAVEDRA.
The analysis of the recruitment indices obtained since 1990 (Fig. 4), together with the faet that the
biomass ofspawners has been at a historicallow (Anon., 1995) shows four years ofvery poor recruitment
(1990, 1992, 1993 and 1995) and two )'ears ofgood reeruitment (1991 and 1994). This implies, a eeitain
level ofindependence between the strength ofreeruitrnent and the spawner biomass, and the existence
of other faetors greatly affeeting the reeruitment proeesses. To studywhieh abiotie parameters may
influence the survival of recruits, the environmental variables from 1993 were studied under t\\'o main
working hypothesis:
1. The eurrent pattern in the slope area eonditions larval d.rift towards reeruitment areas. An intense
geostophic flow towards the western part ofthe Bay ofBiscay carries eggs and Iarvae towards the
oeeanic area, thus increasing mortaIity during the later process of recruitment to the bottom of the
continental shelf.
2. Once the juveniles are recruited to the bottom and are abie to move, their tater development is affected
by the avallability offood. Enriehing processes ofthe surfaee layers which strengthen the food chain may
favour the presence ofrecruits over the bottorri.
ThWLUENCEOFTHESHELFEDGECURRENT
ON THE STRENGTH OF RECRUITMENT
A flow begins to appear towards the interior ofthe BayofBiscay in aunmm in the study area. This flow
grows notably in the 'Ninter months due to the polewar'd current. This phenomenon ean be appreciated
in the results obtained in the central area ofthe Cantabrian Sea in 1995 (Fig. 5). Ifthe intensity ofthis
flow, which trrinsports warnl and sa1ty Vfaters, is intense enough to last unti! the spring, the salinity ofthe
2
•
surra~e la~ers iS observed to be abnonnally high. as happened in April in 1992 and 1993 (Fig, 6),
According to hypothesis 1, thiS is reflected iri very 10\" recruitment indices in these years (Fig. 4):
IMPORTANCE OF ~1ESOSCALE ACTIVrry IN THE LOCATION
OF AGGREGATIONS OF RECRUITS .
Thesalinityobserved at 20 m in the autumn surveys shows 1993 as being radIcally different to 1994 and
1995 \\ith respect to circulation over the continental shelf-slope area (Fig. 7). The high va.Ities of saliriity
observed in 1993 are a consequence ofa ctirrent pattern towards the eastemmost panofthe Bay of
Bis~ay. To the contraIy~ the low salinity fourid in 1994 and 1995 is due to the absence ofthe supply of
salty \vatersfrom the west. In the eastemmost area, it is seen how waters ofvery 10w saliriity appear in
these last two years, originating from flows corirlng from the French contirieniril shelf. In 1994 these areaS
were paniC1llaIly conspicuous with nucIei cf very low scilinity (34.6) with clearly mesoscale
characteristics.
.
•
,
The temperature distnbu~ionS at 90 m clearly show the different originS ofthe water masses of 1994 arid
1995 With respect to1993 (Fig. 8). In the three )'ears studied, the important degree ofmesoscale activitY
iri the
ofanticyclonic eddies is clear, notably the repetition ofgyres over the coritinentril shelfwith
negative vomcity offui Coruila, Estaea-Ribarleo arid Cape peiias. This is due to the fact thai the entrance
ofthe relatively warm waterS ofthe poleward current is mtroduced between the cold coastal up\vellirig
\\aters and the cold stratmed waters outside the continentaI shelf in a geostrophic balance with negative
vorticity (anticYclonie). The fact tMt the presence oftheSe eddies is repeated in the same lireas in the
ihree years studied may be down to topographie faetors which induce negative vortiCity in the water
masses previously described.
form
The dynarnic associated to the eddies with important variations in vonicity causes an outstaridmg
ageOStrophie \'erticril motion. The Q vector fonnulation ofthe Omega equation (Hosldns et al., 1978) is
tiSed to stUdy the relationshiP between mesoScaIe StructUres ärid the induced ageoStrophic vertical forcing.
The convergerice ofthis veetor mdicates the regions where upward fOfCing eXiSts (Hoskins arid Pedder,
1980). 'I'he vertical motions are rei'eated to a gi-eatei or lesser eXtent in the areas of La Corufui, R1ba.deo
and Peiias in the three yearsStudied. In 1994 thei'e Was an intense vertical mesoscale activiij throughout
the area ofthe Cantabiian Sea (Fig. 10).
.
•
In accordance with the hypOthesis 1 and 2, the years of weak eastWard flows and intense vertical
mesoscale activity, regenerating the photie layer, may be favourable to hake recrwtment. In 1993 the
flows were clearly e3StwaId aIid mesoscale aetMty Was reciUced to the tmee areas ofLa Coruiia, Rtbadeo
and Peiias (Fig. 9), Which as a consequence gave nse to low recruitment indices and rhe presence ofhake
juveniles only mthese three areaS. 1994 was favoUrable mthe two hypotheses mentioned, .with weak
flows over the slope and great mesoscale aetivity thl"oughout the Cantabtiari Sea, which led to high
recruitment indices arid high density of recl"l.lits in the eoneentratioDs in this sea (RIbadeo, Peiias and
Guetalia). Thils, the originofthe concentration ofGuet3.riä.lies in the supply oflarvae fromspawners in
the French contmental shelt: In 1995, although the eastward floWs in Spririg were not very intense, leading
to a mvourable larv3l drift, the areas ofvenical motion were very scarce in the Cantabnan Sea (Fig. 11),
which caused the fall in recrWtmerit indices arid low den.sity ofconcentratioris.
REFERENCES
Anon. 1995. Repert oftbe Working Giciup on the ASsessment ofSouthem ShelfDemel"sal StockS. 6-15
3
September 1994. ICES Doe.CM 1995/Asses., 6, 545-598.
Hoskins, B.!., 1. Draghici and H.C. Davies.1978. A new look at the Omega equation. Quart. J. Ray.
Meteor. Soe., 104: 31-38.
Hoskins, B.!., and M.A. Pedder. 1980. The diagnosis of middle latitude synoptie development. Quart.
J. Roy. Meteor. Soe., 106: 707-719.
Olaso, 1. 1990. Distribucion y abundancia deI megabentos invertebrado en fondos de la platafonna
cantabrica. Inf. Tee. Inst. Esp. Oeeanogr. 5: 128 pp
Sänehez, F. 1995. Distnbuci6n espacial de los principaIes recursos demersales deI norte de Espaiia. Actas
IV Col.Int.Oeeanogr. Golfo de Vizcaya: 39-47.
•
•
4
- -
---------------------
so-
~-
FRANCE
46-
SPAIN
•
40-
4-
Figure 1. Studied area.
,....-----------------------------,0
Seit Edge Current
Mesoscale hydrographie anomalies
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-c
fNight
,,~
LARVAE
,
March-Apnr
EGGS
•
~ -+ 100
200
300
400
500
o
5
10
15
20
25
NiutiCalrriJeS
Figure 2. Lire cycle of euroPean hSke.
·5
30
--"""--------l600
40
35
Perias
44-
'\,~.'.:::;"":"";;""""'\
~
:::
.
Guetaria ::,
~ •••• ;
0" " . '
C.Ajo
:::
..
R. Bidasoa
HAKE RECRUITMENT
Main nursery areas (September-November)
•
5-
Figure 3. Main nuisery areas
750
Catch (nO) per trawt hour
,
, ' "
RECRUITMENT INDICES PER AREAS
500
•
250
o
1990
1991
1992
1993
1994
_Rias Bajas .La Coruria .Ribadeo DPetias .Guetaria
Figure 4. RecruitmeIlt indices per areas
6
1995
".0- .
43.S- .
•
. september 1995
.
GeOstiOPh~ CUrrel"lt at1 0 m (level 200)
~
SOcln's
Temperature at 90 m
43.2- '---_ _~:__---.J...-----L-----l---~.....J
4-
43.S-
•
..
43.S·
.3.4- .
DeC:8inbei 1995
GeOstfOphic current at 10m (leVel 200)
43.2- \--
Temperature
at 90 111
L......J.-
.-
_
--L-_~
_I.
....J
Figure S. Geostrophic eurrent and tei'IlperatUrc during September arid December of 1995.
7
44·
APRIL 1992
Salinity at 20 m
34.734.834.935.035.1 35.235.335.4 35.5 35.6 35.7 35.835.936.0
.......... ,
•
.
44·
APRIL 1993
Salinity at 20 m
34.7 34.8 34.9 35.0 35.1 35.2 35.3 35.4 35.5 35.6 35.7 35.8 35.9 36.0
42·
Figure 6. Salinity in April of 1992 and 1993.
8
•
AUTUMN 1993
Salinity cit 20 m
34.7 34.8 34.9 350 35.1 35.2 35.3 35.4 355 35.6 35.7 35.8 35.9
10"
9"
S"
7"
6"
5"
4"
3"
2"
4·
3"
2"
•
AUTUMN 1994
Salinity at 20 m
10"
9"
S·
6"
5"
•
AUTUMN 1995
Salinity at 20 m
'4'-ta·..· !.:~~~
t.
:"",":
.
_
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.'.
~
34.7 346 34.9 35.0 35.1 352 35.3 35.4 35.5 35.6 35.7 35.8 35.9
10"
9"
S"
Figur~ 7. Salinity at ~O In during autumn
9
cf 1993, 1994 Y 1995.
,s
I
,-
.i4.
43·
AUTUMN 1993
Temperature at 90 m
:~
11.5
10·
9·
8·
12.0
12.5
LJ'~·~·~~_
13.0
13.5
14.5
5·
6·
7'
14.0
15.0
'C
3·
4·
2'
•
AUTUMN 1994
Temperature at 90 m
12.0
10·
9'
12.5
13.0
6'
7'
13.5
14.0
14.5
5'
15.0
4·
°C
3'
2·
AUTUMN 1995
Temperature at 90 m
~"~';J.
~~~0~
12.5
10·
9·
13.0
S'
13.5
S·
-.
14.0
.::.
'.
14.5
15.0
'C
3'
Figure 8. Temperature (Oe) at 90 m during autumn of 1993, 1994 y 1995.
10
2'
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,
,
~
~.
.
~.
....
44'
,
43'
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.. ,
,,,..
I
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4
,
AUTUMN 1993
Geostrophic current at 10m (level 200 m)
--+40 cmls
S'
9'
10'
7'
6'
5'
3'
2'
•
AUTUMN 1993
Water vertical motion
Divergence of Q at SO m (level 200 m)
43'
·5
-4
-3
-2
·1
o
1
2
3
4
5
42'
10'
7'
S'
9'
:
. 5'
6'
3'
4'
2'
.
44'
•
43'
AUTUMN 1993
Hake recruitment (nO I hour)
....
100
250
500
750
1000
1500
2000
4000
42'
10'
9'
2'
8'
Figure 9. Geostrophie euerent, water vertieal motion and hake recruitment during 1993.
11
.,\
.
/,.\:',.,
... ,,,
....
,
~I
...
O'
44'
AUTUMN 1994
Geostrophic current at 10m (level 200 m)
---.
30 cmls
10'
9'
S'
7'
6"
4'
5"
3"
2"
•
AUTUMN 1994
Water vertical motion
Divergence of Q at 50 m (level 200 m)
43"
1
42"
10'
9"
S'
7"
6"
2
3
4
4'
5'
5
3"
2"
•
44'
43'
AUTUMN 1994
Hake recruitment (nO I hour)
I
I
100
250
I:~;:'t·:::;;t _
500
750
1000
_
1500
2000
4000
42'
10"
9'
S"
2"
7"
Figure 10. 6eostrophic current, water vertical motion and hake recruitrnent during 1994.
12
44°
r
.... -
.. '..
,
43"
\
I
( ...
/
/'
I
•
\
I
__
..
,
.,.
I
I
,
\
•
...
-
~
•
• ~ .•
.
AUTUMN 1995
Geostrophic current at 10m (level 200 m)
,
-+30 cmls
10"
9°
5°
6"
S"
4"
3°
2"
3"
2"
.'
AUTUMN 1995
Water vertical motion
Divergence of Q at 50 m (level 200 m)
43"
42"
10"
9"
S"
so
6"
7°
4"
•
44"
43°
AUTUMN 1995
Hake recruitment (nO I hour)
11··-
100
250
500
750
1000
1500 2000
4000
42"
10"
9"
S"
Figure 11. Geostrophic current~ water vertical motion and hake recruitment during 1995.
13