Deep-Sea Research II 53 (2006) 1363–1376 Meteorological and oceanographic factors influencing Engraulis encrasicolus early life stages and catches in the Gulf of Ca´diz Javier Ruiz a, , Eva Garcia-Isarch b , I. Emma Huertas a , Laura Prieto a , Ana Jua´rez b , Jose´ Luı´s Mun˜oz c , Alfonso Sa´nchez-Lamadrid c , Susana Rodrı´guez-Ga´lvez a , Jose´ Marı´a Naranjo c , Francisco Baldo´ d a Departamento de Oceanografı´a, Instituto de Ciencias Marinas de Andalucı´a, Consejo Superior de Investigaciones Cientı´ficas, 11510, Puerto Real, Ca´diz, Spain b Instituto Espan˜ol de Oceanografı´a, Apt. 2609, 11006 Ca´diz, Spain c Centro de Investigacio´n y Formacio´n Pesquera y Acuı´cola El Torun˜o, Junta de, Andalucı´a, C/ Nac. IV, Km 654, 11500 El Puerto de Santa Marı´a, Ca´diz, Spain d Instituto Espan˜ol de Oceanografı´a, C/Corazo ´n de Marı´a, 28002 Madrid, Spain Received 1 February 2005; accepted 4 April 2006 Abstract The meteorological and oceanographic factors that have clear influence on the distribution of Engraulis encrasicolus eggs and larvae in shelf waters of the northeastern sector of the Gulf of Ca´diz has been analyzed. Very high concentrations of anchovy eggs and larvae were found in this area during the spawning period (from March 2002 to September of 2002). The shallowness of the water column, the influence of the Guadalquivir River, and the local topography favor the existence of warm and chlorophyll-rich waters in the area, thus offering a favorable environment for the development of eggs and larvae. However, during spring and early summer intense easterlies were observed to cause a decrease of the water temperature by several degrees. Easterlies generate oligotrophic conditions in the area, and their persistence forces the offshore transport of waters over this portion of the shelf, advecting early life stages away from favorable conditions. These negative influences on the development conditions of anchovy eggs and larvae can impact on the recruitment of this species in the Gulf of Ca´diz. Therefore, inter-annual fluctuations in the duration of intense easterly winds is analyzed and discussed in connection with anchovy catches. r 2006 Elsevier Ltd. All rights reserved. Keywords: Anchovy; Engraulis encrasicolus; Gulf of Ca´diz; Fisheries oceanography 1. Introduction The northeastern side of the Gulf of Ca´diz possesses a relatively wide shelf, a feature quite unusual in the southern Iberian Peninsula where the shelf is typically very narrow. This shelf region is delimited to the east and west by the Strait of Gibraltar and Cape Santa Maria, respectively (see Fig. 1). In a previous study, empirical orthogonal function (EOF) decomposition of thermal images revealed that the shelf is statistically different with respect to the deeper parts of the basin, with warmer ARTICLE IN PRESS www.elsevier.com/locate/dsr2 0967-0645/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2006.04.007 Corresponding author. Tel.: +34 956 832612; fax: +34 956 834701. E-mail address: [email protected] (J. Ruiz).
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ARTICLE IN PRESS
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doi:10.1016/j.ds
�Correspondifax: +34956 83
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Deep-Sea Research II 53 (2006) 1363–1376
www.elsevier.com/locate/dsr2
Meteorological and oceanographic factors influencing Engraulisencrasicolus early life stages and catches in the Gulf of Cadiz
Javier Ruiza,�, Eva Garcia-Isarchb, I. Emma Huertasa, Laura Prietoa, Ana Juarezb,Jose Luıs Munozc, Alfonso Sanchez-Lamadridc, Susana Rodrıguez-Galveza,
Jose Marıa Naranjoc, Francisco Baldod
aDepartamento de Oceanografıa, Instituto de Ciencias Marinas de Andalucıa, Consejo Superior de Investigaciones Cientıficas, 11510,
Puerto Real, Cadiz, SpainbInstituto Espanol de Oceanografıa, Apt. 2609, 11006 Cadiz, Spain
cCentro de Investigacion y Formacion Pesquera y Acuıcola El Toruno, Junta de, Andalucıa, C/ Nac. IV, Km 654,
11500 El Puerto de Santa Marıa, Cadiz, SpaindInstituto Espanol de Oceanografıa, C/Corazon de Marıa, 28002 Madrid, Spain
Received 1 February 2005; accepted 4 April 2006
Abstract
The meteorological and oceanographic factors that have clear influence on the distribution of Engraulis encrasicolus eggs
and larvae in shelf waters of the northeastern sector of the Gulf of Cadiz has been analyzed. Very high concentrations of
anchovy eggs and larvae were found in this area during the spawning period (from March 2002 to September of 2002). The
shallowness of the water column, the influence of the Guadalquivir River, and the local topography favor the existence of
warm and chlorophyll-rich waters in the area, thus offering a favorable environment for the development of eggs and
larvae. However, during spring and early summer intense easterlies were observed to cause a decrease of the water
temperature by several degrees. Easterlies generate oligotrophic conditions in the area, and their persistence forces the
offshore transport of waters over this portion of the shelf, advecting early life stages away from favorable conditions.
These negative influences on the development conditions of anchovy eggs and larvae can impact on the recruitment of this
species in the Gulf of Cadiz. Therefore, inter-annual fluctuations in the duration of intense easterly winds is analyzed and
discussed in connection with anchovy catches.
r 2006 Elsevier Ltd. All rights reserved.
Keywords: Anchovy; Engraulis encrasicolus; Gulf of Cadiz; Fisheries oceanography
1. Introduction
The northeastern side of the Gulf of Cadizpossesses a relatively wide shelf, a feature quite
front matter r 2006 Elsevier Ltd. All rights reserved
unusual in the southern Iberian Peninsula where theshelf is typically very narrow. This shelf region isdelimited to the east and west by the Strait ofGibraltar and Cape Santa Maria, respectively (seeFig. 1). In a previous study, empirical orthogonalfunction (EOF) decomposition of thermal imagesrevealed that the shelf is statistically different withrespect to the deeper parts of the basin, with warmer
Fig. 1. Locations of the current meter (�), stations of CTD profiles and discrete samples of chlorophyll, nutrients, zooplankton,
ichthyoplankton (d), and CTD stations plus discrete samples of chlorophyll and nutrients (� ).
J. Ruiz et al. / Deep-Sea Research II 53 (2006) 1363–13761364
surface waters than the rest of the Gulf duringthe summer period (Vargas et al., 2003). Thermalimage analysis carried out in that study alsoshowed that the prevailing zonal winds in the areagenerate up- and downwelling conditions in theshelf when blowing from west and east direction,respectively.
Unlike the open ocean, the warm waters thatoccupy the shelf during the summer period are notoligotrophic, especially those at the inner shelfinfluenced by the Guadalquivir River (Navarroand Ruiz, 2006). These warm and biologicallyproductive waters are highly suitable for thereproduction of fish species such as the anchovy,since early life stages are expected to grow rapidly inan environment where biological productivity re-mains elevated within an otherwise very oligo-trophic basin (Navarro and Ruiz, 2006). In fact,previous cruises performed in the northern Gulf ofCadiz have confirmed that this sector is particularlyrelevant for anchovy spawning (Rubın et al., 1997),and additional studies carried out over an annualcycle have revealed the importance of the Guadal-
quivir River and its vicinity as a nursery area forthis species (Garcıa-Isarch et al., 2003). Besides thisproductive inner shelf, the lower reaches of theGuadalquivir River also seem to offer an adequateenvironment for anchovy post-larvae since highconcentrations of juveniles during the autum havebeen observed in the area (Baldo and Drake, 2002).This is similar to other spawning grounds foranchovy frequently located in regions influencedby river outflows, such as the Rhone, Ebro(Aldebert and Tournier, 1971; Palomera, 1992;Palomera et al., 1995; Garcıa and Palomera, 1996;Sabates et al., 2001), Po (Piccinetti et al., 1980;Coombs et al., 2003), Loire (Arbault and Lacroux-Boutin, 1977), Gironde, Adour and Bidasoa rivers(Motos et al., 1996). Moreover, other estuariessituated on the Atlantic coast of the IberianPeninsula are known to be spawning and nurseryareas for anchovy (Lopez-Jamar, 1977; Re, 1984,1987, 1991, 1996; Re et al., 1983; Ferreiro andLabarta, 1984; Ribeiro, 1991; Ribeiro et al., 1996),where enhanced pelagic production favors thegrowth and survival of fish larvae.
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The continental shelf in the region under study isabout 80 km long with a velocity field directedmainly alongshore as well as with intense hydro-dynamic processes taking place at its east and westboundaries. The occurrence of all these featuresplays an important role in the permanence of fishlarvae in the region. For instance, larvae leaving thearea by its east side are likely injected by theAtlantic jet flowing through the Strait of Gibraltarinto the Alboran Sea, where waters are either cold ifnot oligotrophic or oligotrophic if not cold (Ruizet al., 2001). On the other hand, on the west side ofthe shelf the extremely steep bathymetry of Fosa deAlvarez Cabral located near Cape Santa Mariawould generate complex slope processes with thepotential of transporting eggs and larvae towardsopen-ocean oligotrophic waters (Garcıa-Lafuenteet al., 2006). Regardless the direction of thetransport, the low productivity of surroundingwaters would cause the larvae to be exposed tohigh or even total levels of mortality.
The aim of this study was to analyze theinfluences exerted by the oceanographic features inthe Gulf of Cadiz in determining both the spatialand temporal distribution of anchovy eggs andlarvae in this shelf area during the reproductionperiod of this species. Basic conclusions drawn fromthis analysis allow us to explore hypotheses thatmight facilitate a better understanding of the originof interannual fluctuations observed in the catchesof this species in the Gulf of Cadiz.
2. Material and methods
2.1. Cruises
The study area was the shelf located between themouths of the Guadalquivir and Guadiana rivers.This sector was selected on the basis of previousknowledge of its particular oceanographic behaviorobtained through the analysis of satellite images (seeVargas et al., 2003 or Navarro and Ruiz, 2006) andon additional exploratory studies (Sanchez-Lama-drid et al., 2004) which indicated that the area islikely to contain high concentrations of anchovyeggs and larvae.
Seven cruises were carried out each month in 2002from March to September, on thee 22–24, 2–5,7–10, 11–14, 2–5, 7–10 and 11–14, respectively, onboard the vessel Regina Maris. For each cruise, agrid of 30 stations was sampled (Fig. 1) and CTDprofiles were obtained. Samples for inorganic
nutrient analysis (30ml), total chlorophyll (500ml)and chlorophyll in the size fraction above 20 mm (3 l)were also taken. Nutrients were analyzed with anautoanalyser TRAAC 800, and both total andfractioned chlorophyll measured by standardfluorometric methods (Parson et al., 1984) with aTurner Design Model 10 fluorometer.
Ichthyoplankton samples were collected on 26stations (Fig. 1). At each station, double-obliqueplankton hauls were conducted using a Bongo netwith 40-cm mouth diameter and 200-mm mesh size.All tows were performed at a vessel speed of2–2.5 knots and to a maximum depth approximately5m above the bottom. Two independent flowmeters‘‘General Oceanics 2030’’ fixed on the mouth ofeach net were used to measure the volume of waterfiltered. Immediately after collection, planktonsamples were fixed in formaline at 4% bufferedwith borax. Samples from one cod-end werepreserved for zooplankton quantification, whilesamples from the other cod-end were used forichthyoplankton studies. Once in the laboratory,anchovy eggs and larvae were sorted, identified andcounted from the ichthyoplankton samples under abinocular microscope. Zooplankton was quantifiedby estimations of sedimented plankton volumes.Ichthyoplankton and zooplankton data were stan-dardized to 100m3 of filtered water (eggs or larvae/100m3 and ml/100m3, respectively).
Additional CTD casts were carried out in thecourse of the vessel on the stations marked in Fig. 1,this allowed to obtain high spatial resolutioninformation of the hydrological properties of shelfwaters. Due to computer malfunction, CTD data inAugust is missing.
2.2. Meteorology and currents
Hourly values for the wind vector, daily recordsof rainfall, and air humidity were obtained frommeteorological stations located at Huelva andCadiz. Additional meteorological and water tem-perature data at the surface of the open-sea areaclose to the Strait of Gibraltar were obtained from apermanent SEAWATCH mooring held by Puertosdel Estado (450m depth, 36.481N 6.961W). Currentswere recorded by an Aanderaa RCM9 at a site of17m depth situated around the center of thesurveyed area and with little exposure to fishing(Fig. 1). The current meter was moored at 5mabove the bottom and recorded water temperatureas well as current direction and magnitude every
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10min. Data recorded by this instrument showedthat tidal variability was about 27% of the totalvariance. These periodic and weak tidal currentswould produce a negligible net transport ofbiological properties when compared with the moreimportant role played by the subtidal currents.Consequently, tidal contribution was removed fromthe velocity observations and only the low-passed(subinertial) series has been considered in theanalysis.
3. Results
3.1. Winds and currents
The zonal component of the wind vector wasdominant in the area during the sampling period,and this component has been plotted in Fig. 2(A).This dominance, as will be shown later, hasimportant implications for the generation of cur-rents as well as for the biological productivity andtemperature of the northern shelf of the Gulf. Thecoastline delimiting the north border of the shelf hasa zonal orientation that favors inshore and offshore
Fig. 2. (A) Zonal component of the wind velocity recorded at Huelva
values correspond to east and westerlies. Temperature recorded in the m
the periods when cruises were carried out. (B) Components of the curr
line). Positive and negative values indicate southeastward and northw
(alongshore component) and on the right (cross shore component) axis
surface currents in the presence of easterlies andwesterlies, respectively, due to Ekman transport.During the sampling period the most frequentdirection of the wind was from the west, blowingduring 62% of the time. Although less frequent,easterlies occur in intense bursts, particularly at theCadiz meteorological station, which is closer to thestrait (Fig. 2(A)).
Currents recorded at the mooring site were highlysensitive to the wind direction (Fig. 2(B)). A plot ofmeridional and zonal components of the currentsregistered in the area over this period exhibited aparallel to the coast preferential direction (Fig. 3).Time series of the non-tidal components of thecurrents parallel and perpendicular to the coast(which coincide with the major and minor axis ofthe ellipse in Fig. 3) clearly showed that westerlywinds caused currents directed towards the south-east and offshore. In contrast, when easterlies blewthe currents were towards the northwest andinshore. This finding is consistent with the overallsummer increase/decrease in biological productivityof the shelf remotely sensed by Navarro and Ruiz(2006) under westerlies/easterlies conditions using a
(thick solid line) and Cadiz (broken line). Positive and negative
ooring site is also shown (thin line). Bars at the bottom indicate
ent velocity vector, alongshore (thick line) and across shore (thin
estward flow, respectively. Notice the different scale on the left
.
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statistical analysis of a long series of SeaWiFSimages. In addition, the current system in the regionseemed to overshoot when intense westerlies sud-denly shifted to easterlies (as in early April in Fig.2(B)). This pattern is expected to result from a
Fig. 3. Plot of meridional versus zonal components of the current
vector recorded at the mooring site during the sampling period.
Rai
nfal
l (m
m/d
)
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15
25
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e (ºC
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x (M
J/d)
-20
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(A)
(B)
(C)
Fig. 4. (A) Zonal wind (thick line), relative humidity (thin line), and te
Cadiz. (B) Net balance of energy (incoming short-wave radiation plus l
line), incoming radiation (upper thin line) and heat fluxes (sensible plus
calculated according to Gill (1982). (C) Precipitation (bars) and Guada
period of study.
pilling up of waters at the area around theGuadalquivir River mouth during periods ofwesterlies, as this portion of the coastline is quasi-perpendicular to the overall direction. Pilled waterswould then be released when wind shifts from westto east.
3.2. Temperature and salinity
Changes of wind direction are not translated in astraightforward response of water temperature atthe RCM9 mooring (Fig. 2(A)). This probablyresults from temperature responding to both heatfluxes and vertical mixing generated by windintensity as well as to changes in upwelling ordownwelling conditions generated by wind direc-tion. Thus, easterly bursts from March to June areassociated to synchronous decreases of temperature.This is consequence of the sensitivity that the heatbalance has to easterlies during this period (Fig. 4).Easterlies are frequent, intense, warm and dryduring this period, which increases heat loss to theatmosphere. In addition, the stratification of the
Alc
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ío D
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Zon
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/h)
-20
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40
e July August September
mperature difference between air and sea surface (broken line) in
atent and sensible heats plus long-wave radiation emission, thick
long wave radiation emission, lower thin line). Heat fluxes were
lquivir River discharge (line) from Alcala del Rıo dam along the
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water column (see below) is weaker than in summer.Therefore, a large quantity of momentum istransferred to the sea surface (and heat to theatmosphere) when the potential energy necessary toproduce mixing is lower than in summer. Significantstratification has developed after June (see below),bursts are less frequent and do not produce largedrops of temperature. At this time, temperatureresponse to wind forcing is consistent with theupwelling/downwelling regime described above.Cooling and warming of shelf waters are associatedto off- and inshore currents, respectively.
In addition to this response to wind forcing, bothmooring (Fig. 2(A)) and CTD data (Fig. 5) show aprogressive heating of surface waters in the area. Awarming event seems to have occurred duringMarch when a great portion of the area wasoccupied by waters warmer than 17 1C. This is
-7.30 -7.10 -6.90 -6.70 -6.50
36.60
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(A) Mar 02
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Guadiana R .Huelva R.
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Gua
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Guadiana RHuelva R.
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Temperature (°C)
(E) Jul 02
Fig. 5. Salinity (contour lines) and temperature (1C) ma
consistent with the absence of easterly bursts priorto that cruise, thereby allowing stratification todevelop as can be seen in the transect quasi-perpendicular to the coast at Fig. 6(A). Thewarming event disappeared during April when mostof the area was occupied by waters below 16 1C.This is also consistent with a cruise carried out afteran intense easterly burst (Fig. 2(A)) and under weakstratification (Fig. 6(B)). Water warmed up again inMay, accelerating during June when the overalltemperature rose about 2 1C (Fig. 5). From July toSeptember most of the area had surface tempera-tures over 20 (Fig. 5), and a stable thermoclinesettled along the shelf (Fig. 6).
Fig. 5 shows a region west of 71W withtemperatures that are systematically lower than therest of the area. In March and September cruises, thetemperature decrement occurred together with a
-7.30 -7.10 -6.90 -6.70 -6.50
(F) Sep 02
(B) Apr 02
(D) Jun 02
Guadiana RHuelva R.
Gua
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Guadiana RHuelva R.
Gua
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Guadiana RHuelva R.
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.
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ps at 5m depth for the different cruises performed.
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11 10 c 9
Station
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Dep
th (
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(C) May 02 (F) Sep 02
(E) Jul 02 (B) Apr 02
(A) Mar 02 (D) Jun 02
Temperature (ºC)
Fig. 6. Temperature (1C) profiles along a single transect covering stations 9–11.
J. Ruiz et al. / Deep-Sea Research II 53 (2006) 1363–1376 1369
salinity decrease in that area. This joint decrementof temperature and salinity is also observed withincreasing depths at the open-ocean waters of theGulf (Criado-Aldeanueva et al., 2006). This makesvery probable that the cooling observed in thissector results from the influence of deep watersupwelled west of the sampled area (nearby CapeSanta Marıa; Navarro and Ruiz, 2006) if weconsider that this upwelling is specially strongduring westerlies when currents parallel to the coastflow eastward (Fig. 2).
In contrast to the western section, salinity at theeastern side is influenced by the Guadalquivir Riverdischarge. The March cruise was preceded by a
rainy period, and river discharge generated the lowsalinity pool shown in Fig. 5(A). The river mouthwas colder than the rest of the area during April andbecame warmer afterward, when the overall heatingof the shelf was manifest. This pattern of watercolder than the rest of the area during winter andwarmer during summer is probably the result ofcontinental influence on a very shallow area, closeto the river mouth, and with a topography (thecoast draws an almost right angle at the rivermouth) that does not favor efficient exchanges withthe rest of the basin. This influence is not seen insalinity signals associated to the river mouth since itnever falls dramatically below typical values of
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open-ocean waters (minima are of the order of 34,observed very close to the river mouth). This isconsequence of the intensive use that farmers makeof freshwater in the river, whose discharge iscontrolled by the Alcala del Rıo dam located110 km upstream of the river mouth. The damexerts a tight regulation on river discharges that arevery low during the analyzed period (Fig. 4).
3.3. Nutrients and chlorophyll
Fig. 7 shows the surface distribution of nitrateand chlorophyll. Both concentrations are generally
.
-7.30 -7.10 -6.90 -6.70 -6.50
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(A) Mar 02Guadiana R Huelva R.
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(C) May 02Guadiana R Huelva R.
Gua
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(E) Jul 02
(G) Sep 02
Guadiana R
Guadiana R
Huelva R.
Huelva R.
Gua
dalq
uivi
r R
.G
uada
lqui
vir
R.
Fig. 7. Spatial distribution of Chlorophyll-a (mg/l, contour lines) and nit
low except for singular events occurring at theeastern and western section of the study area.Patches of concentrations above background valuesat the west sector (e.g. stations 18–22 in April andMay) are probably associated with the upwellingnear Cape Santa Marıa, as described above. Thisupwelling brings colder, less saline waters withhigher nutrient concentrations than offshore surfacewaters during this period of the year (Navarro et al.,2006). However, high nutrient and chlorophyllconcentrations in the east must have a riverineorigin. For instance, the high nitrate concentrationsfound during March at the eastern stations are
0 2 4 6 8 10 30 50
Nitrate (mM)
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(B) Apr 02Guadiana R Huelva R.
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(D) Jun 02Guadiana R Huelva R.
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(F) Aug 02Guadiana R Huelva R.
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rate (mM) below 5m of surface for the different cruises performed.
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clearly associated with freshwater inputs since theywere observed after a strong rain event (Fig. 4),which left a distinguishable low-salinity patch ofwater in that area (Fig. 5(A)). In fact, the stationsclosest to the river mouth persistently show highernutrient concentrations than those observed else-where in the area. High nutrients, however, are notalways associated with significant reductions insalinity, suggesting that nutrient enrichment of thearea may not only depend on freshwater input fromthe dam at Alcala del Rıo but also on the lowerreaches of the Guadalquivir River.
Since the shelf surrounding the Guadalquivirmouth is very shallow and the water is warm duringthe summer period, it is not surprising that the inputof nutrients bears a persistently high chlorophyllconcentration in these areas (Fig. 7). This occurswhile the rest of the shelf and the basin experiencesevere oligotrophic conditions (Navarro and Ruiz,2006). During the warm period, chlorophyll occa-sionally can have high concentrations in veryshallow coastal stations away from the river in apattern that is, nevertheless, much less persistentthan the rather permanent feature observed at themouth of the Guadalquivir.
Fig. 7 also shows the existence of an importantphytoplankton bloom in March, which must be aconsequence of the riverine influence on this portionof the shelf since it is clearly associated to salinityminima close to the Gualdalquivir as well as theGuadiana and Huelva rivers (Fig. 5). As discussedabove, the minima were consequence of theimportant rain event during the days preceding theMarch cruise, which was followed by a significantfreshwater discharge from the dam (Fig. 4).
3.4. Zooplankton and anchovy eggs– larvae
distribution
The pulse of March phytoplankton production(Fig. 7) did not result in subsequent high zooplank-ton concentrations (Fig. 8). However, the persistentabundance of phytoplankton at the Guadalquivirmouth seems to support high zooplankton andlarvae concentrations in this area, especially duringthe summer. Anchovy spawning areas were locatedat offshore stations and, to a lesser extent, in theinner stations between the Huelva and the Guadal-quivir rivers (Fig. 9). The offshore patches of highegg concentration were manifest at the spawningpeaks in March and June.
4. Discussion
The oceanographic pattern diagnosed in thisstudy evidenced the existence of current, thermaland trophic regimes suitable for the survival ofanchovy early life stages in the northeastern shelf ofthe Gulf of Cadiz, except when persistent easterliesblow in the area. Anchovy reproduction has beenshown to be sensitive to temperature, with thespawning period being usually associated to warmwaters with surface temperatures above 14 1C(Motos et al., 1996) and a maximum activitybetween 19 and 23 1C in the adjacent Alboran Sea(Garcıa and Palomera, 1996). Temperatures above14 1C were measured on this portion of the Iberianshelf during the whole period of study, exceeding20 1C after June, and coinciding with the anchovyspawning peak. In addition to this in situ study ofone spawning season, an analysis of thermal imagesof 7 years conducted by Vargas et al. (2003) showedthat waters on this portion of the shelf are warmerthan the rest of the basin from about March toNovember.
However, these suitable thermal conditions werenot associated with the severe oligotrophy thatcharacterizes the open sea waters of the basin insummer. In fact, Navarro and Ruiz (2006) per-formed an analysis of several years of SeaWiFSimages and confirmed that this unusual combina-tion of warm and productive waters during summeroccurs on regular basis in the shelf, being particu-larly relevant in the inner shelf surrounding theGuadalquivir River mouth. This trend also could beobserved over the sampling period of this study(Figs. 5 and 7). Obvious chlorophyll signals close tothe mouth persistently appeared in all cruiseswithout apparent connection to a particular windregime or significant modifications of salinity (Figs.2, 5 and 7). These features indicate that thephytoplankton accumulation around the area couldbe associated with the tidal mixing (mainly oftemperature and inorganic nutrients) in and out ofthe river estuary. The constant tidal mixing wouldresult in a permanent chlorophyll signal that wouldinfluence other components of the food web such aszooplankton, whose highest concentrations were,indeed, observed in this area (Fig. 8), and anchovylarvae that are known to feed on copepods in theestuary (Baldo and Drake, 2002).
Therefore, both thermal and trophic conditionsseem to be adequate for the growth and feeding ofanchovy in their early stages. Moreover, the
Fig. 8. Anchovy larvae concentration (] larvae/100m3, contour lines) and zooplankton biovolume (ml/100m3) for the different cruises.
J. Ruiz et al. / Deep-Sea Research II 53 (2006) 1363–13761372
intensity of the currents is usually low in the area(Fig. 2), which also would favor the permanencewithin the shelf of these smaller stages with reducedcapacity to control their spatial position. However,this set of suitable conditions can be significantlymodified during easterly winds. The intensity,temperature and humidity of these bursts contributeto create several negative effects for the spawningand early survival of anchovy. The flux of latentheat to the atmosphere increases considerablyduring the occurrence of these bursts, with thepotential to reverse the seasonal warming of surfacewaters. This situation was observed between March
and April, when a severe burst (Fig. 2) rendered anegative energy balance on the ocean surface (lateMarch early April in Fig. 3(B)) and a subsequentdelay in the spawning that already had commencedin March (Fig. 9). Subsequent bursts until June wereassociated with a rise in heat fluxes to the atmo-sphere (Fig. 3(B)) and a drop in water temperature(Fig. 2(A)). Since the overall energy balance of theocean surface was not negative in the late April,May and June bursts (thick line in Fig. 3(B)), thedrops must have been the result of the combinedaction of a reduced energy flux towards the oceansurface (Fig. 3(B)) and the severe vertical mixing
Fig. 9. Anchovy egg concentration (] eggs/100m3) for the different cruises.
J. Ruiz et al. / Deep-Sea Research II 53 (2006) 1363–1376 1373
generated by the intense bursts. The mixing ought tobe particularly effective in a water column that isnot strongly stratified at this time (Fig. 6). NeitherMay nor June cruises were carried out in anadequate time able to resolve the effect of thesebursts on the spawning, but the potential effect wasprobably similar to that recorded during the Aprilsurvey. Bursts were less frequent from late June toSeptember (Fig. 2), when the water column wasstrongly stratified (Fig. 6), preventing mixing andsignificant cooling events related to easterlies duringthis period (Fig. 2).
After June, when the concentration of larvaepeaks (Fig. 8), the influence of easterlies on thermalconditions of the shelf is lessened although wind stillstrongly affects the trophic condition (Navarro andRuiz, 2006). Color images of this area over thesampling period have shown that westerlies producean overall increase of primary productivity as aresult of offshore Ekman transport, whereas east-erlies cause the opposite effect (Navarro and Ruiz,2006). This effect also can be indirectly inferred herefrom the data recorded with the shelf mooring(Fig. 2(B)). When westerlies blow, offshore currents
ARTICLE IN PRESSJ. Ruiz et al. / Deep-Sea Research II 53 (2006) 1363–13761374
occurr, usually associated to upwelling inputs ofdeep and nutrient rich waters, whereas the oppositesituation can be detected when easterlies predomi-nated. In this latter case, in addition to a generalimpoverishment of the shelf, westward alongshorecurrents were generated, which were very intense ina sudden shift from west to easterlies after theformer had blown persistently (early April at Fig.2(B)). The strong alongshore currents have thepotential to force larvae away from the productivewaters located around the river mouth. Persistentand intense easterlies cause the spilling of shelfwaters west of Cape Santa Marıa or even west ofCape San Vicente to oligotrophic waters in the openAtlantic (Relvas and Barton, 2002). Water advectedby easterlies would then carry suspended ichthyo-plankton along this westward path.
A clear signature of this transport has beendetected by Catalan et al. (2006) for ichthyoplank-ton patches in two synoptic surveys performed only
Tim
e (d
ays)
wit
h E
aste
rlie
s>30
km/h
10
15
20
25
30
35
Year1984 1986 1988 1990 1992 1994 1996
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reci
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atio
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)
200
400
600
800
1000
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(B)
Fig. 10. A. Anchovy catches (black circles) in the Gulf of Cadiz (ICES
fishing trip) in Barbate single purpose pursue-seine fleet. Barbate is cons
data; Anon., 2004). Catch data for year 2000 are not included in the gra
fleet. Bars accumulate the time when easterlies stronger than 30 km/h hi
bars indicate North Atlantic Oscillation index and annual precipitation
several days apart but under west and easterlyregimes. According to these results, it can beconcluded that current, thermal and trophic regimessuitable for the spawning and subsequent develop-ment of anchovy are modified during easterly burstsin the shelf of the Gulf of Cadiz. Anchovy catches inthis area (the vast majority in ICES Division IX.a)must be then sensitive to burst occurrence if thenegative alteration of the oceanographic regimeproduced by the wind action significantly affects therecruitment.
Fig. 10 shows the evolution of the ICES reportfor anchovy catches, and the catch per unit effort inthe Gulf of Cadiz together with the frequency ofstrong easterlies during the period from March toSeptember of several years. The first 8 years of the1990s was a period of intense easterlies precededand followed by intervals when these winds were notas frequent. This period seemed to lead to decreasedanchovy catches, which rose again once the intense
An
cho
vy C
atch
es (
To
nn
es)
An
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vy C
PU
E (
kg/f
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tri
p)
0
2000
4000
6000
8000
10000
0
500
1000
1500
2000
2500
3000
3500
1998 2000 2002 2004
NA
O In
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-4
-2
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4
report for sub-division IX.a south) and CPUE (white circles, kg/
idered as a reference harbor for catches in the Gulf of Cadiz (ICES
ph as catches were not representative due to social conflicts in the
t Cadiz over the period from March to September. B. Circles and
, respectively.
ARTICLE IN PRESSJ. Ruiz et al. / Deep-Sea Research II 53 (2006) 1363–1376 1375
easterlies concluded. Although the connectionbetween the intensity of easterlies and anchovycatches cannot be resolved on annual basis, thesevere drop of catches recorded in the mid-90sresembled more the easterly signal than the NorthAtlantic oscillation (NAO) or precipitation. Pre-cipitation and associated river discharges are knownto influence other clupeoid fisheries (Lloret et al.,2001), and in this may have played a role in therecovery of 1997 catches since the rain fall peak atthe year 1996 clearly reflected the dramatic changeof the NAO index occurring that year. In contrast,the intensity of easterlies was not sensitive to NAOshifts and interannual fluctuations were smoother.
Even though further work is necessary to resolvethe connection between the environmental forcingand anchovy catches in the area, this study points tothe significant modifications generated by easterlieson the oceanographic regime in the Gulf of Cadiz,which can influence markedly the reproductivesuccess of this species. Such changes in the oceano-graphic regime show the close correlation betweeninterannual fluctuation of easterly wind intensityand anchovy catches in this area.
Acknowledgements
We are grateful to the DLR (German RemoteSensing Data Center) for providing the SST imagesas well as to SeaWiFS Project (code 970.2) forSeaWiFS images and to Puertos del Estado for themeteorological information from his ‘‘Boyas Rayo’’observational program. Remote sensing imageshave been processed in Ocean Colour RemoteSensing Service (SeTCO) at ICMAN-CSIC. Thiswork was supported by two projects MAR99-0643-C03-02 (CICYT, Spanish National Program ofMarine Science and Technology) and CTM2005-01091/MAR (Spanish National Program of Envir-onment and Natural Resources) and by Consejerıade Agricultura y Pesca of the Junta de Andalucıa.We thank contributions of two anonymous re-viewers on a previous version of the manuscript.
References
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