Spatial and temporal distribution of European hake, Merluccius merluccius (L.), eggs and larvae in...

Spatial and temporal distribution of European hake,Merluccius merluccius (L.), eggs and larvae in relation to

hydrographical conditions in the Bay of Biscay

Paula Alvarez*, Lorenzo Motos, Adolfo Uriarte, Joseba EganÄaAZTI, Technological Institute for Fisheries and Food, Av. SatruÂstegui 8, 20008 Donostia-San SebastiaÂn, Basque Country, Spain

Abstract

The distribution patterns for hake eggs and larvae were determined by analysing samples collected on 12 cruises in the Bay

of Biscay from February to June both in 1983 and 1995. The majority of the eggs and larvae were found in late winter and

early spring. The expected northern displacement of both hake eggs and larvae as the season progresses was observed only in

the case of larvae. A particular spatial pattern of larval distribution was found. Small larvae (2±4 mm) were found close to the

spawning grounds at the shelf break. Mid-size larvae (4±8 mm) showed a more widespread distribution from the continental

shelf and to areas well beyond the shelf break. Finally, the biggest larvae sampled (>8 mm) were recorded only on the

continental shelf. This pattern seems to be a consequence of the combined action of different mechanisms. Wind-induced

transport in the Ekman layer favoured an onshore transfer in 1983, whereas prevalent wind and geostrophic circulation

favoured a northern, onshore transfer in 1995. # 2001 Elsevier Science B.V. All rights reserved.

Keywords: Hake eggs; Hake larvae; Distribution; Hydrography; Advection

1. Introduction

The European hake, Merluccius merluccius (L.), is

the main commercial demersal species in the Bay of

Biscay, being exploited by several Spanish and French

¯eets, with an annual average catch of 61 500 t in the

period 1978±1990. These catches are worth an average

of 319 million ECU at ®rst sale (Casey and Pereiro,

1995). Despite its economic importance, studies on

the early life history of hake are few.

The spawning area of the European hake population

in Atlantic waters extends all along the western margin

of Europe from Portugal to north Scotland. Spawning

has also been recorded in Norwegian Fjords and in the

Skagerrak and Kattegat.

The peak spawning time of hake is earlier in southern

waters, being later as the latitude increases (Casey and

Pereiro, 1995). In this way, peak spawning is closer to

winter months (February±March) in the Portuguese

coast. It ranges from January to May in Galician waters

(PeÂrezandPereiro,1985),Cantabriansea(Alcazaretal.,

1983) and Bay of Biscay (Sarano, 1983; Martin, 1991).

In the Celtic Sea (ICES subarea VII) peak spawning

takesplacefromApril toJune(Clark,1920;Coombsand

Mitchell, 1982; Horstman, 1988; Fives et al. this issue).

In western Ireland, spawning occurs from April to July

and from May to August for sea areas west of Scotland

(Hickling, 1930; O'Brien, 1986). Ripening hake has

been also recorded in the West Coast of Norway in

August (Kvenseth et al., 1996).

Fisheries Research 50 (2001) 111±128

* Corresponding author. Tel.: �34-43214124;

fax: �34-43212162.

E-mail address: [email protected] (P. Alvarez).

0165-7836/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 5 - 7 8 3 6 ( 0 0 ) 0 0 2 4 5 - 9

ICES (1980) presented the results of the 1977

triennial survey for hake larvae. Larvae were mainly

distributed along the shelf edge from south Biscay to

south Ireland, and displaced to the north as the season

progresses. This latitudinal variation in peak spawning

time is common to other ®sh species. In the case of

hake, it can be explained by a migration of spawners

from south to north, or alternatively, by a spawning

wave going from south to north as local population

components reach maturity and spawn, or it can be a

combination of both. The phenomenon seems to be

related to changes in environmental factors such as

temperature and the consequent changes in biological

features of the spawning habitat. Hake eggs show a

temperature preference in the range of 10.5±128C(Arbault and Lacroix-Boutin, 1969; Coombs and

Mitchell, 1982).

The ®nal spatio-temporal distribution pattern of

eggs and larvae will depend on the initial position

of the spawning, and after the release of eggs by the

parental stock, on the hydrographical features acting

upon this initial distribution. Authors have expressed a

variety of interpretations to explain the cross-shelf

distribution of eggs and larvae. Arbault and Lacroix-

Boutin (1969) concluded that the spawning in the Bay

of Biscay is initiated close to the coast and it spreads

towards deeper waters as the season progress. In more

northern latitudes, Horstman (1988) and Fives et al.

(this issue) suggested the contrary, i.e. spawning starts

along the shelf edge (200 m depth contour) and

spreads onto the shelf afterwards.

The in¯uence of the hydrographical features on the

dispersion of eggs and larvae is of increasing interest.

Several species spawn, as hake does, in areas far away

from the coast but, at the same time, have coastal

nursery areas where the young individuals are

recruited. The European hake occupy different nur-

sery areas, all of them located in coastal, relatively

shallow waters such as the muddy bottoms of the

northern Bay of Biscay known as the `̀ Grand VasieÂreÁ''

(Bez et al., 1995). Consistent mechanisms of retention

over the shelf, and furthermore, transport towards the

coast should occur in the area to ensure the main-

tenance of the population in the long term (Heath,

1992; Koutsikopoulos et al., 1991; Koutsikopoulos

and Lacroix, 1992). Oceanographic events that con-

tribute to the dispersion of ELH stages towards open

ocean waters can cause high larval mortality and

substantially decrease successful recruitment (Lae-

vastu and Hayes, 1981; Hollowed and Bailey, 1989).

Several studies have described the main hydrogra-

phical features in the Bay of Biscay. The general

circulation is weak and the presence of cyclonic

and anticyclonic eddies is frequent (Koutsikopoulos

and Le Cann, 1996). A consistent poleward seeking

slope current is apparent but it shows marked seasonal

changes (Pingree and Le Cann, 1990). The residual

currents over the shelf are principally governed by the

wind, the tide in the northern part and the water

density. The meteorological conditions show marked

spatial and temporal heterogeneity which can have a

notable in¯uence in ®sh populations (Koutsikopoulos

and Le Cann, 1996; Borja et al., 1996; SaÂnchez, 1994).

This paper describes the spatio-temporal pattern of

distribution of hake eggs and larvae in the Bay of

Biscay during the main spawning period for the years

1983 and 1995. Based on the distribution pattern

found, a mechanism to ensure the retention or trans-

portation of eggs and larvae towards the putative

nursery areas where juveniles are recruited is sug-

gested. The spatio-temporal distribution pattern is

analysed in the light of the available data on con-

current oceanographic features. The in¯uence of the

environmental conditions where hake eggs and larvae

develop is assessed by comparing the events described

in the study to the eventual hake recruitment levels

achieved in the year of study (ICES, 1996).

2. Material and methods

Table 1 summarises the cruises analysed in this

study. Hake eggs and larvae were sorted from the

samples collected in a series of cruises carried out

within the frame of the ICES triennial egg survey, both

in 1983 and in 1995. Additionally, two cruises were

carried out to speci®cally collect hake eggs and larvae

in February and March 1995. For comparison pur-

poses, the cruises were grouped by month. Results

were available for 3 months in 1983, i.e. March, May

and June, and for 5 months in 1995, i.e. February,

March, April, May and June.

During the surveys, a set of single oblique plankton

tows were made using a BONGO net, 60 cm mouth

diameter (Mcgowan and Brown, 1966) in a grid of

stations arranged in transects perpendicular to the

112 P. Alvarez et al. / Fisheries Research 50 (2001) 111±128

shelf edge contour. The sampling methodology used in

the cruises followed standard procedures. Tempera-

ture and depth pro®les were recorded at each haul by

means of a MINILOG probe. Ancillary hydrographi-

cal data, namely temperature, salinity and chlorophyll

`̀ a'' were recorded during the cruises using a SEA-

BIRD 25 CTD.

The standard plankton net used in the ICES triennial

egg surveys is the GULF III. The procedures used in

these surveys are described in detail elsewhere (ICES,

1994). Double oblique tows were carried out in a grid

of stations located at the mid-point of 0:5� � 0:5�

rectangles along the whole mackerel and horse mack-

erel spawning area. Only samples collected south of

488, i.e. within the Bay of Biscay, were processed in

this work. Ancillary hydrographical data were also

collected during the ICES cruises.

On completion of the hauls, plankton was preserved

in a 4% buffered formaldehyde solution. Hake eggs

and larvae were sorted from the samples collected in

the speci®c hake cruises. ICES samples were re-sorted

for hake eggs and larvae at AZTI, after separating the

eggs and larvae of the cruise-speci®c target species,

i.e. mackerel and horse mackerel, in the laboratory of

origin. All hake eggs sorted were classi®ed by devel-

opmental stage and hake larvae were measured to the

nearest lower 0.1 mm. In this study no correction was

made to allow for shrinkage of larvae due to ®xation

procedures. Bailey (1982) gave a shrinkage ®gure of

9±22% for small Paci®c hake larvae (<4 mm) ®xed 5±

10 min after collection, and he assumed smaller

shrinkage rates for bigger larvae.

Egg and larval abundance in both BONGO and

GULF III sampler hauls were converted to numbers

per 10 m2 following standard procedures (Smith and

Richardson, 1977; ICES, 1994).

The distribution of hake eggs and larvae were

mapped to show the spatio-temporal pattern of dis-

tribution. Furthermore, this pattern was summarised

by mapping the centroids of distribution and their

variance (Hollowed, 1992). The centroids were esti-

mated as the mean position (latitude and longitude) of

the positive stations for hake eggs or larvae weighted

by the abundance of eggs or larvae found at each

station. An ellipse shows the centre and the orientation

of the animal's distribution in space and the amount of

dispersion about the centre (Kendall and Picquelle,

1989). The secondary axis is at right angles to the

principal (Sokal and Rholf, 1980; Kendall and Pic-

quelle, 1989; Hollowed, 1992). The ellipse is the two-

dimensional analogue of mean and standard error bars

(Kendall and Picquelle, 1989). The major and minor

axes are each two standard deviations long. The

centroids of the station grid covered in each survey

were also calculated to compare them with the respec-

tive centroids of hake egg and larval distribution and to

test if the gravity centre of the sampling area could

confound the comparison.

Table 1

Summary of the main characteristics of the surveys carried out

Cruise Survey dates Bay of Biscay Period Gear Samples

Year 1983

Cirolana 4 18 March±9 April 18 March±9 April March GULF III 53

Scotia 5 4±18 May 8±12 May May GULF III 35

Tridens 5 4±31 May 4±31 May May GULF III 70

Scotia 6 8±28 June 18±22 June June GULF III 35

Tridens 6 31 May±7 June 31 May±7 June June GULF III 45

Year 1995

Lebal 2 15±24 February 15±24 February February BONGO-60 40

Lebal 3 22 March±1 April 22 March±1 April March BONGO-60 62

Walter Herwig III 4 26 March±12 April 2±9 April April GULF III 40

Cirolana 4a 22 April±17 May 15±17 May May GULF III 17

Scotia 5 23 April±12 May 1±7 May May GULF III 28

Tridens 5 18±31 May 22±31 May May GULF III 58

Tridens 6 13±26 June 13±21 June June GULF III 60

P. Alvarez et al. / Fisheries Research 50 (2001) 111±128 113

Wind data for the area and period of study was

supplied by NOAA as obtained from FNMOC (Fleet

Numerical Oceanographic Center). Geostrophic wind

data is derived from surface pressure ®elds measured

every 6 h. To approximate geostrophic wind to real

wind data, the magnitude has to be reduced by 30%

and the wind vector has to be turned 158 anticlockwise

(Bakun, 1973).

The data used in 1983 were obtained at position

458N, 58W. We assumed that the wind does not change

substantially between two points, if the distance

between them is less than 38 of latitude or longitude.

This is a good approximation to reality (Bakun, 1973).

The data used in 1995 were obtained at 458N, 28Wsince wind data at 458N, 58W were not available. Both

positions were approximately centred in the survey

area. Drift vectors were calculated by using Ekman's

theory (Pond and Pickard, 1983) to depict wind-

induced drift on hake eggs and larvae. Results were

produced for the reproductive periods of 1983 and

1995 at 15 and 100 m water depth. All the six hourly

drift vectors were added to produced one monthly

vector. This vector indicates the potential displace-

ment of eggs and larvae during that period assuming

that the displacement of hake eggs and larvae only

depends on this factor.

The wind-induced current velocity decreases expo-

nentially with water depth as shown by Ekman's

theory:

u � t

Aa��2p

� �ea�ÿz� cos az� p

4

� �(1)

v � t

Aa��2p

� �ea�ÿz� sin az� p

4

� �(2)

a ��rf

2A

r(3)

where z is the water depth, t the wind stress, A the eddy

viscosity, r the water density, f the Coriolis parameter.

The variation of tide-induced current velocity

with water depth has been described by Postma

(1988):

Vz � v0z

h

� �0:2(4)

where v0 is the surface current velocity, h the water

depth, z the depth at which velocity is calculated.

3. Results

3.1. Distribution of eggs

The distribution of hake eggs by month/year is

shown in Fig. 1, together with the distribution cen-

troids. In February 1995, a low abundance of hake

eggs was noted but widely distributed throughout the

study area. A relative abundance maximum (�500

eggs/10 m2) occurred close to the shelf edge at around

478450N latitude. Eggs were also found in the south-

eastern corner of the Bay of Biscay, although in very

low abundance. In March, eggs were more abundant

and widespread than in February. Peak abundance

(�700 eggs/10 m2) was located close to the shelf

break between 458450N and 478450N. No eggs were

found south of 448N in that month. Egg abundance

continued to be relatively high in April. Eggs occurred

very close to the shelf edge in most of the sampling

area covered (45±488N) and were absent in inner shelf

stations (Fig. 1). Abundance peaks were located close

to the shelf break from 468 to 478450N (282 eggs/

10 m2). Egg abundance was lower later, both in May

and June. They occurred over the outer continental

shelf of northern Biscay. Hake eggs in the shelf break

area are almost absent in those months, although an

unusual presence was identi®ed in oceanic waters off

the shelf at 478450N latitude.

The cruises covered only 3 months in 1983, March,

May and June. In general, the seasonal pattern of egg

distribution found that year was similar to that in 1995.

Abundance peaks were located in March. Peak abun-

dance in this month (�1000 eggs/10 m2) were located

around and close to the shelf break, but eggs were

widespread from near the shelf edge to the inner shelf

stations. They were also found in southern waters of

the Bay of Biscay (Cantabrian Sea) although in very

low abundance. Hake eggs appeared in lower quan-

tities both in May and in June 1983. The peak abun-

dance was 10 times lower in May (158 eggs/10 m2)

than it was in March and was located over the external

part of the continental shelf at a latitude of 478250N.

Low abundance egg patches were also observed at

latitudes of 448250N and at 468750N in June 1983.

The distribution centroids of hake eggs summarise

the displacements along the season (Table 2 and

Fig. 2). In 1983, the egg distribution centroids occur

at 46.728N, 4.128W in March and at 47.048N, 5.868W


Fig. 1. Proportional representation of egg densities of hake, M. merluccius, in the Bay of Biscay in 1983 and in 1995. Dot size indicates the

abundance (numbers/10 m2) of eggs relative to the maximum observed in that year (1156 eggs/10 m2 in 1983 and 700 eggs/10 m2 in 1995).

Crosses indicate station where hake eggs were not observed. Distribution ellipses are also plotted.


in May, ending with a location more southerly and

onshore in June (45.458N, 2.998W). In 1995, the egg

centroid is located at 46.138N, 3.658W in February

and it remains in a similar position in March (46.018N,

3.798W). The centroid goes up and outwards in April

(46.628N, 4.998W), and May (46.618N, 5.058W), and

®nally it continues northwards in June (47.228N,

4.238W), but with a remarkably onshore movement.

Most observations of hake eggs in May 1995 were

placed well inside the continental shelf. However, the

corresponding distribution ellipse was clearly in¯u-

enced by an offshore presence (Fig. 1).

Comparing the displacement of the distribution

centroids of the stations sampled in each survey, the

displacements observed in the egg centroids show

something different. The northern displacement shows

in most cases a similar magnitude for both egg dis-

tribution and station distribution centroids. A southern

movement was observed only in June 1983. Regarding

the onshore/offshore movements, however, the evolu-

tion in 1983 was to go out from March to May and to

go back to the continental shelf in June. In 1995, the

centroids stayed similarly placed in longitude from

February to April but showed a clear onshore displa-

cement in May and June (Fig. 2). So, no net movement

to the north can be unequivocally inferred from the

data gathered in this study, but it shows a clear coast-

ward movement in May (only in 1995) and June (both

years).

The orientation of the principal axis of the egg

distribution ellipses is in a direction similar to that

of the shelf break in the area, i.e. NW±SE, along the

whole sampling period. The sizes of the ellipses

increased somewhat from February to May in both

directions (N±S and W±E).

3.2. Distribution of larvae

Hake larvae occurred in low abundance in February

1995 (Fig. 3). An hot spot (�30 individuals/10 m2)

was found in the southern corner of the sampling area

that month. Hake larvae were much more abundant in

March 1995 and occurred more northerly. Abundance

peaks (�50 individuals/10 m2) occurred over the shelf

break between 468450N and 478450N. Hake larvae also

occurred in the south-eastern corner of the Bay of

Biscay during this period but in a smaller numbers

(four individuals/10 m2) than in February 1995. A

similar distribution pattern was found in the April

1995 cruise, although hake larvae occurred in lower

abundance than in March 1995. In May 1995, hake

larvae were restricted to the area north of 468300N.

Hake larval abundance decreased in May 1995 with

relation to March±April 1995, although an abundance

peak (�70 larvae/10 m2) appeared in shelf waters and

a patch of lower abundance occurred close to the shelf

break. Larvae were absent in June.

The highest larval abundance in 1983 were found in

March (Fig. 3), when a peak of abundance was located

in shelf waters off Brest (52 larvae/10 m2) in the

northern corner of the Bay of Biscay. No larvae

appeared south of 468N in that cruise. The distribution

pattern was roughly similar in May 1983, when a

relative abundance peak (44 larvae/10 m2) was situ-

ated off Brest. Hake larvae were still present in June

1983, although in very low abundance and centred

more southerly.

The centroids of hake larval distribution showed

perceptible monthly changes (Figs. 3 and 4, Table 2).

Again, the movements of the cruise station distribu-

tion masked the apparent movements of the larval

Table 2

Summary of the position of the centroids calculated for both egg and larvae distribution

Period 1983 1995

Survey Eggs Larvae Survey Eggs Larvae

Latitude Longitude Latitude Longitude Latitude Longitude Latitude Longitude Latitude Longitude Latitude Longitude

February ± ± ± ± ± ± 45.50 ÿ3.21 46.13 ÿ3.65 45.40 ÿ3.16

March 46.05 ÿ4.29 46.72 ÿ4.12 47.09 ÿ4.24 45.70 ÿ3.51 46.01 ÿ3.79 46.66 ÿ4.42

April ± ± ± ± ± ± 46.67 ÿ4.88 46.62 ÿ4.99 47.23 ÿ5.5

May 46.48 ÿ5.21 47.04 ÿ5.86 47.45 ÿ4.51 46.55 ÿ5.40 46.61 ÿ5.05 47.42 ÿ5.25

June 46.14 ÿ4.58 45.45 ÿ2.99 45.86 ÿ3.06 46.76 ÿ6.94 47.22 ÿ4.23 ± ±


Fig. 2. Centroids and associated ellipses of hake eggs, M. merluccius, in the Bay of Biscay in (a) 1983 and (b) 1995. Centroids of distribution

(months within blank square) are placed in the mid-point of the associated ellipse. The centroids of the sampling stations covered each cruise

(months within black squares) are also represented for comparative purposes.


Fig. 3. Proportional representation of larvae densities of hake, M. merluccius, in the Bay of Biscay in 1983 and in 1995. Dot size indicates the

abundance (numbers/10 m2) of larvae to maximum observed both years (70 eggs/10 m2).


Fig. 4. Centroids and associated ellipses of hake larvae, M. merluccius, in the Bay of Biscay in (a) 1983 and (b) 1995. Centroids of distribution

(months within blank square) are placed in the mid-point of the associated ellipse. The centroids of the sampling stations covered each cruise

(months within black squares) are also represented for comparative purposes.


Fig. 5. Distribution of mean length of hake larvae, M. merluccius, in the Bay of Biscay in 1995 and 1983. Bar size indicates the mean length

(mm) of larvae relative to the maximum observed in that year (11.4 mm in 1983 and 9 mm in 1995). Crosses indicate station where hake larvae

were not observed.


distribution centroids. However, a close comparison

between both distributions permits a distinction

between the northern movement recorded between

February and March 1995, which was still noticeable

from April to May (Fig. 4b) and the onshore move-

ment which was noticeable from May to June 1983

(Fig. 4a) and from April to May 1995 (Fig. 4b). During

the ®rst survey period (February) of 1995, the centroid

of larval distribution was in front of the Gironde

Estuary, the most southerly location of the centroids

of either eggs or larvae found in this study. Afterwards,

they moved from 458400N in February 1995 to

478420N in May 1995, with a main contribution from

February to March (160 km in the northern direction).

At the same time, the larval distribution centroid

moved onshore from April to May, after having

remained in a similar position with relation to the

shelf break from February to April.

Concerning the relative position of larval distribu-

tion centroid in relation to the egg distribution cen-

troid, the former are generally located more to the

north and to the east (coastward) than the latter

(Figs. 1±4). This feature is clearer in 1983 than in

1995. The only remarkable exception to this rule is the

situation observed in February 1995, when the larval

distribution was much more southerly than in any of

the eggs and larval distribution observed in the

remainder of the cruises.

The distribution of larval mean length per station is

shown in Fig. 5. There was no apparent pattern in the

spatial distribution of this parameter in February.

However, during the remainder of the cruises there

was a consistent pattern, with the largest larvae gen-

erally occurring closer to the coast than the smaller.

When all the observations are pooled, the spatial

distribution of larvae by size shows a particular pattern

(Fig. 6). Small larvae (2±4 mm total length, 40% of the

larvae observed) occurred close to the spawning

grounds over the shelf break. Mid-sized larvae (4±

8 mm, 54% of the larvae observed) show a more

widespread distribution over the continental shelf,

shelf break and even beyond it. Finally, the biggest

larvae (>8 mm, 6% of the larvae observed) were

recorded only over the continental shelf. Furthermore,

the largest larvae were mostly recorded over the inner

part of the continental shelf.

Fig. 6. Summary of the hake larvae length spatial distribution derived from the results obtained in this study. All the observations of hake

larvae in both years 1983 and 1995 are pooled together and the distribution shown by size groups: <4 mm (circles), 4±8 mm (diamonds) and

>8 mm (squares).


Fig. 7. Progressive vectors of wind-induced Ekman drift for the 1983 and 1995 spawning season of hake. Wind data were obtained every 6 h

from 1 April to 31 May (60 days) at 458N, 58W in 1983 and at 458N, 28W in 1995. The magnitudes of the vectors are calculated following

Eqs. (1)±(3) in the text. They are applied in a daily basis to the egg distribution centroids estimated in late March 1983 (left panel) and 1995

(right panel). The resultant potential drift is shown both for surface waters (15 m) and for a deeper depth layer (100 m). The latter is negligible.

Fig. 8. Histograms of egg and larval abundance by 0.58C temperature class. Temperature as recorded at 20 m water depth.


Data on vertical distribution of hake eggs and larvae

(Coombs and Mitchell, 1982; Motos et al., 2000)

indicate that both eggs and larvae occur in the upper

150 m of the water column. Furthermore, Motos et al.

(2000) found that egg and larval distribution was

substantially deeper in February 1995 than in March

1995. In the former, hake eggs occurred at a mean

depth of 120 m (not enough data were gathered for

hake larvae). In March 1995, hake eggs occurred in the

0±50 m water depth range, whereas hake larvae were

found mostly in the 50±100 m water depth range.

Based on these previous ®ndings, 15 and 100 m were

chosen as the range of water depths potentially rele-

vant for egg and larval transport during the 1983 and

1995 reproductive season (Fig. 7). We applied the

wind-induced drift vector at 15 m depth to the egg

distribution centroid of March during 60 days (from

late March to late May). This resulted in a displace-

ment of about 200 km in the SE direction (mean

current velocity of about 5 cm sÿ1) in 1983 and

52 km in the NW direction in 1995 (mean current

velocity of about 2.5 cm sÿ1). Wind-induced drift in

the 100 m depth layer was one order of magnitude

weaker than in the 15 m depth layer, in both years,

showing very little potential for wind-induced advec-

tion at that layer.

Fig. 8 shows the frequency distribution of egg and

larval abundance by 0.58C temperature classes (20 m

depth temperature). Most eggs and larvae occurred in

the 10.5±138C temperature range. This range is very

similar to the optimum temperature range for hake

spawning stated in the literature, i.e. 10±12.58C(Arbault and Lacroix-Boutin, 1969; Dicenta, 1979;

Coombs and Mitchell, 1982; Valencia et al., 1989).

4. Discussion

Ideally, all the putative spawning season and areas

of the European hake population have to be covered

for a comprehensive study of the patterns of egg and

larval distribution. This study was restricted to the Bay

of Biscay, a main area of hake spawning but only a part

of the geographical range of spawning of the European

hake, and no information was available on hake egg

and larval distribution in other hake spawning areas.

Within the Bay of Biscay, the main spawning season

of hake is assumed to extend from February to May

(Martin, 1991). Since the data used in this study, came

from cruises directed to other target species they

provided an incomplete spatio-temporal coverage.

In 1983, there was no cruise in February and the data

from the April cruise were not available for further

processing, giving an uneven temporal coverage in

that year. However, the spatial coverage of the 1983

cruises was satisfactory and they covered the areas of

the Bay of Biscay where the putative hake spawning

®elds are located. In 1995, the temporal coverage was

satisfactory, with cruises all along the main spawning

season of hake in the Bay of Biscay, but the spatial

coverage was not full. Both in February and in March,

the northern part of the area, namely the shelf edge

north of 478150N, was not covered, and in February

bad weather conditions further impeded the comple-

tion of transects in the northern area (Motos et al.,

2000). However, these two surveys were speci®cally

directed to hake and the sampling methodology used,

namely the high volumes ®ltered by the 60 cm

BONGO nets, aimed at getting a more representative

picture of the distribution of hake eggs and larvae in

these cruises than in the other cruises where GULF III

samplers were used.

However, the de®cits in the spatio-temporal sam-

pling pattern do not reduce the value of the data set. It

constitutes a valuable illustration of hake eggs and

larval distribution patterns in the Bay of Biscay along

the reproductive season in 2 years, 1983 and 1995.

The seasonal variation of hake egg distribution in

the Bay of Biscay shows a similar sequence in both

years 1983 and 1995. Peak egg abundance occurred in

March and decreased both in density and in the

proportion of positive hauls as the reproductive season

progressed. Although the temporal coverage is not

uniform between years, these results indicate that the

peak spawning period of hake in the Bay of Biscay is

centred in March. This general pattern of distribution

is consistent with previous data on hake spawning

periodicity in the area. Several authors reported that

the presence of hake eggs, and hence spawning, is

particularly intense during the ®rst quarter of the year

(late winter and early spring months) in the Bay of

Biscay (Anon., 1980; Arbault and Lacroix-Boutin,

1969; Dicenta, 1979; SolaÂ and Franco, 1985; Valencia

et al., 1989; Martin, 1991; Motos et al., 2000). How-

ever, some spawning still remains in the Bay of Biscay

throughout the ®rst half of the year (PeÂrez and Pereiro,


1985; Martin, 1991) as con®rmed by the presence of

hake eggs in June both in 1983 and in 1995, although

in very low abundance.

In quantitative terms and regarding the maximum

abundance period (March), our results show that hake

eggs in 1983 were about twice as abundant as in 1995

(Motos et al., 1997). This decrease in egg abundance is

consistent with the general decline in hake population

abundance recorded in the same period (ICES, 1996).

Concerning the spatial distribution of spawning, the

differences in spatial coverage in the different sam-

pling periods should be taken into consideration.

However, two aspects can be highlighted. First, the

platform close to the shelf edge appears as a prefer-

ential region for spawning in the months of peak

spawning, i.e. maximum egg abundance occur in

stations close to the 200 m isobath. Second, the shelf

break seems to represent a natural barrier for the

spawning of this species, since eggs are almost absent

in stations located outside the continental shelf.

The seasonal evolution of the distribution of egg

and larval centroids (Table 2 and Figs. 2 and 4), give

some insights about the spatio-temporal pattern of

hake spawning in the Bay of Biscay. An initial, north-

ward movement is followed by a coastward and

eventually southward movement at the end of the

study period. This pattern can be better demonstrated

in 1995, the year with a better coverage. This pattern is

also consistent with the current knowledge of hake

spawning along the North Atlantic coast concerning

the northern displacement of the spawning as the

season progresses (Coombs and Mitchell, 1982; Casey

and Pereiro, 1995; O'Brien and Fives, 1996; Horstman

and Fives, 1994; Fives et al., this issue). Either a

northward migration of the spawners or a maturation

wave going northwards as the season advances can

explain this fact. On the other hand, the observed

movement of the distribution centroids towards the

coast in May and/or June is consistent with the

description of a spawning starting near the shelf break

and extending later in the season towards the con-

tinental shelf (Horstman, 1988; Fives et al., this issue).

However, as mentioned in Section 1, it contradicts the

observations of Arbault and Lacroix-Boutin (1969)

about hake spawning starting close to the coast and

spreading towards deeper waters later in the season.

Spawning time is the result of an adaptive evolu-

tionary process leading to the population sustaining

itself in the long term (Heath, 1992). The northward

movement of spawning shown in this study should be

related to particular hydrographical conditions favour-

ing maturation and spawning. Temperature appears to

be a determinant factor driving the timing Ð both

beginning and duration Ð of the spawning season

(Hoar and Randall, 1988). In the case of hake, this

paper con®rms that the optimum temperature range

for spawning lies between 10 and 12.58C (Fig. 8). This

temperature range is common in Bay of Biscay waters

in the depth range of the hake habitat along the

reproductive season of this species, i.e. from January

to June (Koutsikopoulos and Le Cann, 1996). There-

fore, the observed displacement of the spawning areas,

although within the temperature range mentioned

above, has to be related to other factors (Valencia

et al., 1989). These authors suggested that vertical

homogeneity and the winter peak of the phytoplankton

cycle were environmental factors related to hake

spawning. In more northern latitudes the spawning

period of hake is located in late spring, early summer

months (O'Brien and Fives, 1996; Horstman and

Fives, 1994), coinciding with water temperature Ð

or related hydrological conditions Ð close to the

optimum range.

For both eggs and larvae the position of the dis-

tribution centroids in the highest abundance month,

i.e. March, indicated that in 1983 the spawning centres

were displaced to the north earlier than in 1995 (78 km

for eggs, and 48 km for larvae) (Fig. 3). Horstman and

Fives (1994), observed an earlier spawning of hake

and other species in the Celtic Sea and West Irish

waters in 1983 and suggested an earlier warming up of

sea water that year as the factor giving rise to early

spawning.

The distribution of larvae was similar in both years.

It was widespread all along the study area during the

two ®rst period intervals (February and March) and

concentrated in the north-eastern corner of the Bay of

Biscay in April and May, giving evidences of a north-

easterly movement of larvae as indicated above.

The spatial distribution of average length of hake

larvae (Fig. 5) shows a size gradient in the east±west

direction. With some exceptions, the largest larvae

appear generally in those stations closer to the coast.

Again, this suggests a coastward displacement of the

larvae from the main spawning areas close to the shelf

break, which is consistent with general residual


current maps presented by Castaing et al. (1993) and

Koutsikopoulos and Le Cann (1996) for the Bay of

Biscay. Nevertheless, the distribution of mean length

of hake larvae in the northern-most part of the study

area in March 1995 showed a different pattern, with

bigger larvae both in the offshore and the coastal part

of the northernmost transects and smaller larvae in the

mid-part. There, the geostrophic ¯ow ®elds show the

presence of a cyclonic eddy placed outside the shelf

break (Motos et al., 2000). This gyre could be trapping

part of the larvae coming from the shelf break spawn-

ing area and could be responsible of the retention and

offshore drift of the trapped larvae. However, the

pattern explained above would be similar in the rest

of the transect, where eggs and larvae would be

advected onto the shelf, giving rise to the observed

increasing gradient of larval sizes from the shelf break

to the coast.

The occurrence of the largest sized larvae (<8 mm)

exclusively over the continental shelf (Fig. 6) suggest

that larvae advected offshore would be trapped in a

unfavourable environment and will eventually die as

already pointed out by Hollowed and Bailey (1989) for

Merluccius productus. Our results suggest that if hake

larvae are not transported onto the continental shelf

before they reach 8 mm, it is likely that they will

eventually die. The oceanographic conditions prevail-

ing during the ELH stages will be of primary impor-

tance concerning the transport of eggs and larvae

onto the shelf, in order to ensure survival success

for the egg and larval cohorts, and hence, recruitment

success.

The present paper and data elsewhere (Anon., 1980;

Arbault and Lacroix-Boutin, 1968, 1969; Motos et al.,

2000) show that there is usually a high abundance of

hake eggs and larvae pears in the northern part of the

Bay of Biscay, i.e. in waters off southern Brittany

(Bretagne). The European hake has several documen-

ted juvenile nursery areas (ICES, 1996). In the Bay of

Biscay, a main nursery area is indeed located in the

northern part, where hake juveniles (0 group) concen-

trate from 120 m depth-line to the coastline from late

spring to the autumn (Bez et al., 1995). However, the

main spawning centre of hake in this area is well

offshore, mainly over the shelf edge, as described in

this paper. Eggs and larvae must therefore be trans-

ported towards the coast over the shelf into the nursery

areas. We have also show that hake larvae tend to be

localised more north-easterly than hake eggs at the

same season of the year. Prevalent residual currents in

the area (Castaing et al., 1993; Koutsikopoulos and Le

Cann, 1996; Barscht et al., 1996; Motos et al., 2000)

and passive diffusion (Koutsikopoulos et al., 1991)

processes will determine whether the eggs and larvae

move towards the coast, remain in the spawning areas

or are dispersed offshore.

Any further analysing of net transport of hake eggs

and larvae, would have to consider tidal currents,

wind-induced currents and geostrophic currents.

The most relevant forcing factor in the upper water

(15 m) is generally the wind, which can induce cur-

rents on the order of 5±10 cm sÿ1. Net transport due to

tides is negligible at the shelf break, although, it

increases coastwards. The wind-induced current velo-

city decreases exponentially with water depth accord-

ing to Ekman's theory. Therefore, the wind-induced

current velocity at 100 m water depth is of the order of

between 0.05 and 0.1 cm sÿ1.

The variation of tide-induced current velocity with

water depth (Postma, 1988) shows that the current

velocity decreases slowly at the surface layers and

then decreases rapidly towards the bottom. In the Bay

of Biscay, the average tidal range is of about 4.6 m and

tide-induced currents are larger in shallow waters than

in the open sea. Nevertheless, tide-induced transport

can be considered negligible at any depth.

The magnitudes of geostrophic currents (0±100 m)

in the Bay of Biscay analysed in this study has been

derived from CTD pro®les, and are in the range

2±3 cm sÿ1 (Motos et al., 2000). Some exceptions

can be found in some eddies, were velocities can be

in excess of 10 cm sÿ1.

Therefore, in the upper layers of the water column,

the net dominant current is the one induced by the

wind. On the other hand, at 100 m water depth, the

dominant net ¯ow is determined by geostrophy.

Nevertheless, in order to get a good approximation

of the magnitude of the currents in the area, the three

forcing factor components (wind, tide and geostrophy)

should be added.

The wind-induced drift data shown here for 1983

favoured an onshore transport (SE) toward coastal

nurseries (Fig. 7). Data from studies carried out in

the Bay of Biscay in 1983 (Castaing et al., 1993)

pointed out that residual currents moved in a north-

easterly direction during mid-winter to spring that


year. The distribution pattern of hake larvae found that

year appears to be coherent with the hydrographical

processes observed during the study period.

The geostrophic ¯ow observed during the March

1995 cruise showed clear ¯uxes in the north-eastern

direction (Motos et al., 2000). Wind-induced drift was

much weaker in 1995 than in 1983 and possibly had a

very low contribution to the net ¯ow. During 1995,

hake eggs and larvae were advected from the spawn-

ing areas at the shelf edge, towards the north and

towards the inner shelf by the dominant geostrophic

¯uxes, leading eventually to the spatial distribution of

larvae found in March. This advection pattern was also

con®rmed by a three-dimensional physical model of

the southern European shelf area (Barscht et al., 1996).

Finally, meso-scale events such as gyres or ®laments

can trap slope water where hake eggs and larvae

inhabit and advect them offshore (Barscht et al.,

1996; Motos et al., 2000). As already stated above,

larvae advected offshore are removed from the popu-

lation suggesting that the in¯uence of slope water

®laments and gyres can be detrimental to recruitment

success.

Another known nursery area usually appears in the

south-eastern corner of the Bay of Biscay (28W)

(SaÂnchez, 1994). The highest abundance of hake

larvae in this area was found during the February

cruise in 1995. In addition, the larval distribution

found there in March 1995, although low, seems to

suggest that eggs spawned in February were retained

and developed into large larvae by the end of March

(Barscht et al., 1996). Alternatively, the occurrence of

large larvae can result from the accumulation of

material coming from the coastal areas of the Cantab-

rian Sea. The general circulation in the area shows an

eastward transport (Barscht et al., 1996) which can

result in accumulation of material in the eastern

Cantabrian Sea (SaÂnchez, 1994; Cabanas, 1994),

where the direction pattern of the currents is not clear

and the intensity slackens.

Based on the patterns of distribution described here,

several issues are worth emphasising. By comparing

distribution of eggs and larvae, we inferred dispersion

(advection plus diffusion) in the north and northeast

directions. These drift patterns were similar for both

years and are consistent with the available data on the

physical oceanography of the area. The pattern of

oceanographical processes in the peak season of hake

egg production in the Bay of Biscay leads to the

maintenance of the integrity of the population by

linking the main spawning areas located in the shelf

break with the known nursery areas located in shelf

waters closer to the coast. These kinds of processes

have been postulated as one important regulatory

mechanism of recruitment, and hence, population

sustainability (Sinclair, 1988). The average current

patterns on the spatio-temporal window of hake peak

spawning in the study area may have resulted in the

Table 3

Summary of recruitment variation for several fish populations

Species Population na CVb (%)

Hake M. merluccius ICES northern 14 22

ICES southern 9 60

M. productus North Pacific 32 138

M. capensis South Africa 1.6 20 34

M. bilinearis NAFO 4VWX 20 54

M. gayi Peru 10 57

M. gayi Chile 14 29

Cod G. morhua NAFO 2J3KL 31 57

NE Arctic 43 72

Haddock M. aeglefinus NAFO 3NO 25 185

NE Arctic 42 129

Anchovy E. encrasicholus Bay of Biscay 15 103

Mackerel S. scombrus NE Atlantic 19 50

Horse mackerel T. trachurus NE Atlantic 9 226

a Number of recruitment estimates available for a given species.b Coefficient of variation of the mean recruitment. The data is taken from Myers et al. (1995).


evolution of reproductive patterns such that hake

spawn in winter and early spring months when

onshore transport prevails in northern Biscay waters.

Hake recruitment shows rather low variability when

compared with other ®sh species. Table 3 shows the

coef®cients of variation (CV) of recruitment for a

series of populations (Myers et al., 1995). The CV

of hake recruitment of the northern population unit in

the ICES area, was 20%, one of the lowest in ®sh

populations. This suggests that the pattern of hake

recruitment described above is consistent year after

year and leads to a relatively low variation in recruit-

ment success in the long term.

Based on the pattern of egg and larval distribution

described here, we inferred advection processes in

particular directions. The drift patterns were compa-

tible with the physical oceanography of the area.

These results re-inforce the need for a good knowledge

of the fundamental physical processes so as to under-

stand better the complex physical/biological interac-

tions in¯uencing recruitment of marine populations

(Sinclair, 1988; Kendall and Picquelle, 1989; Francis

et al., 1989; Heath, 1992; Koutsikopoulos and Le

Cann, 1996; Motos et al., 2000; ValdeÂs et al., 1996).

Acknowledgements

The results presented in this study were obtained

within the European Union SEFOS project (contract

AIR2-CT93-1105). SEFOS is the acronym for Shelf

Edge Fisheries and Oceanography Studies. The

authors very much appreciate the bene®ts of the

contrast of opinions with the various colleagues

involved in the SEFOS project.

Ichthyoplankton samples were provided by MAFF

Fisheries Laboratory (Lowestof, UK) SOAFD Marine

Laboratory (Aberdeen, Scoland) and RIVO-DLO

(Ijmuiden, the Netherlands). Inmaculada MartõÂn and

InÄaki Rico (AZTI) carried out the sample sorting

work.

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