Spatial Patterns of Ichthyoplankton Assemblages Along the Eastern English Channel French Coast during Spring 1995

Estuarine, Coastal and Shelf Science (1999) 49, 141–152 Article No. ecss.1999.0483, available online at http://www.idealibrary.com on

Spatial Patterns of Ichthyoplankton Assemblages Along the Eastern English Channel French Coast during Spring 1995 A. Griochea, P. Koubbi and X. Harlay Laboratoire d’ichtyoe´cologie marine de l’Universite´ du Littoral—Coˆte d’Opale, 17 avenue Ble´riot, B.P. 699, 62 228, Calais, France Received 19 October 1998 and accepted in revised form 17 February 1999 Two surveys were conducted during April and May 1995 along the French coast of the eastern English Channel. They aimed to describe the influence of environmental features—water advection through the Straits of Dover and the hydrological coastal front—on ichthyoplankton assemblages. Multivariate analysis, using the non-metric multi-dimensional scaling and based on the Bray-Curtis dissimilarity index, was associated with multiple regression to characterize the spatial larval distribution and to relate it to the environmental factors. During the April survey only young larvae were found. They were separated into a coastal larvae assemblage, characterized by high water temperature and fluorescence of chlorophyll a and two offshore assemblages limited to high salinity water. During the May survey, young larvae were mainly located in offshore waters, in the south of the Picarde Bay and in the Straits, where high offshore chlorophyll a fluorescence was found. In contrast old larvae were found along the coast, near estuaries. This pattern shows a spatial segregation of developmental stages of larvae in this area due to adult spawning migration, larval drift and ontogentic migration. Larval distributions can be explained by salinity, temperature and concentration of chlorophyll a, but the spatial evolution of those parameters during spring have also to be taken into account for the comparison of the distributions between both surveys.  1999 Academic Press Keywords: ichthyoplankton; drift; migration; multivariate analysis; English Channel; North Sea

Introduction The eastern English Channel is characterized by a permanent advection of water masses related to large tides and the presence of the Straits of Dover (Figure 1). Residual outflow is from the English Channel to the North Sea, except during northeasterly wind (Salomon & Breton, 1991). As shown by hydrology and the zooplankton community (Brylinski, 1986), currents create a separation along the French coast between continental waters maintained to the coast, and offshore waters of Atlantic influence. The boundary is a frontal zone varying in intensity and shape with the tidal cycle (Brylinski & Lagadeuc, 1990). On a reduced scale study, Grioche and Koubbi (1997) have shown the frontal influence on the evolution of ichthyoplankton assemblages. Thus, depending on the shape of the front, fish larvae are either accumulated on the frontal area or diverge from it to induce offshore–inshore segregation of assemblages. In this hydrological context, and according to the existing knowledge about the plankton dynamic of the a

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area (Brunet et al., 1996; Brylinski et al., 1996) it was interesting to study the influence of water advection on the life-cycle of fish species spawning in the Channel. This paper presents the results of two surveys conducted along the French coast of the eastern English Channel, during both day and night, between 11 to 13 April and 2 to 5 May 1995. In this study larval distributions are related with environmental factors, the evolution of larval assemblages are followed and larval transfers between the English Channel and the southern bight of the North Sea are described. Materials and methods Sampling design In April, 45 stations were investigated. During this time a strong northerly wind (7–10 m s
1) limited sampling along the North Sea coast. Three weeks later, in May, the network was extended to 60 stations (Figure 1). Transect lengths and positions were defined in accordance with Brunet et al. (1996) to cover the total range of hydrological features.  1999 Academic Press

142 A. Grioche et al.

60 rs

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F 1. Position of the stations in the study area. Arrows indicate the travel of the boat from the first station (1) to the last one (60). Stars indicate transects 4 and 9 (refer to Figure 3).

Salinity, temperature (C), and chlorophyll a fluorescence (unit of fluorescence: 1 u.f.=1 mg Chla l
1) were recorded at each station using a CTD profiler. Turbidity (percentage of light transmission) and irradiance (PAR) were only recorded during the first survey. Ichthyoplankton was sampled from the surface to the bottom by oblique tows using a bongo net fitted with 500 ìm mesh. Flowmeters were placed in the mouth of each net to measure the filtered volume (50–100 m3) and to check for clogging according to water flux. Samples were fixed in 5% formaldehyde buffered seawater and identification was carried out to species level (Russell, 1976). All larvae were sorted and staged according to Koubbi et al. (1990); adapted from Ryland (1966): stage 1: yolk sac larva; stage 2: pre-flexion larva, stage 3: flexion larva and stage 4: transition larva fin-rays formed. Data analysis The methodology was adapted from Field et al. (1982) and Kruskal and Wish (1978) and was applied to marine plankton ecology by Hosie (1994). For the analysis, only species or stages of species which accounted for more than 1% of total catches were used. Fourteen categories of larvae and 44 stations

were used for the April analysis. The first station (1) was excluded from this analysis because of the lack of larvae. For the May survey, 21 categories of larvae and all 60 stations were used. Classifications allowed description of the links between: (a) the species composition of stations (normal classification) for which data were log10 (x+1) transformed to reduce the share of abundant species; and (b) spatial distribution of species and stages (inverse classification), for which data were standardized. This means that abundance at each station was expressed as the percentage of the total abundance of the species for the survey (Field et al., 1982). The transformation places dominant and rare species at the same level, so that clustering only considered the spatial trend of larval distribution. To classify both sets of transformed data, the Bray-Curtis dissimilarity index was used (Bray & Curtis, 1957) and ‘ flexible sorting ’ (â=
0·3) (Lance & Williams, 1967). This method is an intermediate between simple and complete linkage and it allows classification of observations without influence of the numbers of stations in the existing groups. To define the relations between the groups of stations and the larval assemblages an ANOVA was used. This analysis, using the log10 (x+1) transformed matrix, characterized the groups of stations according

Spatial patterns of ichthyoplankton assemblages 143

to larval abundance. The Student Newman-Keuls (SNK) multiple range test of means (Scherrer, 1984) allowed identification of the most significant relations between the groups of stations and larval abundance. Moreover, as data used for clusters were also taken for statistical tests, the assumption of independence is not true. Therefore, the probability values of the ANOVA are biased and have to be considered as indicative even if they are necessary to the SNK validity. Graphic representation was done using non-metric multi-dimensional scaling (MDS) (SYSTAT software). The distance matrix between stations was used to represent data in a reduced space. The quality of the projection was given by a stress value, which is a measure of the distortion of the original data and determines the number of axes needed. MDS coordinates of stations, on selected axes, were then tested to each environmental variable using multiple regression. The value of the adjusted coefficient of determination (adj. R2) was representative of the variance accounted for each environmental variable in the MDS space (Jongman et al., 1987). In contrast to canonical analysis or partial regression analysis, this technique is neither based on a priori ecological hypothesis, nor does it remove the variance explained by previously used variables. Each variable is tested independently and their number is not limited, thus percentages of explained variance are not additive. By comparing the coefficient of each variable, the factor that explained best the larval distribution could be determined. The influence of the environmental variable over the dispersion of stations could be represented as a gradient in the MDS plane. It was represented as the direction of the maximum correlation of the regression, which is at an angle Èr with the rth axis. The formula of Kruskall and Wish (1978) estimated the direction cosine cr of the angle: cr =br/√(b21 +b22 +. . . b2m) where b1, b2, br, . . . bm are the regression coefficients of the multiple regression a+b1x1 +b2x2 +. . .+ br x r +. . .+bmxm, where m is the number of independent variable and xi the axes. Results

found near the coast, showing the influence of continental waters from the various rivers along the Opale coast (Figure 1). Three hydrological areas can be described. The first area was located between Dieppe and the Authie estuary, over Picarde Bay which was under the influence of the Somme River. Surface water of coastal origin spread out over the whole bay [Figure 2(a)] whereas salinity of bottom water conformed more to the coastline [Figure 2(b)]. In contrast, chlorophyll a was more confined to the coast in the surface layer than it was for deep waters [Figure 2(c,d)]. This area was also characterized by a strong vertical stratification of the water mass, strengthened by a thermocline [Figure 3(a)]. The second area was located along the Opale coast between the Authie estuary and Cape Blanc-Nez. Isohalines were parallel and close to the shore [Figure 2(a,b)]. This area was separated from Picarde Bay by a vertical oblique frontal structure marked at the surface by the 33 isohalines [Figure 2(a)]. It was also associated with an offshore extension of fluorescence (>5 u.f.) [Figure 2(c,d)]. Vertical thermal or haline stratification was low or absent [Figure 3(b)]. The third hydrological area was located in the North Sea. It was characterized by cold (7C) and salty water (greater than 35) (Figure 2(a,b)] with no vertical stratification. This area was separated from the previous one by another frontal structure, perpendicular to the coast, off the Cape Blanc-Nez. For the May survey, in spite of good weather, no vertical stratification was observed in the English Channel, due to spring tides. However, a coastal to offshore gradient of salinity was pronounced in the south of the Channel, i.e. in the Picarde Bay [Figure 4(a)]. In the North Sea, off Flanders, a frontal structure was observed. As shown by salinity reducing to the east, it is probably linked to the Scheldt estuarine water. Temperature was again inversely correlated to salinity with high temperature (10·4–11·5 C) and low salinity (32–33) along the coast. The chlorophyll a fluorescence pattern followed salinity. A gradient could be observed, with the highest concentration along the coast and nearby estuaries, such as the Somme and the Scheldt rivers [Figure 4(b)]. An offshore extension of chlorophyll a was noticeable in the southern part of the Picarde Bay, i.e. along the first two transects.

Hydrology During April, a coastal to offshore gradient, underlining the boundaries of the coastal waters, was observed [Figure 2(a,b)]. Temperature and salinity were strongly and inversely correlated. Highest temperature (9·3 to 9·8 C) and lowest salinity (32 to 33) were

Species composition The regular slope of the rank-frequency diagram indicated a relatively even distribution of abundance of the dominant species, except for Sprattus sprattus (Figure 5). For both surveys S. sprattus was the

144 A. Grioche et al.

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F 2. Hydrological characteristics during April survey. Italic names indicate rivers of the Opale coast. (a) Surface (
5 m) salinity; (b) bottom salinity; (c) surface (
5 m) fluorescence of chlorophyll a; (d) bottom fluorescence of chlorophyll a.

50°14' N 1°28' E 0

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4 6 8 10 12 Distance to the coast (miles)

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F 3. Vertical thermal structure of the water, dotted lines indicate CTD profiles. (a) transect number 4 (Figure 1); (b) transect number 9 (Figure 1).

dominant species, having about 50% of the total abundance. While dominant species among surveys were similar, there were some changes in frequency

rank. Mean total larval abundance for each survey was important with about 3·6 larvae m
3 in April and 9 larvae m
3 in May. Mean larval abundance doubled

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Sprattus sprattus Merlangius merlangus Ammodytidae Callionymidae Solea solea Pleuronectes flesus Limanda limanda Trisopterus luscus

0.6 0.8 1.0 Log(rank)

1.2

1.4

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F 5. Rank–frequency diagrams for both surveys. April survey: ; May survey: .

20 miles

(b)

Cumulative frequency (larval abundance)

Spatial patterns of ichthyoplankton assemblages 145

Somme 20 miles

F 4. Hydrological characteristics during May survey. Italic names indicate rivers of the Opale coast. (a) Mean salinity; (b) mean fluorescence of chlorophyll a.

between the surveys for S. sprattus, callionymidae, Merlangius merlangus, Limanda limanda, Solea solea and Trisopterus luscus, whereas abundance of Pleuronectes flesus and ammodytidae remained constant. Spatial trend For the April survey, clustering of stations showed three main groups (I to III) [Figure 6(a)]. Coastal stations (groups IIb and III) were separated from offshore stations (groups I and IIa) [Figure 6(b)], but clusters also separated in the south to north eastern

English Channel, with a first feature off Dieppe (group II), a second off estuaries (groups Ia and IIIb) and a last one near the Straits (groups Ib and IIIa). Three larval assemblages (A to C) were found [Figure 6(a)]. Highest abundance, for all larvae, was in group Ia in the Picarde Bay. Abundance for the first assemblage (A) was essentially found in group I [Figure 6(a)], i.e. in most of the offshore area in the eastern English Channel. Lowest abundance was found in groups IIb and IIIb, along the southern Opale coast. The larvae of the second assemblage (b) were mostly present offshore in groups Ia and IIa, geographically limited to the south of the Canche estuary even if some stations were found in the north of the Straits [Figure 6(b)]. The last assemblage of larvae (C) was abundant in groups Ia, and III [Figure 6(a)]. These larvae were the most coastal, close to estuaries but as the other assemblages, they were also particularly abundant offshore from the Picarde Bay [Figure 6(b)]. Clustering of stations for the May survey revealed five main groups (IV to VIII) [Figure 7(a)]. English Channel and Straits offshore stations (groups IV and VI) are separated from the coastal ones (group V) [Figure 7(b)], whereas northern stations, along the Flanders coast, did not show this separation and were found apart (groups VII and VIII). Two main clusters (D and E) were found for the larvae. Older larvae (assemblage D) were clearly separated from the youngest (E) except for S. solea stage 2 and S. sprattus stage 3–4 [Figure 7(a)]. Assemblage D1 showed the highest abundance of the larvae stage 3 in groups IV and V, localized in the coastal and offshore area off Dieppe and off the Opale coast in the eastern English Channel [Figure 7(b)]. Assemblage D2 comprised transition larvae (stage 4) which were mainly coastal

146 A. Grioche et al. Stations clusters (a)

(b) 100 0

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0

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Merlangius merlangus Trisopterus luscus Pleuronectes flesus Callionymus spp. Trisopterus spp. Lotinae Limanda limanda Pleuronectes flesus Ammodytidae Solea solea Sprattus sprattus Solea solea Gobiidae Pleuronectes flesus

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Dieppe

F 6. Relationships between station clusters, larval assemblages and larval abundance for April survey. (a) Results of SNK tests about mean larval bundance by stations clusters (Ia to IIIb). Probabilities of the ANOVA are only indicative: *<0·05; **<0·005; ***<0·0005. indicate significant high abundance and indicate significant low abundance. Clustering was done using the Bray-Curtis dissimilarity (Bray & Curtis, 1957) and the flexible linkage (Lance & Williams, 1967). (b) Mapping of the groups of stations defined by the cluster analysis. Symbols indicated the membership to groups.

Stations clusters 100 (b)

(a)

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D2 E1 E2 E3

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Stages

Merlangius merlangus Trisopterus luscus Callionymus spp. Solea solea Solea solea Limanda limanda Pleuronectes flesus Trisopterus luscus Merlangius merlangus Solea solea Lotinae Callionymus spp. Pleuronectes flesus Gobiidae Sprattus sprattus Sprattus sprattus Ammodytidae Limanda limanda Merlangius merlangus Trisopterus luscus Limanda limanda

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3 3 3–4 3 2 4–5 4–5 4 4 4–5 2 2–3 2 3–4 2 2 2 3

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rs

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Canche Authie Somme

Dieppe

F 7. Relationships between stations clusters, larval assemblages and larval abundance for May survey. (a) Results of SNK tests about mean larval abundance by stations clusters (IV to VIII). Probabilities of the ANOVA are only indicative: *<0·05; **<0·005; ***<0·0005. indicated significant high abundance and indicated significant low abundance. Clustering was done using the Bray-Curtis dissimilarity (Bray & Curtis, 1957 and the flexible linkage (Lance & Williams, 1967). (b) Mapping of the groups of stations defined by the cluster analysis. Symbols indicated the membership to groups.

(group V). Young larvae (assemblage E) [Figure 7(a)] showed their main abundance offshore, in the south of the Picarde Bay and near the Straits of Dover (group IV). In addition, assemblage E3 was found off the Somme estuary (group VI), whereas assemblage E2 was the most extensive, present in all the eastern English Channel, as well in coastal than in offshore area. The spatial pattern of the larval abundance, for both surveys, assumed these descriptions (Figure 8).

As previously described for P. flesus in this area (Grioche et al., 1997) it allowed observation of the migration of M. merlangus to the coastal nurseries. Non-metric multidimensional scaling and multiple regression The discontinuity of the physicochemical gradient in the northern frontal zone resulted in a non-linear

M. merlangus st 2 Assemblage A

L. limanda st 2 Assemblage B

S. solea st 2 Assemblage C

Callionymus spp. Assemblage A

Lotinae Assemblage B

S. sprattus st 2 Assemblage C

Callionymus spp. st 2 Assemblage E1

M. merlangus st 2 Assemblage E3

M. merlangus st 3 Assemblage D1

S. solea st 2 Assemblage D1

M. merlangus st 4 Assemblage D2

May

April

Spatial patterns of ichthyoplankton assemblages 147

S. sprattus st 2 Assemblage E2

F 8. Examples of larval distributions for both surveys, April and May. Species and developmental stage (st) are indicated on the map as well as the assemblage. Circle area is proportional to the abundance: 10, 50, 100 ind/100 m3, except for S. sprattus in April (assemblage C) where the scale is divided by 2 and S. sprattus in May (assemblage E2) where the scale is divided by 10.



Axis 2

148 A. Grioche et al.

Ia IIIb

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1 4 IIa 8

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F 9. Planes of the April MDS using Bray-Curtis dissimilarities. Groups of stations (I to IIIb), and directions of multiple regressions explaining the strongest part of the distribution of the stations in the planes were drawn. Position of the station in the planes are indicated. Black dots are for the stations not used in the regression. 1: Bottom salinity; 2: bottom temperature; 3: surface fluorescence; 4: surface light transmission; 5: wind speed; 6: distance to the coast; 7: bottom fluorescence; 8: bottom irradiance; 9: depth.

relationship. For this reason and because of a weak sample survey, the northern stations were not taken into account in the multiple regression analysis for April, but their positions were indicated in the MDS planes (Figures 9 and 10). Three MDS axes were chosen for April analyses with a stress value of 0·076. Group IIb was eliminated from the MDS as it was too dissimilar from the others. Indeed, it was characterized by strong irradiance and light transmission. Because of local vertical variations in the network, environmental parameters were taken for the surface (
5 m) and the bottom. The percentage of explained variability accounted to each variable is given in Table 1. The first MDS plane (Axes 1–2) (Figure 9) discriminated group I of high bottom salinity and low

surface turbidity from group III having high bottom temperature and high surface fluorescence of chlorophyll a. This clearly points out the coastal to offshore hydrological gradient. Associated with estuaries, group IIIb appeared to be the most ‘ coastal ’. Stations in group IIa were separated from the other groups by axis 2: this was characterized by strong bottom irradiance. The third axis discriminated groups Ia, IIa and IIIb with high bottom fluorescence, from groups Ib and IIIa having strong wind. The multiple regression analysis also showed that group Ib was characterized by higher salinity and deeper waters. For the May survey, three axes were kept for the MDS and all stations were retained. Stress value was 0·092. As irradiance in the water was not measured, and the weather was constantly sunny during this survey, estimation of the light influence was attempted during the sampling by using the sun angle with respect to the nadir. As no stratification of the water column was observed, mean values of environmental factors were used. Temperature or salinity explained more than 60% of the data variability [Table 2(a)]. These variables were essentially linked to the coastal to offshore gradient as shown by the depth and the distance to the coast [Figure 10(a)]. This gradient dominated the results and it was difficult to estimate the effects of any other variable. Axes 2 and 3 remained weakly explained by environmental variables, as shown by the low coefficients in Table 2(a). MDS planes only allowed discrimination of coastal groups V and VIII by their low salinity, high temperature and fluorescence. To explain the offshore area better, these groups were removed and a second MDS was achieved [Figure 10(b)]. The two dimensional MDS gave a stress value of 0·138. The environmental factors which explained most of the data variability were now the difference in salinity, temperature and fluorescence between both surveys [Table 2(b)]. On the MDS plane [Figure 10(b)], group VII was separated from groups IV and VI because of an increase in temperature. On the second axis, group IV was linked to an increase in chlorophyll a fluorescence and this was opposite to group VI. Group IV was also characterized by an important sun angle with respect to the nadir, which meant that these stations were mainly sampled during the night. Discussion As the sampling was carried out during days and nights, the influence of light was previously controlled by t-tests. Tests have shown that for larvae used in the analysis, day/night differences between catches were

Spatial patterns of ichthyoplankton assemblages 149

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F 10. Planes of the April MDS using Bray-Curtis dissimilarities. Groups of stations (IV to VIII), and directions of multiple regressions explaining the strongest part of the distribution of the stations in the planes were drawn. Position of the station in the planes are indicated. (a) MDS with all stations clusters. (b) MDS without clusters V and VIII. Black dots are for the stations only sampled in May and not used in the regression. 1: Mean salinity; 2: mean temperature; 3: distance to the coast; 4: mean fluorescence; 5: depth; 6: difference of salinity; 7: difference of temperature; 8: difference of fluorescence; 9: sun angle.

not significant (P>0·05). Light influenced few station clusters of the analysis, and was not a prevailing feature in the results. Concerning the eastern English Channel, environmental structuring factors were similar between surveys as the coastal to offshore segregation of the larvae. This segregation is explained by the gradient of temperature and salinity characterizing the coastal frontal area along the Opale coast. Secondly, south to

north variations in assemblages were related to variations of chlorophyll a fluorescence. For April, some species were found in coastal and offshore areas off the Somme estuary. The elevated bottom value of fluorescence is the only common feature for the two areas that seems to explain this result. Bottom fluorescence also explained the segregation of the southern offshore larvae from the northern one which were found in low fluorescence waters. The bottom

150 A. Grioche et al. T 1. Coefficients of the multiple regression analysis between stations co-ordinates and environmental variables, for each axis of April MDS Variable

axis 1

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adj. R2

F

P

Bottom salinity Bottom temperature Surface fluorescence Surface light transmission Wind velocity Distance to the coast Bottom fluorescence Surface salinity Log of bottom irradiance Depth Log of surface irradiance Surface temperature Bottom light transmission

0·78
0·18
8·00 2·61 0·30 3·70
4·19 0·60 0·44 5·26 0·13
0·09 4·74

0·09 0·01 3·74
0·50
0·20 0·49 3·65 0·06
0·93 2·08
0·22 0·03
2·25


0·38 0·04 4·54 0·22
3·68 4·44 16·44
0·68
0·72
9·34
0·40 0·19 4·39

82·8 66·9 65·6 61·6 58·0 52·8 44·3 42·0 39·3 17·5 13·9 8·0 1·7

57·32 24·64 23·26 19·73 17·11 14·03 10·29 9·47 8·57 3·47 2·89 2·01 1·20

** ** ** ** ** ** ** ** ** * — — —

Percentage of explained variability (adj. R2), and probability of the regression are indicated. —: not significant; *:<0·005; **:<0·0005.

T 2. Coefficients of the multiple regression analysis between stations co-ordinates and environmental variables, for each axis of May MDS. (a) MDS with all groups of stations; (b) MDS without clusters V and VIII axis 1

axis 2

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F

P

(a) Mean salinity Mean temperature Mean fluorescence Depth Distance to the coast Fluorescence variation Sun angle Salinity variations Temperature variation

1·40
0·53
8·08 7·99 2·97
4·04
8·28
0·37
0·13

0·04 0·14 3·45
3·47
2·36 2·99 8·01 0·11 0·13


0·13
0·02
2·74 1·05
0·43
0·43
8·08
0·13 0·01

69·42 62·37 54·32 33·22 29·89 11·52 3·35 1·32 0

43·39 31·94 23·20 20·29 9·38 2·52 1·65 1·18 0·15

** ** ** ** ** — — — —

Variable

axis

axis

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(b) Salinity variations Temperature variation Fluorescence variation Sun angle Mean fluorescence Mean temperature Mean salinity Depth Distance to the coast


0·84
0·65
1·60 17·13
3·13
0·12 0·04 0·74 0·07

Variable

1

2

0·35 0·04 5·06 0·49 0·47
0·03 0·14 0·78 0·25

32·81 23·69 23·69 13·28 8·00 4·75 0 0 0

R2

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8·33 5·66 5·05 4·14 2·78 2·02 0·82 0·14 0·04

P

** ** * * — — — — —

Percentage of explained variability (adj. R2), and probability of the regression are indicated. —: not significant; *: <0·005, **: <0·0005.

influence of environmental variables did not mean the larvae were found near the sediment. As young stages had essentially been found, larval distribution was linked to abiotic characteristics of the spawning areas

(Grioche & Koubbi; Grioche et al., 1997) and thus, were better linked to the bottom hydrological factors. In May, larvae were fewer in the Picarde bay, even with important fluorescence of chlorophyll a. The

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Spatial patterns of ichthyoplankton assemblages 151

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Spawning area

F 11. Synthetic scheme of the ecological functioning of the French coast of the English Channel during spring time.

highest abundance of young larvae was found in the neighbouring areas, particularly to the north, corresponding to the fluorescence increase between surveys. The North Sea coast was characterized by highest variations of temperature and salinity between surveys in relation to shallow depths and to the influence of wind. The Scheldt stations might be related to an extension of estuarine water (Hezq et al., 1992), with indicative larvae such as Gobiidae and transition larvae of P. flesus and S. solea. A continuity in larvae assemblages was visible between the English Channel and the southern bight of the North Sea. It underlined a northern drift up to the frontal zone, separating the Opale coast from Flanders. Then, waters originated in the English Channel seemed to be directed to the north (Nihoul et al., 1989), bringing fish larvae to the northern area as shown for herring (Bu¨ckmann, 1942) and plaice (Talbot et al., 1978). Indeed, during north-eastern wind periods inverse drift of water (Anonymous, 1968) and planktonic organisms (Belgrano et al., 1990) have been observed. Accordingly, during the April survey, the temporary northern wind would have reduced the outflow from the eastern English Channel to the North Sea (Salomon & Breton, 1991) and disrupted the vertical stratification of the water.

From this study and the previous studies in the area, a synthetic scheme is now able to be built up of the functioning of the eastern English Channel ecosystem during the spring period (Figure 11). The Picarde Bay appeared to be a productive area, enhanced by hydrological stability (vertical stratification) and nutrient enrichment from the Seine estuary (90 km to the south) (Brylinski et al., 1996). As the phytoplanktonic bloom is initiating, maximum abundance of larvae was found in the bay as well as best larval conditions, as shown by morphological and histological indices (Grioche, 1998). Later on, as production is taken to the north by currents, ecosystem maturation occurs along the Opale coast (Brunet et al., 1996) and young fish larvae were found offshore. This offshore distribution of fish larvae corresponds to the localization of spawning grounds. Because of tide mixing between coastal and offshore waters (Dupont et al., 1991; Grioche & Koubbi, 1997), planktonic production is exported to the offshore and young larvae benefit from this exported production as they are drifting to the Strait. Following this northern drift, better larval conditions (estimated by various morphological and physiological indices) were shown to exist in the Strait and in the North Sea than in the Picarde Bay during May 1995 (Grioche, 1998). Then planktonic production,

152 A. Grioche et al.

through chlorophyll a fluorescence, explained the distribution of the youngest larvae in the offshore area. When larvae become older, they stop their passive drift and migrate to the coastal nurseries as shown by Grioche et al. (1997) for P. flesus in this area. Nevertheless, some larvae still remained and settled in the south of the Picarde Bay. Referring to water stratification and important fluorescence values, this area seemed to be a retention zone, as confirmed by hydrodynamic model (Salomon, 1991). Looking to the match–mismatch theory (Cushing 1975; 1990), it appears that the spawning strategy of fish is adapted to water circulation so that young larvae are transported to water masses of great production, as shown by Fortier et al. (1992) in the Gulf of St Lawrence. However, as noted during April, the wind seemed to interfere with the residual drift. It will be interesting to observe if this phenomenon could be durable enough to lead to a better settlement of recruits, confining them to the eastern English Channel.

Acknowledgements Sincere thanks are due to the crews of the Coˆte d’Aquitaine and the Coˆte de Normandie for their help during the surveys, to Dr G. Meaden for correcting the English, and to the students who have participated in the collection of data. This study was supported by a grant from the De´partement du Pas de Calais and Re´gion Nord-Pas de Calais. It is included in the French contribution to GLOBEC, the European Programme INTERREG II, and the regional programme DYSCOP.

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