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Influence of physical environmental factors on the composition and horizontal distribution of summer larval fish assemblages off Mallorca island (Balearic archipelago, western Mediterranean) ´ PEZ-JURADO1, J. JANSA ` 1, M. PALMER2 AND I. PALOMERA3 F. ALEMANY1*, S. DEUDERO2, B. MORALES-NIN2, J. L. LO 1 ` FIC DE BALEARS/INSTITUTO ESPAN ˜ OL DE OCEANOGRAFI´A, MOLL DE PONENT S/N, 07015 PALMA DE MALLORCA, SPAIN, 2GRUPO DE CENTRE OCEANOGRA OCEANOGRAFI´A INTERDISCIPLINAR DEL INSTITUT MEDITERRANI D’ESTUDIS AVANC ¸ ATS, UIB/CSIC, 21 MIGUEL MARQUES, ESPORLES, 07190 MALLORCA, 3 SPAIN AND CENTRE MEDITERRANI D’INVESTIGACIONS MARINES I AMBIENTALS, CMIMA-CSIC, PASSEIG MARI´TIM DE LA BARCELONETA, 37-49 E-08003
BARCELONA, SPAIN
*CORRESPONDING AUTHOR:
[email protected]
Received June 1, 2005; accepted in principle August 25, 2005; accepted for publication December 22, 2005; published online January 30, 2006 Communicating editor: K.J. Flynn
Two ichthyoplankton surveys were carried out in two areas off Mallorca island (Balearic archipelago, western Mediterranean) in June 1996 and August 1996, respectively. The aim of both surveys was to assess the influence of physical environmental factors on the horizontal distribution of larval fish assemblages, focusing on larvae of large migratory pelagic fish species. Canonical correspondence analysis (CCA) indicated that in the June survey, the patterns of horizontal distribution of fish larvae were mainly conditioned by depth and by the distribution of two surface water masses: the Modified Atlantic Waters (MAW), of recent Atlantic origin, and the older Surface Mediterranean Waters (SMW). The effect of depth gradient was clear in mesopelagic and neritic species, but it was not so evident in the larvae of large pelagic species which presented a highly patchy distribution. Contrastingly, the August patterns of horizontal fish larvae distribution were significantly correlated with the surface salinity gradient resulting from successive MAW inflows, whereas depth did not show any significant effect, probably linked to the bottom topography with a very narrow shelf area. The data obtained highlight that the Balearic Islands constitute an important spawning ground for most of the large pelagic fishes inhabiting Mediterranean waters, both highly migratory and resident species.
INTRODUCTION The hydrodynamics around Balearic Islands are characterized by a high mesoscale activity, related to its condition of transitional region between the two main western Mediterranean sub-basins: the Ligurian–Provenzal and the Algerian. During summer, the dynamics of surface water masses are mainly conditioned by the interaction between Surface Mediterranean Waters (SMW), moving southward from the northern part of western Mediterranean, and Modified Atlantic Waters (MAW), flowing northwards from Algerian Basin, leading to a complex hydrographic situation. Surface currents are relatively strong, and several mesoscale oceanographic
features like fronts and eddies are present in the area (Millot, 1994; Garcia-Lafuente et al., 1995; Lo´pez-Jurado et al., 1995, 1996; Garcı´a-Ladona et al., 1996; Pinot et al., 2002). Since mesoscale advective processes affect planktonic communities (Haury et al., 1978), it could be expected that the horizontal distribution of fish larvae in the studied areas would reflect the complex pattern of summer surface circulation around Mallorca Island (Balearic archipelago). Because of these physical environmental conditions, fish larvae assemblages in Balearic archipelago (Alemany, 1997) differ in species composition and distribution from those found in nearby areas
doi:10.1093/plankt/fbi123, available online at www.plankt.oxfordjournals.org Ó The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please email:
[email protected]
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of the Spanish Peninsula coasts (Sabate´s, 1988; Sabate´s and Maso´, 1992; Pe´rez de Rubı´n, 1996). These differences are remarkable during the summer because of the presence around the islands of larvae from large migratory pelagic species. The Balearic Islands represent one of the main spawning grounds for species like bluefin tuna (Thunnus thynnus), albacore (Thunnus alalunga) (Duclerc et al., 1973; Dicenta et al., 1975, 1983; Dicenta 1977; Alemany, 1997) and dolphin fish (Coryphaena hippurus) (Alemany and Massutı´, 1998). Fish larvae from other large pelagic species like istiophorids (Tetrapturus sp.) and swordfish (Xiphias gladius) that inhabit in the Mediterranean all year round are also present. Adults of migratory species enter the Mediterranean through the Strait of Gibraltar at the end of spring. Spawning takes place during the summer months, when the thermocline can reach 50-m depth and surface water temperature rise up to 268 C (Rey, 1983; Massutı´, 1997). All these species are targeted in the Balearic Islands by seasonal professional fisheries, purse seiners and long liners, exploiting the spawning stock and/or the young recruits, and also by a game fishery that has been developed in recent years (Massutı´ et al., 1997). Several authors have described the preference expressed by oceanic pelagic fishes to spawn around islands (Miller, 1979; Leis et al., 1991). This phenomena could be attributed to the ‘island mass effect’ (Doty and Oguri, 1956) that produces an increment of planktonic biomass around oceanic islands as a result of perturbations inflicted on marine currents. Although the Balearic Islands are not considered typical oceanic islands, since they are located in a semi-enclosed sea and are relatively near mainland, they strongly affect the flow of the water masses in the western Mediterranean basin. As a consequence, fronts and eddies appear in the channels between islands and between the mainland and the islands, and they may promote larval retention, larval dispersion or act as fertilization mechanisms. The continental slope also contributes to the generation of hydrographic instability (Lo´pez-Jurado et al., 1996; Jansa` et al., 1998). Larval fish assemblages in the Balearic Sea have been previously described. However, in most cases sampling consisted of exploratory surveys, with stations widely separated, or monthly sampling over the same few neritic stations (Alemany, 1997). Other ichthyoplankton surveys carried out in the area focused on the capture of tuna larvae (Duclerc et al., 1973; Dicenta et al., 1975, 1983; Dicenta, 1977). However, none of these sampling strategies permitted a clear definition of horizontal patterns of larval fish distribution or their relation to the hydrographic conditions. The main objectives of this study
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were to extend our knowledge about the faunal composition of the summer larval fish assemblages and to assess the influence of physical environmental factors on the general pattern of their horizontal distribution.
METHOD Sampling Two areas, located in the west and east of Mallorca Island, were surveyed (Fig. 1A). In the western region, a grid of 25 stations was sampled between 20 and 26 June 1996. Stations, 5 nautical miles apart, were located in five transects of 25 nautical miles long. In the eastern region, a grid of 16 stations were sampled between 6 and 10 August 1996. Stations, 1.5 nautical miles apart, were located in three transects of 9 nautical miles long. All stations were sampled during daytime. Ichthyoplankton samples were collected with a Bongo net 40 cm in diameter with a 333-mm mesh size. A flow-meter installed in one of the mouths of the net was used to measure the volume of filtered water. Hauls were oblique, from 100-m depth to surface, and samples obtained were preserved immediately after capture in 4% seawater-buffered formalin. In the laboratory, fish larvae were sorted and identified under a stereoscopic microscope to the species level when possible. Vertical profiles of temperature, conductivity, pressure and fluorescence, between the surface and a maximum of 250-m depth, were obtained at every station with an SB25 Conductivity, Temperature and Depth (CTD). Geostrophic currents were estimated from those CTD data. Although the values of the geostrophic current could be slightly biased because of the depth of the reference layer, only 250 m, the current directions and velocity values obtained can be considered representative of geostrophic circulation in the study areas. Therefore, the hydrographic scenario was completed with information on the regional circulation pattern in the area available from two previous hydrographic surveys. These cruises covered the Mallorca and Ibiza Channels and were performed only 1 week before both of our larval cruises. Data provided by current meters moored in the points A3 and A4 were also available (Fig. 1A) (Pinot et al., 2002).
Preliminary data processing and ecological indices The number of fish larvae caught at every station was standardized to number/1000 m3. The abundance and frequency of occurrence of each species were calculated. Shannon–Weaver diversity index, expressed as P H 0 ¼ Si¼1 pi ln pi , where S is the total number of species in the community and pi is the proportion of S
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Fig. 1. (A–C) Study areas in the June and August surveys. Points A3 and A4 represent the location of moored currentmeters. In panels B and C, lines with arrows represent geostrophic currents and isolines the fluorescence values at 80-m depth.
made up of the ith species, and the Pielou’s evenness 0 0 index, J 0 ¼ H 0 =Hmax , where Hmax ¼ ln S, were also calculated for each sampling station.
Statistical analysis related to patterns of horizontal distribution of fish larvae A cluster analysis was performed from the relative abundance of species that represented >1% of the total to explore the horizontal distribution of larval fish assemblages. Unpaired Weight Group Average was used as the aggregation algorithm on the matrix of pairwise Euclidean distances.
Independence of samples was tested for spatial autocorrelation effects. Between-hauls distance ranged from 19.6 to 1.1 km (August survey) and from 70.9 to 6.0 km (June survey). Two Mantel tests, one per survey, were performed to evaluate the spatial autocorrelation. The Mantel statistics (i.e. the observed Pearson correlation coefficient, r) between geographic distance and species composition difference (chi-squared distance) were compared with those obtained by 999 random permutations of the data. Horizontal patterns of larval fish distribution were explored with a correspondence analysis (CA) (Legendre and Legendre, 1998) to summarize betweensample differences (measured by w2 distance) in a few
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dimensions. Rare species (found in less than five hauls) were excluded. The number of species included was 36 for the June survey and 27 in August. A canonical analysis (Legendre and Legendre, 1998) was performed to address the relationship between species composition and physical environmental variables. The exploratory variables considered were depth (lntransformed depth), temperature and salinity at 50-m depth and temperature and salinity at the surface waters. In the analysis of the June survey, as two different water masses were found (MAW or SMW), a categorical variable was assigned to each station according to the type of water. Two canonical CAs (CCAs), one per survey, were performed. Forward stepwise selection procedure of explanatory variables was applied in all cases. Since spatial autocorrelation in larval composition was not significant, the spatial variation of the variables measured and the sample scores resulting from different CA and CCA runs were mapped using a simple (lineal) interpolation procedure.
RESULTS Local hydrographic conditions The dynamics of surface water masses, estimated from CTD data from June to August surveys, are shown in Fig. 1B and C, respectively. In June, the estimated surface geostrophic currents coming from the Algerian Basin in a northward flow, crossed the Mallorca channel. This current was deflected to the East but later resumed its northerly flow (Fig. 1B). The survey was carried out under early summer conditions when thermal
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stratification was well defined. Surface (Fig. 2A) and 50-m depth (Fig. 3A–C) water temperatures and salinities were colder and saltier in the north of the sampling area than in the south, with differences of 2.5 and 0.38 C, respectively. Density values defined two surface water masses separated by a front, with SMW in the north and MAW in the southern region. This front was oriented in a south-west—north-east (SW–NE) direction in the upper few meters and in a N–S direction in depth, down to 90 m. In the latter instance, the warmer and lighter waters were close to the shoreline and the less warmer and saltier waters were located offshore. A layer of warm and lighter water (MAW), reaching 50 m in depth, was observed at the north-west (NW) part of the study area, close to the Mallorca coast. This confirmed the presence of a northward MAW flow, coming from the Algerian basin, close to Mallorca Island. Fluorescence maximum values were detected between 65- and 85-m depth. Spatial distribution of fluorescence values at 80-m depth, which is representative of the distribution of the maximum of fluorescence, is shown in Fig. 1B. In August, the surface geostrophic currents, estimated from CTD data (Fig. 1C), showed an inflow of northern flowing waters affecting the southern part of study area. Surface temperatures were high (Fig. 2B), close to the maximum values for summer stratification. Surface salinity values were lower than in the previous survey (Fig. 2B). These salinity values corresponded to MAW, which were found in the whole area at the surface. However, it could be observed that warmer and lighter MAW (Fig. 2B) was overlaying a somewhat colder and saltier MAW (Fig. 4A–C). The MAW layer was thicker in the southern and eastern stations than in onshore stations. The depths of the maximum of fluorescence ranged between 70 and
N 40.0
N 39.60
A
B 1000 m
39.9 39.8
39.55
37.94 37.90 37.86 37.82 37.78 37.74 37.70 37.66 37.62 37.58 37.54 37.50
200 m
39.7 39.6 39.5 39.4
37.77
39.50
37.74 39.45
37.71 37.68
39.40
37.65 37.62
39.35 50 m
37.59
39.30 39.25
37.56 37.53
200 m 1000 m
39.3 1.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
39.20 3.20
E
3.25
3.30
3.35
3.40
3.45
3.50
3.55
E 3.60
Fig. 2. (A and B) Surface salinities (grey scale) and temperatures (white lines) in both study areas. Different station labels, crosses and points, respectively, have been used to distinguish in both surveys the stations in which the two larval assemblages identified through cluster analysis were distributed.
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40.0
Neritic species larvae
N
>600
MEDITERRANEAN SUMMER ICHTHYOPLANKTON
321–360
A 1000 m
281–320
501–600
38.10 39.8
200 m
38.02
39.7
161–200
37.90
39.4 39.3 1.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
37.82
81–120
37.78
41–80 1–40 0
37.76
1000 m
39.60
38.10 38.06
200 m
2401–2700
38.02 39.7
37.98
3.25
3.30
3.35
3.40
3.45
3.50
E 3.60
3.55
Mesopelagic species larvae
N
39.50
37.88
39.45
37.86 37.84
2101–2400 39.40
37.82
37.94
501–600 39.6
1801–2100 39.35
37.90
401–500 39.5
1501–1800 39.30
37.82 37.78
1.9
901–1200 601–900
2.1
2.2
2.3
2.4
2.5
E
>400
C
181–200
38.10 39.8
1000 m 3.25
161–240
38.02 39.7
37.98 37.94
39.6
37.90
41–80
37.86 39.5
3.35
3.40
3.45
3.50
3.55
E 3.60
Large pelagic species larvae
N
C 39.55 39.50
37.88
39.45
37.86 37.84
81–160
39.40
1–80 0
39.35
37.82 37.80
50 m
37.78
39.30
37.82
0
39.25
37.78
37.76
200 m
39.4 39.3 1.8
3.30
240–320
38.06
200 m
161–180
39.20 3.20
321–400
1000 m
39.9
37.76
200 m
39.25
39.60
Large pelagic species larvae
N 40.0 >200
2.0
37.78
1201–1500
39.4 39.3 1.8
37.80
50 m
37.86
101–200
1–40
37.78 200 m
2701–3000
1000 m
601–700
81–120
37.80
50 m
B
39.8
121–160
39.35
39.55
39.9
1–100
37.84 39.40
39.20 3.20
>3000
701–800
201–300
37.86
39.25
B
301–400
39.45
39.30
E
Mesopelagic species larvae
N 40.0
37.88
121–160
37.86
39.5
39.50
37.82
37.94
101–200
>800
201–240
37.98
201–300 39.6
39.55
241–280
38.06
401–500
1–100 0
Neritic species larvae
N
A 39.9
301–400
39.60
1000 m 39.20 3.20 1.9
2.0
2.1
2.2
2.3
2.4
2.5
3.25
3.30
3.35
3.40
3.45
3.50
3.55
E 3.60
E
Fig. 4. (A–C) Distribution of salinities (grey scale) and temperatures (white lines) at 50-m depth corresponding to August survey. Black dots represent the abundances of neritic, mesopelagic and large pelagic species larvae, respectively.
Fig. 3. (A–C) Distribution of salinities (grey scale) and temperatures (white lines) at 50-m depth corresponding to June survey. Black dots represent the abundances of neritic, mesopelagic and large pelagic species larvae, respectively.
105 m. The spatial distribution of fluorescence values at 80-m depth is shown in Fig. 1C.
Faunistic composition, abundance of species and ecological indices From the June survey, 2113 fish larvae were sorted. A total of 52 different taxa were identified, 43 to species
level, 4 to the genera level, 4 to family, 1 to order, and 1 remained unidentified. Mean abundance of larvae per station was 542 larvae/1000 m3 ± 276 SD. Density values by taxa are detailed in Table I. The most abundant species was Trachurus mediterraneus (87 larvae/1000 m3 per station), followed by the two mesopelagic species Cyclothone braueri (70 larvae/1000 m3) and Ceratoscopelus maderensis (50 larvae/1000 m3). Larvae
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Table I: Mean densities by species, expressed in individuals/1000 m3, in June 1996 and August 1996 surveys June Species
Mean
August SD
F
%
Mean
SD
F
%
Sardinella aurita
0.81
4.06
0.04
Engraulis encrasicholus
4.03
10.73
0.24
0.76
Cyclothone braueri
70.44
101.41
0.76
13.83
61.94
56.99
0.94
3.16
Cyclothone pygmaea
19.91
43.35
0.44
3.91
1155.33
566.60
1.00
58.92
Vinciguerria attenuatta
1.11
3.91
0.08
0.22
Stomias boa
0.79
2.72
0.08
0.15
0.54
2.15
0.06
0.03
50.32
56.65
0.80
9.88
281.32
124.66
1.00
14.35
Ceratoscopelus maderensis Diaphus holti
0.16
1.34
5.96
0.08
0.26
1.35
3.69
0.13
0.07
Hygophum spp.
34.72
56.77
0.56
6.82
213.17
203.82
1.00
10.87
Lampanyctus crocodilus
14.45
31.23
0.32
2.84
11.86
11.87
0.69
0.60
Lampanyctus pusillus
3.39
7.77
0.24
0.67
13.61
19.45
0.50
0.69
Lobianchia dofleiini
7.96
14.58
0.32
1.56
4.07
8.48
0.25
0.21
Myctophum punctatum
0.44
2.20
0.04
0.09
Notoscopelus elongatus
0.36
1.80
0.04
0.07
Symbolophorus veranyi
1.79
5.21
0.16
0.35
0.71
2.84
0.06
0.04
Lestidiops jayakari
0.01
0.04
0.04
0.00
1.55
4.46
0.13
0.08
Paralepididae
0.20
0.99
0.04
0.04
2.10
3.77
0.25
0.11
Anacanthini
0.01
0.04
0.04
0.00 0.52
2.06
0.06
0.03
0.80
2.80
0.08
0.16
6.16
9.77
0.38
0.31
0.52
2.06
0.06
0.03
Apogon imberbis Anthias anthias Epinephelus sp. Serranus cabrilla
9.62
16.44
0.44
1.89
1.87
4.06
0.19
0.10
Serranus hepatus
15.04
23.55
0.52
2.95
2.89
5.39
0.25
0.15
Cepola rubescens
6.77
11.45
0.36
1.33
Trachurus mediterraneus
87.18
119.08
0.68
17.12
2.82
6.48
0.19
0.14
Trachurus trachurus
0.33
1.59
0.08
0.07
0.54
2.15
0.06
0.03
Coryphaena hippurus
0.98
3.55
0.08
0.19
Mullus barbatus
9.54
22.18
0.32
1.87
2.22
3.99
0.25
0.11
Mullus surmuletus
0.18
0.92
0.04
0.04 7.42
11.24
0.38
0.38
Diplodus sp.
0.93
4.56
0.08
0.18
Oblata melanura
0.79
3.97
0.04
0.16
Pagrus pagrus
5.97
16.39
0.24
1.17
Sparidae
5.90
19.16
0.16
1.16
11.82
26.16
0.40
2.32
4.54
11.50
0.28
0.89
Brama raji
Spicara smaris Coris julis Xyrichthis novacula
19.26
28.64
0.63
0.98
8.90
12.69
0.50
0.45
Symphodus sp.
0.66
2.39
0.08
0.13
Chromis chromis
3.21
7.21
0.20
0.63
3.07
5.75
0.25
0.16
Trachinus draco
5.75
9.56
0.44
1.13
1.78
3.88
0.19
0.09
29.50
44.76
0.72
5.79
55.04
64.67
0.81
2.81
Euthynnus alleteratus
0.48
2.38
0.04
0.09
Thunnus alalunga
3.20
7.54
0.20
0.63
51.54
48.45
0.69
2.63
Thunnus thynnus
1.81
7.31
0.12
0.36
12.50
26.80
0.84
2.46
5.24
14.01
0.38
0.27
Auxis rochei
Gobiidae
(continued)
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Table I: Continued June Species
Mean
August SD
F
%
Lebetus guilletti
1.14
3.24
0.12
0.22
Callionymiidae
1.07
2.83
0.16
0.21
Lipophris pholis
0.37
1.86
0.04
0.07
Parablennius tentacularis
0.37
1.86
0.04
0.07
Lepidotrigla cavillone
0.20
0.99
0.04
0.04
Blennidae
Mean
SD
F
%
0.56
2.24
0.06
0.03
2.13
8.52
0.06
0.11
0.69
2.78
0.06
0.04
Helicolenus dactilopterus
0.69
2.78
0.06
0.04
Scorpaena sp.
0.54
2.15
0.06
0.03
Dactylopterus volitans
0.52
2.06
0.06
0.03
0.23
Arnoglossus ghromanni
0.74
2.56
0.08
0.15
Arnoglossus laterna
2.18
6.12
0.16
0.43
Arnoglossus spp.
3.54
9.61
0.24
0.69
Arnoglossus thori
1.13
4.15
0.08
0.22
4.58
7.68
0.31
Bothus podas
0.02
0.08
0.04
0.00
18.90
30.79
0.69
0.96
0.56
2.25
0.06
0.03
Lepidorhombus sp. Xiphias gladius
1.95
5.85
0.13
0.10
2.23
6.43
0.12
0.44
0.69
2.78
0.06
0.04
Unidentified yolk-sac larvae
40.92
54.45
0.64
8.03
3.24
9.10
0.13
0.17
Broken larvae
25.96
38.87
0.64
5.10
8.61
25.48
0.25
0.44
Unidentified larvae
%, relative abundance of fish larvae; F, frequency of occurrence; SD, standard deviation.
from three other mesopelagic species (Hygophum sp., Cyclothone pygmaea and Lampanyctus crocodilus) ranged in abundance from 15 to 34 larvae/1000 m3. Among the neritic species, the most abundant were Spicara smaris, Serranus cabrilla, Serranus hepatus and Mullus barbatus, with density values between 9 and 15 larvae/1000 m3. Adults of these species are commonly found in the Balearic shelf areas (Alemany, 1997). The most abundant large pelagic species was Auxis rochei (29 larvae/1000 m3), a middle-sized scombroid. All these species appeared in >32% of the samples. The horizontal distribution of larvae of neritic, mesopelagic and large pelagic species is shown in Fig. 3A–C. Shannon–Weaver diversity index ranged between 1.04 and 2.84, with a mean value of 1.93, whereas Pielou’s evenness index showed values between 0.52 and 0.97, with a mean of 0.81 (Table II). During the August survey, a total of 3059 larvae were captured and 39 taxa identified, 31 to species level, 4 to genera level, 3 to family level, and 1 remained unidentified. Mean abundance was 1960 larvae/1000 m3 (±627). The most abundant species was C. pygmaea, with a mean value of 1155 larvae/1000 m3, followed by other mesopelagic species like C. maderensis, Hygophum sp. and C. braueri (281, 213 and 62 larvae/1000 m3, respectively). Two scombroid larvae from A. rochei and T. alalunga were also
relatively abundant showing mean densities of 55 and 52 larvae/1000 m3. Among the neritic species, the most frequent were Coris julis, Bothus podas, Brama raji and Anthias anthias, with a mean abundance ranging from 20 to 6 larvae/1000 m3 (Table I). The horizontal distribution of the larvae of neritic, mesopelagic and large pelagic species is shown in Fig. 4A–C. Shannon’s diversity index presented lower values than in the previous survey, between 0.79 and 2.22, with a mean of 1.35. Pielou’s evenness index also showed lower values between 0.38 and 0.73 with a mean of 0.55 (Table II).
Similarity between stations In the June survey, cluster analysis separated stations in two main groups (Fig. 5A). The first group included only the shelf and upper slope stations (Fig. 2A), and the larval assemblage was characterized by higher abundances of T. mediterraneus (9–344 larvae/1000 m3) and the relative scarcity of mesopelagic species, such as C. braueri (0–34 larvae/1000 m3) or C. maderensis (0–67 larvae/1000 m3) (Fig. 6A). The second group included deeper stations (Fig. 2A). This group contained high values of C. braueri (18–327 larvae/1000 m3) and C. maderensis (12–241 larvae/1000 m3) but low densities of T. mediterraneus (0–9 larvae/1000 m3) (Fig. 6B).
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Table II: Total larval density (individuals 1000 m3) at the June and August surveys, Shannon–Weaver’s diversity index (H0 ) and Pielou’s evenness index (J0 ) June Sampling station
August Depth
Individuals
1000 m
3
H0
J0
Sampling station
Depth
Individuals
1000 m3
H0
J0
1
74
354
2.14
0.93
1
73
1595
1.12
0.42
2
90
774
2.11
0.8
2
95
2417
1.92
0.64
3
110
752
1.25
0.52
3
252
2344
2.22
0.73
4
180
328
2.03
0.72
4
483
1653
1.77
0.62
5
400
601
2.84
0.91
5
1170
710
1.58
0.72
6
75
398
2.01
0.78
6
73
1118
1.71
0.71
7
110
252
2.04
0.82
7
79
3227
1.04
0.43
8
190
401
2.36
0.77
8
250
2506
0.86
0.44
9
600
888
2.21
0.76
9
332
1420
0.86
0.41
10
1100
1020
2.65
0.81
10
726
1966
1.29
0.47
11
70
11
1190
2305
1.5
0.54
12
150
409
2.25
0.88
12
71
1372
1.74
0.63
13
220
945
2.03
0.75
13
75
1847
1.2
0.48
14
850
1033
1.76
0.74
14
224
2173
1.21
0.49
15
1000
626
1.85
0.75
15
718
2311
0.88
0.38
16
70
771
1.62
0.74
16
1012
2410
0.79
0.41
17
150
488
1.3
0.67
18
370
489
1.7
0.82
19
880
548
2.08
0.84
20
1100
460
1.86
0.85
21
70
191
1.74
0.97
22
200
791
2.11
0.8
23
750
256
1.78
0.91
24
1050
167
1.47
0.91
25
1200
87
1.04
0.95
Total mean
543
1.93
0.81
1961
1.35
0.53
Neritic sampling station
465
1.89
0.78
1930
1.45
0.55
Oceanic sampling station
609
1.95
0.83
1980
1.3
0.52
Depth, total depth.
In the August survey, as in the previous one, two main groups of stations were identified by cluster analysis, although in this survey, the station depth apparently was not the main factor affecting the larval fish assemblages distribution (Fig. 5B). The first larval assemblage, including stations 2–6 and 12 (Fig. 2B), was characterized by relatively low densities of C. pygmaea (290–668 larvae/1000 m3) (Fig. 7A). The second assemblage (Fig. 2B) showed an opposite trend, with high abundance values for C. pygmaea (Fig. 7B). These abundances ranged from 1151 to 2125 larvae/1000 m3. In spite of some sampling stations being located close to each other similarity in species, composition between sites was not significantly correlated with geographic distance showing no effects of spatial autocorrelation.
The Mantel test results for the June survey presented a value of r = 0.10, with one-tailed probability value of 0.14, and for the August survey, they were r = 0.11, with one-tailed probability value of 0.21.
Spatial patterns The main patterns of species composition were defined, for each survey, by the site scores on the first CA axis. In the June survey, the first axis explained 27.3% of the variability of the composition of the fish larval assemblage. This spatial pattern (Fig. 8A) was mainly related to the depth gradient but also to the relative distribution of the water masses SMW and MAW. These results on species spatial distribution confirm those obtained from
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Linkag e distance
50 40 30 20
21-70 6-75 1 6-70 2 2 -2 0 0 1 2 -1 5 0 4-180 1 8 -3 7 0 2-90 5-400 1-74
3 -1 1 0
2 5 -1 2 0 0 8 -1 9 0 7 -1 1 0 17-150
2 4 -1 0 5 0 9 -6 0 0
19-880 13-220 2 0 -1 1 0 0 1 5 -1 0 0 0 23-750 14-850 1 0 -1 1 0 0
10
40
Linkage distance
35
B
30 25 20 15 10
1-73
15-718
7-79
16-1012
10-726
14-224
8-250
9-332
11-1190
2-95
13-75
3-252
6-73
4-483
12-71
0
5-1170
5
Fig. 5. (A and B) Similarity among stations in June 1996 and August 1996 surveys, based on relative abundances of species which represented >1% of total. Numbers on horizontal axis represent station number and depth, respectively.
the cluster analysis. The isocline of site score 0.15 separated a group of shallower stations characterized by high density of neritic species, such as T. mediterraneus, and a lower density of mesopelagic species from another group of deeper stations in which mesopelagic species dominated the larval assemblage. Unexplained variability was mainly attributable to stations 1 and 21, both being nearer the coastline (Fig. 8B). These stations were characterized by a high density of Gobiidae and by the total absence of mesopelagic species. In the August survey, the first axis explained 40.2% of the variability of the composition of the larval fish assemblage. The resulting pattern (Fig. 9A), as already suggested by cluster analysis, seems to be depth independent but related to surface salinity (Fig. 2B). Stations with lower surface salinities, such as stations 2–4, 6 and 12, were already clumped together in the cluster analysis. This group was characterized by the relatively low density of C. pygmaea, which was the most abundant species in the rest of stations with higher salinity values. Unexplained variability was related mainly to stations 2, 6, 12, 13, 8 and 9 (Fig. 9B).
Correlation patterns between larval and environmental variables Stepwise forward selection was performed looking for the subset of environmental variables that explain the largest
Fig. 6. (A and B) Relative abundances of species in the larval assemblages observed in June survey.
percentage of variability in the larval composition (Table III). Larval composition of samples corresponding to June is significantly correlated with lnDepth and Front. In August, only salinity (surface) shows some significant correlation with faunistic composition. Note that there are no clearly defined different water masses and that faunistic composition is less variable than in the case of June (Table III). The general patterns of the relationships between larval composition of the larval fish assemblage and these two environmental variables are displayed in Fig. 10. In the June survey, both depth (Ln-transformed depth) and water characteristics (the categorical variable Front) have a combined effect on the composition of the larval fish assemblage. The species closely associated to SMW were predominantly mesopelagic, including all the identified species of the families Mycthopidae and Sternoptichidae. Within these group, the mesopelagic species were associated to deeper stations, whereas some neritic species, such as Sparids and A. anthias, were located in the shallower ones. Considering the species associated to MAW, most of them were shelf species. However, within this
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Fig. 7. (A and B) Relative abundances of species in the larval assemblages observed in August survey.
group of larvae, some neritic species, like C. julis or Pagrus pagrus, were relatively more abundant in the deeper stations. Larvae of large pelagic species did not show any special trend, appearing indistinctly in MAW (T. mediterraneus) and SMW (T. alalunga), both in shallow (C. hippurus) and in deeper stations (T. thynnus). In the August survey, the depth gradient did not show any significant relation with larval distribution, and only salinity was significantly correlated with larval composition (Fig. 11). Most of the species were more abundant in stations with low salinity, which indicates recent inflows of MAW. However, neither mesopelagic nor neritic larval species showed any clear correlation with the salinity gradient. Only the larvae of large pelagic species were predominantly located in less salty stations.
DISCUSSION Hydrography The location of the June survey was in the Mallorca channel, at the border between the Algerian basin and
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the Balearic sub-basin. Consequently, the June survey was under the influence of water masses from both basins. Hydrographic data from the hydrographic survey, a week before the June ichthyoplankton survey in the same area (Pinot et al., 2002), indicated that the southward flow of SMW through the Ibiza channel was blocked, because of the presence of an anticyclonic gyre (Pinot et al., 2002). For this reason, the Northern Current, which carries SMW southward along NW Mediterranean Iberian Peninsula coast, was deflected towards the Mallorca channel strengthening the Balearic Current which flows northwards along NW Mallorca coast. On the other hand, northward inflows of MAW reached the Mallorca Channel from the Algerian Basin, reinforcing also the Balearic Current, modifying its path and generating meanders (Pinot et al., 2002). Data from permanent current meters indicated a northward flow of surface water masses during the June survey. Surface layer flow at point A3 (Fig. 1A) during the whole May–June period was NW (10 cm/s on average), which implies that Balearic Current was driven by the MAW inflows entering through the Mallorca channel. At A4 (Fig. 1A), the data set showed weak currents (3–4 cm/s) flowing in a northerly direction during May but turning westward during June. This could be the result of a weak cyclonic circulation developing between the inflow and outflow of the channel. In situ data recorded during the June cruise confirmed this situation, since both SMW and MAW flowing northwards were detected in the study area. During the August survey, all the study area was influenced by the waters of Algerian basin, where the MAW prevailed. Data from the hydrographic survey carried out in the study area a week before the August ichthyoplankton survey and from moorings A3 and A4 indicated inflows of MAW (Fig. 1). These were observed in both channels, being stronger than in the previous survey. At A4, the surface flow continued westward until the middle of August, similar to the June situation. Afterwards, the flow rotated to the North and remained in this direction until late October, with a mean speed of 5 cm/s, indicating a general strengthening of the northward spreading of southern waters in August (Pinot et al., 2002). This situation possibly affected the whole southern coast of Mallorca Island. The data recorded in situ during the August survey confirmed that north-flowing surface waters of recent Atlantic occupied all the study area. Spatial distribution of fluorescence values at 80-m depth seemed to be, in both surveys, influenced by the hydrodynamic situation, since in both cases the higher fluorescence values were detected where MAW inflows
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Bongo Andraitx CA axis 1 (23.5%)
Bongo Andraitx CCA axis 3
25
25
4420
A
24
4410
4420
4410
23
20 19
19
17
4390
14
12
4390
14
1
4370
12
7
8
9
1
4370
2
3
4360
3
4360
4
4
5
410
11 6
10
2
21 16
13
4380
6 7
8
9
17 15
11
4380 10
18
21 16
13
22
4400
18
15
23
20
22
4400
B
24
5
420
430
440
450
410
420
430
440
450
Fig. 8. Maps of larval composition of the June 1996 survey. Geographical units are Universal Transverse Mercator (UTM) coordinates (km). Isoclines are estimated by simple linear interpolation. Main observed patterns of larval composition [site scores on the first axis of a correspondence analysis (CA)] are shown in panel A. Main patterns of unexplained (by environmental variables) variation of larval composition [site scores on the first non-canonical axis from canonical CA (CCA)] are shown in panel B. Isolines are at the same scale in both panels.
Bongo Porto Colom CA axis 1 (40.2%)
Bongo Porto Colom CCA axis 2
13
12
13
12
6
6 14
4360 1
15
A
7
14
4360 1
16
4358
16 8
2
4356
9 3
4354
11
4
4350 532
534
536
9 3 10
4350 538
540
542
544
546
528
11
4
4352
5
530
2
4354
10
4352
528
B
4358 8
4356
15
7
5
530
532
534
536
538
540
542
544
546
Fig. 9. Maps of larval composition of August 1996 survey. Geographical units are Universal Transverse Mercator (UTM) coordinates (km). Isoclines are estimated by simple linear interpolation. Main observed patterns of larval composition [site scores on the first axis of a correspondence analysis (CA)] are shown in panel A. Main patterns of unexplained (by environmental variables) variation of larval composition [site scores on the first non-canonical axes of a canonical CA (CCA)] are shown in panel B. Isolines are at the same scale in both panels.
reach the continental slope, producing dynamic instabilities which would favour vertical mixing and hence enrichment processes.
Species composition Most of the identified species have previously been reported in the Balearic Islands (Alemany, 1997) except
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Table III: Variance decomposition corresponding to a canonical correspondence analysis (CCA) on the larval abundances Trace June 0.274
11.2
