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Keywords:

  • Community structure;
  • fish larvae;
  • hydrology;
  • spatial distribution;
  • Western Mediterranean Sea

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The purpose of this paper was to study the community structure, in terms of species composition, abundance and spatial distribution, of fish larvae in a wide coastal area of Sicily facing the Southeastern Tyrrhenian Sea, extending for 2300 km2 from Cape Cefalù to the west, to Cape Rasocolmo in the east. This study analyses how species are assembled in relation to an inshore–offshore gradient and also how environmental conditions, determined by surface circulation patterns occurring in the Central Mediterranean at the local scale, determine the distribution patterns. Samples from 39 stations were collected using a 60-cm Bongo net during an ichthyoplanktonic survey carried out in June 2006. In all, 62 taxa, representing 32 families, were identified. Cyclothone braueri (59.6%), Engraulis encrasicolus (9.2%) Lampanyctus crocodilus (4.3%) and Lampanyctus pusillus (4.1%) were the most abundant species. The results showed that the highest abundance value (14830.6 fish larvae per 10 m2 sea surface) was observed in the western part of the study area. MDS, SIMPER and CCA analyses revealed well defined groups of stations and assemblages of larvae in accordance with an inshore–offshore gradient. The results of this study could have implications for the management of marine resources because the investigated area has already been identified as a nursery area for many pelagic and coastal fishes and a natural habitat for many species of high commercial interest.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Spatiotemporal patterns of ichthyoplankton communities, with respect to abundance, distribution and species composition, have traditionally been a basic topic of scientific research in fisheries oceanography (Govoni 2005), and more recently of studies on aquaculture production and global climate change. Most demersal marine fishes, inhabiting shallow waters, produce pelagic eggs and larvae, which then recruit to the benthic environment. The advantage of planktonic larvae may be to increase accessibility to food or reduce predation from demersal predators (Swearer et al. 2002).

The structure of the continental shelf is known to play a key role in determining the spatial distribution of fish larvae (Leis & Miller 1976; Young et al. 1986). Horizontal distribution studies have shown that inshore larval assemblages have a different composition from offshore ones (Marliave 1986; Sabatés 1990; Gray 1993). Multispecies larval associations may be adaptive and result in similar responses among species to the pelagic environment (Somarakis et al. 2000).

Marine fish larvae vary in length at each stage of development and in the duration of growth to the juvenile stage (Matarese et al. 1989). The larvae play an important role in studies of the ecology and evolution of fish populations (Moser & Smith 1993).

Larval fish assemblages are temporary components of the zooplankton community, whose structure is dependent upon reproductive cycles within populations of adult fishes. Patterns of larval fish distribution originate from complex spatial and temporal interrelationships that are strongly affected by the seasonality and duration of their meroplanktonic existence (Olivar et al. 1998; Koutrakis et al. 2004; Palomera et al. 2005).

It has been shown that coastal environments often constitute favourable habitats for the early life stages of fishes living in different marine ecosystems. Such environments represent nursery areas for species exhibiting distinct spawning habits, demersal, pelagic or beach spawning (McGowen 1993). Both abiotic and biotic factors influence larval fish settlement, distribution and abundance within an environment (Kingsford et al. 2002). Larval fish assemblages are also affected by temperature and salinity (Ramos et al. 2006), which influence water density. Ichthyoplankton surveys provide useful data for the assessment of important parameters of commercially important fish populations (spawning stock biomass, recruitment) and can also improve our knowledge on the agents structuring larval assemblages (Isari et al. 2008).

This work investigates species the composition and spatial distribution of fish larvae in the Southern Tyrrhenian Sea, a typically oligotrophic area (Carrada et al. 1992; Povero et al. 1998), and their potential relation with oceanographic features.

Hydrobiological characteristics of the Aeolian Archipelago and surrounding basins were studied in the 0–200 m layer during late spring by Azzaro et al. (2003). Previous studies (Astraldi et al. 1996, 2002; Sparnocchia et al. 1999; Gasparini et al. 2005) showed that, due to topographic constraints and the Coriolis effect, the outflow in the Strait of Sicily turns right and enters the Tyrrhenian basin, flowing along the Sicilian continental slope, inducing changes in the Tyrrhenian water column. The thermohaline vertical structure shows the presence of three different water masses: Tyrrhenian Surface Water (TSW), Atlantic Water (AW) and Tyrrhenian Intermediate Water (TIW).

This area has high relevance as a probable nursery area for many commercially important pelagic and coastal fishes such as Seriola dumerili, Xiphias gladius and Thunnus thynnus (Cefali et al. 1997; Bruno et al. 2001). The area is outside the influence of upwelling phenomena, with a deeper thermocline and lower primary production and mesozooplanktonic biomass compared with the adjacent Ionian Sea. A previous study has shown this area is a habitat for mesopelagic fishes and squids and plays a key role in deeper ecosystem energy flux (Granata et al. 2011).

Material and Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Sampling

Samples were collected during an ichthyoplanktonic survey carried out in June 2006 in the Southern Tyrrhenian sea (between Cape Cefalù and Cape Rasocolmo) along the Sicilian coast (Fig. 1), in the framework of ‘MARE’ Project POR SICILY 2000–2006 ‘Model for an integrated coastal management plan aimed at the sustainable use of fishing resources' funded by the Sicily Region. A total of 39 stations, located in 13 inshore–offshore transects, were sampled. Oblique tows, onboard the scientific research vessel Luigi Sanzo, were carried out using a Bongo net with 60-cm diameter towed at a constant speed of two knots, equipped with a 500-μm mesh size for both sides of the frame and one flowmeter (HydroBios, Kiel, Germany) to measure the filtered volume. The physical and chemical features were registered by a multiparametric probe Sea Bird 911 Plus and a Seapoint fluorometer to record temperature, salinity, oxygen and fluorescence continuously. Fluorescence was measured and calculated as equivalent μg chlorophyll a (Chl-a) l−1. The conventional unit (F) for in vivo fluorescence in the range 0–5 volts corresponds to 0–50 mg m−3 for Chl-a with a resolution of 0.1 mg m−3 and an accuracy variability of <10%. Rough data of water depth (m), temperature (°C), salinity and fluorescence were processed with the Ocean Data View (ODV; Alfred Weneger Institute, http://www.awi.de) software to obtain vertical profiles in real time. Sampling details are shown in Table 1. Samples were fixed in 4% borax-buffered formalin immediately after capture and then analysed in the laboratory.

Table 1. Sampling data from the ichthyoplanktonic survey carried out during June 2006 in the Southern Tyrrhenian Sea, and total fish larvae density (number of larvae per 10 m2)
StationDateBottom depth (m)Max sampled depth (m)No. larvae per 10 m2
1D28 June 064701167652.86
2G28 June 068401058149.88
6B28 June 067644282.66
7F28 June 06680983571.50
8G28 June 06910838844.43
12B28 June 066540681.07
13C28 June 06128722356.39
14E28 June 063827614830.63
15G28 June 061000905789.92
20B28 June 068751556.16
21F27 June 06700901335.04
22G27 June 0610001442990.68
26B27 June 0682445881.88
27E27 June 064491551305.69
28G27 June 0610001625239.15
29G27 June 0610001342202.29
30D27 June 063621661596.47
31B27 June 0683474976.84
35G27 June 069241372579.63
36D27 June 064001443158.51
41G25 June 068001372987.97
42G25 June 068231361720.80
43G25 June 068001112417.36
44D25 June 06225946710.46
50G29 June 06879624179.00
51G29 June 068711162029.41
52D29 June 06400803053.35
53D29 June 06359834811.79
54E29 June 06436874037.76
55F29 June 06651621491.47
58G29 June 06882652018.86
59E29 June 06489721362.13
60D29 June 06267693079.62
67E26 June 064311841874.10
68F26 June 066101343475.22
69G26 June 0683072737.31
73E26 June 06500872528.60
74G26 June 06950152191.35
75G26 June 06986871459.40
image

Figure 1. Study area and stations sampled with the Bongo 60 net during ‘MARE’ Project, June 2006.

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Laboratory procedures

Fish larvae were sorted from the plankton, identified to the lowest possible taxonomic level, staged into preflexion, flexion and postflexion larvae, and enumerated. Taxonomic identification was based mainly on an ichthyoplankton database created at the Institute of Coastal Marine Environment (IAMC) of Messina, including existing literature (Sanzo 1913a,b, 1917, 1918, 1928, 1932; D'ancona 1931; D'Ancona et al. 1931–1956; Bertolini et al. 1956) and more than 100 drawings of more than 50 fish species in different developmental stages inhabiting the Straits of Messina and the Southern Tyrrhenian Sea (Spartà 1924–1938, 1939–1963; Cavaliere 1955–1990). When the diagnosis was not assured, biomolecular analyses (DNA extraction, PCR amplification, sequencing, sequences adding into BLAST program) were used to eliminate any doubt (Giordano et al. 2008). The nomenclature and systematic order of the species were updated to the website www.fishbase.org. The number of larvae collected for each station was standardized per 10 m2 sea surface, using the following equation by Smith & Richardson (1977):

  • display math

where C is the number of larvae beneath a unit sea surface area (10 m2 in this case), a is the area of the mouth of the bongo net in square meters, b is the length of the tow path in meters, c is the number of larvae in the sample, and d is the maximum depth of tow in meters.

The data on abundance were averaged, normalized and then mapped using the deterministic interpolation method inverse distance weighting (IDW; Isaaks & Srivastava 1989).

The species frequency of occurrence (in percentage) was estimated as the ratio between number of samples in which the species was found and the total number of samples collected. The use of a combination of a frequency index and abundance is important because certain species may be caught several times but with few specimens, or they may be collected a few times but with a large number of specimens (Granata et al. 2011).

All specimens were measured as total length (TL) and notochord length (NL) for larvae in preflexion stage, and standard length (SL) for flexion and postflexion larvae (Giordano et al. 2004).

Data analysis

Several statistical analyses were performed to determine fish larval composition and abundance, their spatial distribution, potential relation with hydrographical features of the study area and species contribution to stations ordination. PRIMER 5 (Clarke & Gorley 2001) was used to study the spatial distribution pattern. To examine the larval fish assemblage distribution from inshore to offshore waters, data were log-transformed log(x + 1) into a lower triangular resemblance matrix using the multi-dimensional scaling (MDS) ordination method. Analysis of similarity (ANOSIM) was used to test inshore and offshore effects. Analysis of similarities provides a way to test statistically whether there is a significant difference between two or more groups of sampling units. Subsequently, the similarity percentage analysis (SIMPER) was utilized to identify the species contributing the most to the similarity within group factors. A canonical correspondence analysis (CCA; Ter Braak 1986) was performed to analyse species contribution to station ordination and the influence of environmental variables such as temperature, salinity and fluorescence (Lyons 1996). Data were log-transformed prior to CCA calculation. The vegan library (Oksanen et al. 2013) of the R statistical language (R Development Core Team, Vienna, Austria, http://www.R-project.org/), version 2.15, was used to perform CCA.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Environmental conditions

Figure 2 shows the T-S diagram of the vertical profiles collected during the campaign. The structure of the water column was relatively homogeneous at the different stations, resembling typical early summer conditions with a strong vertical stratification, well described by the vertical distribution of the hydrographical and physico-chemical parameters along a transect parallel to the coast (Fig. 3), representative of the overall situation.

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Figure 2. T-S diagram of the vertical profiles collected during the campaign. TSW, Tyrrhenian Surface Water; TIW, Tyrrhenian Intermediate Water.

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Figure 3. Vertical distribution of the hydrographical and physico-chemical parameters along a transect parallel to the coast.

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A clear thermocline was recorded in the upper (0–50 m) layer where the temperature decreased from 27.5 °C at the surface to near 15 °C at about 50 m depth. This layer was characterized by a density lower than 28.4 σt (Fig. 3) and with the bulk of salinity values in the range of 37.8–38.1

Tyrrhenian Intermediate Waters (TIW), with temperatures of 14.2 °C and a salinity of 38.15 (σt > 28.4), were observed from 50 to 150 m (Figs 2 and 3). The T-S vertical profiles indicated a particular situation for St. 60D that exhibited warmer and saltier surface waters (Fig. 2), probably due to a less efficient mixing between coastal and pelagic waters depending on a confinement effect in the Gulf of Patti.

Chlorophyll a concentrations ranged between 0.064 and 0.478 μg Chl-a l−1, with an overall mean of 0.123 ± 0.051 μg Chl-a l−1 in the upper 200 m of the water column. The pattern of vertical distribution of Chl-a was almost uniform throughout the basin, with a prominent deep chlorophyll maximum (DCM) of c. 0.25 μg Chl-a l−1 at 80–90 m. The depth-integrated chlorophyll content of the upper 150 m ranged between 4.96 and 21.54 mg m2, with higher values in the central part of the area between Cape d'Orlando and Cape Milazzo.

Larval fish composition and abundance

The larval fish community consisted of 2184 individuals mainly represented by preflexion stage larvae, with few metamorphosing stages and juvenile fishes. Fish larvae belonging to 33 families were identified (Table 2).

Table 2. Species composition of ichthyoplankton community. Number of collected specimens, relative abundance on total catches, frequency index of positive hauls on the total samples (fi = ni/NT), minimum–maximum range SL and its mean (x) ± SD
FamilySpeciesnRelative abundance (%)Frequency index (%)range SL (mm)x (mm)+SD
ApogonidaeApogon imberbis10.052.66666
BlenniidaeBlennius ocellaris30.142.627–3229.672.52
BothidaeBothus podas110.5020.520–7550.3618.58
Arnoglossus kessleri10.052.67070
Arnoglossus laterna110.5015.440–7457.5512.93
Arnoglossus rueppelii20.092.630–504014.14
Arnoglossus thori190.8712.820–10064.9522.98
CallyonimidaeCallionymus maculatus20.092.622–26242.83
CaproidaeCapros aper50.2312.821–7447.823.94
CarangidaeTrachurus mediterraneus30.147.740–715715.72
Trachurus trachurus60.2712.825–5039.179.41
CentracanthidaeSpicara maena40.185.130–6046.2513.77
Spicara smaris30.147.737–7355.3318.01
CentriscidaeMacrorhamphosus scolopax30.142.629–5041.6711.15
CepolidaeCepola macrophtalma10.052.67373
CitharidaeCitharus linguatula20.095.141–5849.512.02
CynoglossidaeSymphurus ligulatus10.052.63232
Symphurus nigrescens10.052.69090
EngraulidaeEngraulis encrasicolus1958.9320.520–10050.1115.85
GadidaeMicromesistius poutassou20.095.150–6758.5012.02
GobiidaeGobius niger612.795.122–7533.039.06
GonostomatidaeCyclothone braueri125757.5584.620–19063.0023.02
Cyclothone pygmaea462.1112.834–11061.3919.24
LabridaeCoris julis130.6012.820–9070.7520.71
MerlucciidaeMerluccius merluccius20.095.145–6052.5010.61
MullidaeMullus surmuletus10.052.66262
MyctophidaeBenthosema glaciale20.095.165–6866.502.12
Ceratoscopelus maderensis512.3441.020–8048.2413.65
Diaphus holti80.3710.330–5040.516.24
Diaphus rafinesquii10.052.65252
Electrona risso20.095.138–504725.22
Hygophum benoiti90.4117.920–10063.0930.06
Hygophum hygomii90.4112.835–9057.5521.80
Lampanyctus crocodilus904.1256.420–7034.398.26
Lampanyctus pusillus863.9456.410–6032.788.61
Lobianchia dofleini40.1810.340–10365.530.22
Myctophum punctatum612.7959.020–12565.926.6
Notoscopelus bolini10.052.65353
Notoscopelus elongatus30.147.733–5546.6710.41
NemichthyidaeNemichthys scolopaceus10.052.6160160
OphidiidaeOphidion barbatum10.052.6115115
ParalepididaeArctozenus risso10.052.62222
Lestidiops jayakari jayakari100.4612.830–11061.523.53
Paralepis speciosa30.147.769–7069.670.58
PhosichthyidaeVinciguerria attenuata90.4117.921–9764.8822.85
PomacentridaeChromis chromis10.052.66565
ScorpaenidaeScorpaena porcus30.145.132–8552.3328.57
Scorpaena scrofa20.092.65050.000.00
SebastidaeHelicolenus dactylopterus100.4615.437–10059.6718.50
SerranidaeAnthias anthias30.147.765–7563.3312.58
Serranus cabrilla140.6415.425–7145.2914.35
Serranus hepatus30.145.126–45379.85
SparidaeBoops boops30.145.132–1006934.39
Diplodus annularis120.557.715–6041.511.16
Oblada melanura10.052.62525
SternoptychidaeArgyropelecus hemigymnus170.7817.920–6041.888.86
Maurolicus muelleri50.2312.815–8656.226.59
StomiidaeStomias boa boa251.1425.640–25099.2855.80
TrachinidaeTrachinus draco20.095.147–49481.41
TriglidaeLepidotrigla cavillone20.095.137–6048.516.26
UranoscopidaeUranoscopus scaber20.095.145–6052.510.61
Unidentified specimens71      

Considering all the sampled stations, Cyclothone braueri was the most abundant species (57.5% of the total fish larvae abundance). As concerns the Myctophidae family, a large number of species (13) were identified. Among these species, the most abundant were Lampanyctus crocodilus (4.1% of the total), Lampanyctus pusillus (3.9% of the total), Myctophum punctatum (2.8% of the total) and Ceratoscopelus maderensis (2.3% of the total).

Among the pelagic and more coastal species, Engraulis encrasicolus (8.9% of the total) was the second most abundant species throughout the study area. A great number of species, belonging to another 23 families, was found, such as Paralepididae, Sternoptychidae and Stomiatidae, each in low numbers.

Length–frequency distributions of the two most abundant species, C. braueri and E. encrasicolus, are reported in Fig. 4. The total length of C. braueri ranged from 2 to 19 mm. The majority (91%) of the specimens were in preflexion larval phase (size <9 mm) and 6% were in flexion larval phase (10–11 mm SL). Only 3% were in post-larval phase (>12 mm SL). The modal class was at TL 6 mm (Fig. 4). The total length of E. encrasicolus ranged from 2 to 16 mm. Most of the collected specimens (94%) were in preflexion larval phase (size < 7 mm); 5% were in flexion larval phase (8–9 mm SL) and only 1% were in post-larval phase (>10 mm SL). The modal class was TL 5 mm (Fig. 4).

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Figure 4. Length–frequency distribution of Cyclothone braueri and Engraulis encrasicolus larvae.

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Larval fish spatial distribution

The spatial distribution of larval fish abundance in the study area is shown in Fig. 5. Distribution maps for the two most abundant species are shown in Figs 6 and 7, which show the number of each species collected inshore and offshore standardized per 10 m2 sea surface.

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Figure 5. Horizontal distribution of total larval fish abundance (number per 10 m2 of sea surface) in the study area.

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Figure 6. Distribution map for Cyclothone braueri as number of specimens collected inshore and offshore (number per 10 m2 of sea surface).

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Figure 7. Distribution map for Engraulis encrasicolus presented as number of specimens inshore and offshore (number per 10 m2 of sea surface).

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Cyclothone braueri was more abundant around the Gulf of Milazzo both inshore and offshore, and also near Cefalù offshore. Engraulis encrasicolus was more abundant inshore in the Gulf of Patti and near Sant'Agata di Militello, whereas high values were recorded offshore only in front of the Gulf of Patti.

The results showed that the highest abundance value (14830.63 fish larvae per 10 m2 sea surface; Table 1) was observed in the western part of the study area (between Cefalù and S. Agata of Militello).

MDS analysis revealed well defined groups of stations and larval assemblages, primarily related to the distance from the coast, showing a clear segregatihon between inshore and offshore stations (Fig. 8). The stress value of the MDS plot was 0.16, which is lower than the limit (0.2) considered adequately to represent similarity or dissimilarity between samples in MDS plots.

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Figure 8. Multi-dimensional scaling (MDS) ordination method representing inshore and offshore fish larvae assemblages.

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ANOSIM analysis revealed significant differences between inshore and offshore assemblages with a high global R value (global R = 0.904, P < 0.001).

The larval species contributing most to the similarity values of the inshore group, estimated by the SIMPER routine (Table 3), were E. encrasicolus, Arnoglossus thori and Serranus cabrilla. The species contributing most to the similarity within the offshore group factor were C. braueri, Myctophum punctatum and Lampanyctus crocodilus.

Table 3. SIMPER analysis showing the species that contributing mainly to similarity within inshore and offshore groups
SpeciesAv.AbundAv.SimSim/SDContrib%Cum.%
Inshore
Engraulis encrasicolus6.4815.203.0750.9850.98
Arnoglossus thori3.244.130.7513.8364.81
Serranus cabrilla2.052.290.467.6972.50
Arnoglossus laterna2.321.730.475.8178.31
Bothus podas1.941.510.485.0583.36
Anthias anthias1.281.070.263.5886.94
Gobius niger jozo2.410.090.263.0389.97
Trachinus draco1.460.710.262.3992.36
Average similarity: 29.82
Offshore
Cyclothone braueri7.5419.853.5846.2646.26
Myctophum punctatum3.415.710.9013.3159.57
Lampanyctus crocodilus3.545.290.8212.3271.89
Lampanyctus pusillus3.465.150.8312.0083.90
Ceratoscopelus maderensis2.312.430.475.6789.57
Stomias boa1.520.970.292.2691.83
Average similarity: 42.92

Multivariate CCA analysis showed that the CCA1 axis accounted for the greatest percentage (79.8%) of total variance (Fig. 9). Of the different environmental parameters considered, temperature showed the highest negative score with the CCA1 axis (−0.97), followed by Salinity (0.82). In all, 26 larval fish species and 25 stations showed a negative or positive high score with the CCA1 axis. Regarding the species contribution to station ordination, the aggregation of species was linked to the inshore and offshore stations. The species Trachinus draco, Diplodus annularis, E. encrasicolus, Oblada melanura, Serranus cabrilla and Arnoglossus laterna, were mainly present at inshore stations (Fig. 9) and were influenced by temperature. The species Lampanyctus pusillus, L. crocodilus, Benthosema glaciale, Notoscopelus elongatus, Stomias boa, Cyclothone pygmaea, Myctophum punctatum, Lobianchia dofleini, Paralepis speciosa, Lestidiops jayakari, Trachurus mediterraneus and Nemichthys scolopaceus were mainly present at offshore stations and were influenced by Salinity and Fluorescence (Fig. 9).

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Figure 9. Results of CCA analysis of larval fish species and sampled stations. Two first axes (CCA1 and CCA2) are represented. An_a (Anthias anthias), Ap_i (Apogon imberbis), Ar_k (Arnoglossus kessleri), Ar_h (Argyropelecus hemigymnus), Ar_l (Arnoglossus laterna), Ar_r (Arnoglossus rueppelii), Ar_t (Arnoglossus thori), Be_g (Benthosema glaciale), Bl_o (Blennius ocellaris), Bo_b (Boops boops), Bo_p (Bothus podas), Ca_a (Capros aper), Ca_p (Callyonimus maculatus), Ce_m (Ceratoscopelus maderensis), Ce_m1 (Cepola macrophtalma), Ci_l (Citharus linguatula), Co_j (Coris julis), Co_n (Ophidion barbatum), Cy_b (Cyclothone braueri), Cy_p (Cyclothone pygmaea), En_e (Engraulis encrasicolus), Di_a (Diplodus annularis), Di_h (Diaphus holti), Di_r (Diaphus rafinesquei), El_r (Electrona rissoi), Go_n (Gobius niger), He_d (Helicolenus dactylopterus), Hy_b (Hygophum benoiti), Hy_h (Hygophum hygomii), La_c (Lampanyctus crocodilus), La_p (Lampanyctus pusillus), Le_c (Lepidotrigla cavillone), Le_j (Lestidiops jayakari), Lo_d (Lobianchia dofleini), Ma_m (Maurolicus muelleri), Ma_s (Macrorhamphosus scolopax), Me_m (Merluccius merluccius), Mi_p (Micromesistius poutassou), My_p (Myctophum punctatum), Mu_s (Mullus surmuletus), Ne_s (Nemichthys scolopaceus), No_b (Notoscopelus bolini), No_e (Notoscopelus elongatus), No_r (Arctozenus risso), Ob_m (Oblada melanura), Pa_s (Paralepis speciosa). Sc_p (Scorpaena porcus), Sc_s (Scorpaena scrofa), Se_c (Serranus cabrilla), Se_h (Serranus hepatus), Sp_f (Spicara maena), Sp_s (Spicara smaris), Sy sp. (Symphurus nigrescens), Sy_v (Symphurus ligulatus), St_b (Stomias boa boa), Tr_d (Trachinus draco), Tr_me (Trachurus mediterraneus), Tr_t (Trachurus trachurus), Ur_s (Uranoscopus scaber), Vi_a (Vinciguerria attenuata).

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Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

This study analyses the distribution pattern of larval fish assemblages and how environmental conditions occurring in the Central Mediterranean Sea at the local scale, contribute to determining the community composition in this region.

The mesoscale circulation in the Tyrrhenian Sea is generally cyclonic along the Sicilian and Italian peninsula shelves, with a general W-to-E path interacting with a re-circulation cell located on the northern side of Sicily (centered at 39° N, 14° E; Rio et al. 2007). Our results show that the study area which is located at the southern border of this cell, close to the Sicilian coast, presents a water mass structure typical of the early summer season, confirming the results reported by Azzaro et al. (2003). Temperature and salinity conditions between the surface and the 150 m depth, show typical features of the Southern Tyrrhenian Sea (Sanfilippo et al. 2009; Sitran et al. 2009), where the fresher MAW flowing from W to E along the Sicilian coast mix with TSW. TIW are formed here due to the input of Levantine Intermediate Waters (LIW; T ≈ 13.8 °C, S ≈ 38.6), produced in the Eastern Mediterranean Sea, which flow into the Tyrrhenian Sea offshore of the Egadi Islands, very close to the Sicily slope (Sparnocchia et al. 1999; Brugnano et al. 2010). Temperature and salinity conditions were directly related to the seasonal riverine inputs, which are under the influence of the productive industrial activities operating in the area. In spring, salinity showed strong oscillations at the mouth of the major rivers flowing into the Gulf (Rinelli et al. 2007). Also, in this season, an increase in temperature was observed in coastal and offshore surface waters due to the power plant in Milazzo.

Our results indicate that chlorophyll a concentrations were higher near the coast, throughout the central part of the area, between Cape d'Orlando and Cape Milazzo. These values appeared to be quite independent of the thermohaline properties of the water column and were related to the distribution models of the physical structure of the water masses and of the local nutrient concentrations. The DCM layer recorded between 80 and 90 m was also typical of oligotrophic areas such as the Southern Tyrrhenian basin.

The factors determining final larval survival, and probably recruitment, are multiple and interact in a complex way. Abiotic factors such as temperature, salinity and light can influence individual larval survival by directly affecting development rates (Frank & Leggett 1981, 1982; Miranda et al. 1990; Koumoundouros et al. 2001), development patterns (Johnston et al. 2001), adult fecundity (McGurk 2000) and spawning time (Ré et al. 1988; Shelton & Hutchings 1989; Mihelakakis et al. 2001). Temperature and salinity are conservative in nature and integrate to define the water masses in which larvae are present. It is generally believed that the physical processes that structure the water column at different spatial and temporal scales and lead to water movements, have a bigger influence in final larval survival. For example, in upwelling areas, the associated offshore transport of larvae to a less productive zone is well documented (Parrish et al. 1981; Olivar & Shelton 1993). In this study we found that one of the most abundant species was represented by anchovy, Engraulis encrasicolus. Larvae of this species dominated the plankton during the sampling period, with the highest abundance in the Gulf of Patti and near S. Agata of Militello in waters with low salinity. This is in accordance with several studies that have reported high concentrations of anchovy eggs and larvae associated with low salinity waters (Palomera 1992; Olivar & Sabatés 1997; Palomera et al. 2007).

The larval fish density patterns appeared to be related mainly to an inshore–offshore gradient with respect to the hydrographic features and structures determined by the surface circulation path. The horizontal distribution observed in our samples, confirmed by SIMPER and CCA analyses, was in accordance with other studies (Sabatés 1990; Granata et al. 2011) that found typical inshore/offshore assemblages of fish larvae. Mesopelagic fishes, as Lampanyctus pusillus, Lampanyctus crocodilus, Benthosema glaciale, Notoscopelus elongatus, Stomias boa, Cyclothone pygmaea, Myctophum punctatum, Lobianchia dofleini, Paralepis speciosa, Lestidiops jayakari and Nemichthys scolopaceus, seem to prefer offshore waters, like near the Cefalù basin, in which the TIW is present from 50 to 150 m depth, with lower temperatures and higher salinities. In contrast, an assemblage constituted mainly by Trachinus draco, Diplodus annularis, E. encrasicolus, Oblada melanura, Serranus cabrilla and Arnoglossus laterna was present mainly at inshore stations during our study.

In this region, as in many others in the world, different larval assemblages can be distinguished closely related to adult habitat and behaviour: shore fish larvae, which usually dominate in coastal areas (Labridae, Serranidae); larvae found over the continental shelf (Sparidae, Scorpaenidae, Bothidae, Mullidae, Engraulidae, Gobiidae, Trachinidae, Blennidae); larvae of meso- and bathypelagic species, which mainly over slope and open waters (Gonostomatidae, Myctophydae, Sternoptychidae).

In species whose larval development is dependent on successful transport to defined nursery areas (i.e. defined areas where larval and juvenile survival is enhanced, such as estuaries), favourable advection processes associated with the force of wind or pressure gradient may be crucial (Govoni & Pietrafesa 1994).

The presence of highly impacted bays (Gulf of Palermo and Termini Imerese), polluted by spills and shipbuilding industries, leads to high concentrations of ichthyoplankton due to the currents near Cefalù basin (Rinelli et al. 2007). This phenomenon explains the abundance of mesopelagic fish larvae (Cyclothone braueri, Lampanyctus pusillus and L. crocodilus) in this area. In contrast, E. encrasicolus shows a higher abundance in the Gulf of Patti, in which there is a maximum value of Chl-a.

The existence of marked pynoclines, frontal zones (shelf-slope fronts, plumes), upwelling and the effect of eddies may contribute to the concentration of larvae and their prey in aggregates where their survival is greater. The concentration of planktonic organisms is enhanced in frontal zones, which can be determined as the surface expression of sharp gradients in the physical properties of two water masses (Granata et al. 2001). Whereas there is evidence that maximum concentrations of larvae are associated to shelf-break fronts or river plumes (Sabatés 1990; Sabatés & Masó 1992; Govoni & Pietrafesa 1994), frontal zones appear to be the direct cause of larval retention and enhanced production, therefore contributing to final survival (Kiǿrboe et al. 1988). Wind stress can generate highly dynamic conditions that recruit larvae to invest more energy in feeding or maintaining a position in the water column, producing physiologically unfit larvae that have a higher potential mortality (Gallego et al. 1996; Kloppmann et al. 2002).

Among the biotic factors affecting survival, feeding success and predation are thought to be the most important. Predation on fish eggs and larvae is assumed perhaps to be the final cause of mortality, and can be exerted by numerous organisms (Bailey & Houde 1989). There are numerous factors affecting predation, including larval size, age (Litvak & Leggett 1992), level of starvation (Yin & Blaxter 1987), density of predators (Steele & Forrester 2002), ontogeny (Fuiman 1994), temperature and developmental rate (Houde 1987).

In conclusion, our results indicate that the structure of the summer larval fish assemblages is linked to an inshore–offshore gradient in the study area and is influenced by an interaction of the spawning behaviour of marine fishes that inhabit this zone and local oceanographic features. Temperature and salinity trends are linked directly to the seasonal riverine inputs and also to the local industrial activities. During summer, the salinity remains steady, showing few dilution phenomena near the thermoelectric plant wastes that lead to clear temperature increases. In fact, the highest concentration of larvae is present in the area of Cefalù, due also to the movement of water from the adjacent areas of Termini Imerese and Palermo Gulfs (highly polluted areas). In the Western Mediterranean, several studies have observed high numbers of mesopelagic fish larvae in the open ocean (Sabatés & Masó 1990; García-Lafuente et al. 1998) in accordance with the high abundance of adults in these waters (Goodyear et al. 1972). The diversity of fish species in the Mediterranean is relatively high, as can be expected in an oligotrophic sea, considering that the diversity gradient is the opposite of the productivity gradient (Margalef 1997).

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Material and Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

This study was undertaken in the framework of the project ‘POR SICILY 2000–2006 SFOP Axis IV – Model for a management plan for integrated coastal zone aimed sustainable use of fishing resources – MARE’ initiated by Sicily Region. A special thanks go to Dr Adrianna Ianora, Zoological Station ‘Anton Dohrn’ Naples-Italy, for the English revision of the manuscript. Thanks go also to some persons who have taken part in data elaboration, in particular Mr Alessandro Cosenza (CNR-IAMC Messina, Italy), Dr Marco Pansera and Dr Cinzia Brugnano (DiSBA-UNIME, Messina, Italy).

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  6. Discussion
  7. Acknowledgements
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