| Summary: | 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 south-eastern Tyrrhenian Sea (central Mediterranean). 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 a total of 63 stations were collected using a 60 cm Bongo net, during two ichthyoplanktonic surveys carried out in June 2006 and in April 2007.
The physical and chemical features (temperature, salinity, oxygen and fluorescence) were continuously registered by a multiparametric probe Sea Bird 911 Plus equipped with a Seapoint fluorometer. Rough data of water depth (m), temperature (°C), salinity and fluorescence were processed with the ODV software to obtain vertical profiles in real time. Samples were fixed in 4% borax-buffered formalin immediately after capture and then analyzed in the laboratory.
The number of larvae collected for each station was standardized per 10 m2 sea surface. Data of abundance were averaged and normalized and then mapped using the deterministic interpolation method Inverse Distance Weighting (IDW). The species frequency of occurrence (in percentage) was estimated as the ratio between number of samples in which the species was found by 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. All specimens were measured as Total length (TL), Notochord length (NL), for larvae in preflexion stage and standard length (SL), for flexion and postflexion larvae. A canonical correspondence analysis (CCA) was performed to analyse species contribution to station ordination and the influence of environmental variables (temperature, salinity oxygen and fluorescence). Sixty-five taxa, representing thirty-six families, were identified. <i>Cyclothone braueri</i> (57.5%), and <i>Engraulis encrasicolus</i> (8.9%) were the most abundant species during the survey of June 2006, while <i>Myctophum punctatum</i> (22.4%) and <i>Cyclothone braueri</i> (11.7%) were the most abundant species during the survey of April 2007.
Multivariate CCA analysis during June 2006 sampling showed that the CCA1 axis accounted for the greatest percentage (79.8%) of total variance (Fig. 1). Of the different environmental parameters considered, temperature showed the highest negative score with the CCA1 axis (-0.97), followed by Salinity (0.82). Twenty-six larval fish species showed a negative or positive high score with the CCA1 axis. Twenty-five stations showed a negative or positive high score with the CCA1 axis. Regarding the species contribution to stations ordination, the aggregation of species was linked to the inshore and offshore stations. The species <i>Trachinus draco</i>, <i>Diplodus annularis</i>, <i>Engraulis encrasicolus</i>, <i>Oblada melanura</i>, <i>Serranus cabrilla</i> and <i>Arnoglossus laterna</i>, were mainly present at inshore stations and were influenced by temperature. The species <i>Lampanyctus pusillus</i>, <i>L. crocodilus, Benthosema glaciale, Notoscopelus elongatus, Stomias boa, Cyclothone pygmaea, Myctophum punctatum, Lobianchia dofleini, Paralepis speciosa, Lestidiops jayakari, Trachurus mediterraneus</i> and <i>Nemichthys scolopaceus</i> were mainly present at offshore stations and were influenced by Salinity and Fluorescence (Fig. 1).
Multivariate CCA analysis during April 2007 sampling showed that the CCA1 axis accounted for the greatest percentage (55.4%) of total variance (Fig. 2). Of the different environmental parameters considered, oxygen showed the highest negative score with the CCA1 axis (-0.63), while salinity showed the highest positive score (0.62). Fourteen stations showed a negative or positive high score with the CCA1 axis. Regarding the species contribution to stations ordination, the species <i>Nezumia aequalis, Spicara smaris, Glossanodon leioglossus, Ceratoscopelus maderensis, Paralepis speciosa, Arnoglossus thori, Synchiropus phaeton, Hygophum hygomi</i> were mainly influenced by salinity, while the species <i>Carapus acus, Boops boops, Diaphus holti e Cyclothone braueri, Gobius niger, Trachurus trachurus</i> and <i>Actozenus risso</i> were mainly influenced by temperature and some mesopelagic species as <i>Lampanyctus pusillus, Myctophum punctatum</i> and <i>Notoscopelus</i> were mainly influenced by fluorescence, depth and oxygen (Fig. 2). The results showed that the highest abundance value was observed in the western part of the study area. MDS and CCA analyses revealed well-defined groups of stations and assemblages of larvae in accordance with an inshore-offshore gradient.
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 recirculation cell located on the northern side of Sicily. 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, thereby confirming the results reported by Azzaro <i>et al</i>. (2003). Temperature and salinity conditions, between the surface and the 150 m depth, show the typical features of the Southern Tyrrhenian Sea, where the fresher Modified Atlantic Waters (MAW) flowing from W to E along the Sicilian coast mix with Tyrrhenian Surface Waters (TSW). The Tyrrhenian Intermediate Waters (TIW) are formed here due to the input of Levantine Intermediate Waters, produced in the Eastern Mediterranean Sea, that flow into the Tyrrhenian Sea offshore the Egadi Islands very close to the Sicily slope. 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. Also, in this season, an increase in temperature was observed in coastal and offshore surface waters due to the power plant in Milazzo. The existence of marked pycnoclines, 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 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.
Fig. 1. Results of CCA analysis for larval fish species and sampled stations during June 2006. Two first axes (CCA1 and CCA2) are represented. Species abbreviations in alphabetical order: An_a (<i>Anthias anthias</i>), Ap_i (<i>Apogon imberbis</i>), Ar_k (<i>Arnoglossus kessleri</i>), Ar_h (<i>Argyropelecus hemigymnus</i>), Ar_l (<i>Arnoglossus laterna</i>), Ar_r (<i>Arnoglossus rueppelii</i>), Ar_t (<i>Arnoglossus thori</i>), Be_g (<i>Benthosema glaciale</i>), Bl_o (<i>Blennius ocellaris</i>), Bo_b (<i>Boops boops</i>), Bo_p (<i>Bothus podas</i>), Ca_a (<i>Capros aper</i>), Ca_p (<i>Callyonimus maculatus</i>), Ce_m (<i>Ceratoscopelus maderensis</i>), Ce_m1(<i>Cepola macrophtalma</i>), Ci_l (<i>Citharus linguatula</i>), Co_j (<i>Coris julis</i>), Co_n (<i>Ophidion barbatum</i>), Cy_b (<i>Cyclothone braueri</i>), Cy_p (<i>Cyclothone pygmaea</i>), En_e (<i>Engraulis encrasicolus</i>), Di_a (<i>Diplodus annularis</i>), Di_h (<i>Diaphus holti</i>), Di_r (<i>Diaphus rafinesquei</i>), El_r (<i>Electrona rissoi</i>), Go_n (<i>Gobius niger</i>), He_d (<i>Helicolenus dactylopterus</i>), Hy_b (<i>Hygophum benoiti</i>), Hy_h (<i>Hygophum hygomii</i>), La_c (<i>Lampanyctus crocodilus</i>), La_p (<i>Lampanyctus pusillus</i>), Le_c (<i>Lepidotrigla cavillone</i>), Le_j (<i>Lestidiops jayakari</i>), Lo_d (<i>Lobianchia dofleini</i>), Ma_m (<i>Maurolicus muelleri</i>), Ma_s (<i>Macrorhamphosus scolopax</i>), Me_m (<i>Merluccius merluccius</i>), Mi_p (<i>Micromesistius poutassou</i>), My_p (<i>Myctophum punctatum</i>), Mu_s (<i>Mullus surmuletus</i>), Ne_s (<i>Nemichthys scolopaceus</i>), No_b (<i>Notoscopelus bolini</i>), No_e (<i>Notoscopelus elongatus</i>), No_r (<i>Arctozenus risso</i>), Ob_m (<i>Oblada melanura</i>), Pa_s (<i>Paralepis speciosa</i>). Sc_p (<i>Scorpaena porcus</i>), Sc_s (<i>Scorpaena scrofa</i>), Se_c (<i>Serranus cabrilla</i>), Se_h (<i>Serranushepatus</i>), Sp_f (<i>Spicara maena</i>), Sp_s (<i>Spicara smaris</i>), Sy sp. (<i>Symphurus nigrescens</i>), Sy_v, (<i>Symphurus ligulatus</i>), St_b (<i>Stomias boa boa</i>), Tr_d (<i>Trachinus draco</i>), Tr_me (<i>Trachurus mediterraneus</i>), Tr_t (Trachurus trachurus), Ur_s (Uranoscopus scaber), Vi_a (Vinciguerria attenuata).
Fig. 2. Results of CCA analysis for larval fish species and sampled stations during June 2006. Two first axes (CCA1 and CCA2) are represented . Species abbreviations in alphabetical order: Acto_ris (Arctozenus risso), Arno_tho (Arnoglossus thori), Boop_boo (Boops boops), Auxi_roc Cara_acu Cera_mad (Ceratoscopelus maderensis), Cyclo_bra (Cyclothone braueri), Diap_hol (Diaphus holti), Gobi_nig (Gobius niger), Heli_dac (Helicolenus dactylopterus), Hygo_hyg (Hygophum hygomii), Lamp_cro (Lampanyctus crocodilus), Lamp_pus (Lampanyctus pusillus), Lest_jay (Lestidiops jayakari), Myct_pun (Myctophum punctatum), Nezu_aeq (Nezumia aequalis) Noto_bol (Notoscopelus bolini), Noto_elo (Notoscopelus elongatus), Para_spe (Paralepis speciosa), Sync_pha (Synchiropus phaeton) Spic_sma (Spicara smaris), Trac_tra (Trachurus trachurus).
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