Modeling and predicting potential spawning habitat of anchovy (Engraulis encrasicolus) and round sardinella (Sardinella aurita) based on satellite environmental information

Anchovy and round sardinella are two important small pelagic species in the Mediterranean that spawn during the summer period. This is a first attempt to model and predict the two species’ potential spawning habitats in this area. Generalized additive models (GAMs) were constructed based on satellite environmental variables and presence/absence egg data, available from ichthyoplankton surveys conducted in the North Aegean Sea during early summer (June 2003–2006). These models were subsequently used to predict the probability of anchovy and round sardinella spawning in the Greek Seas as well as the entire Mediterranean and Black Sea during the same month of the year. The interaction of bottom depth and chlorophyll explained most of the deviance in the presence/absence GAMs of both species, indicating spawning over continental shelf areas with increased surface chlorophyll values. Round sardinella spawned closer to coast than anchovy. Predicted potential spawning areas for anchovy and round sardinella in unsampled areas of the Greek Seas and the entire Mediterranean and Black Sea were in good agreement with existing information on the distribution and extent of the spawning grounds, especially for anchovy. Modeling the species’ reproductive activity in relation to easily accessible environmental information and applying the models in a predictive way could be an initial, low-cost step to designate potential spawning fish habitats. Guest editor: V. D. Valavanis Essential Fish Habitat Mapping in the Mediterranean     

Modelling potential habitat of the invasive ctenophore <Emphasis Type="Italic">Mnemiopsis leidyi</Emphasis> in Aegean Sea

The invasive ctenophore Mnemiopsis leidyi was accidentally introduced into the Black Sea in the early 1980s and it was first sighted in the Aegean Sea (Eastern Mediterranean) in the early 1990s. This article presents a first attempt to develop a predictive spatial model based on M. leidyi presence data and satellite environmental data from the Aegean Sea during early summer, in order to identify those areas in the Greek Seas and the entire Mediterranean basin that could serve as potential habitat for the species. Generalized additive models (GAM) were applied. The final GAM model indicated higher probability of finding M. leidyi present in depths of 65–135 m and sea surface temperature values of 21–25°C. Furthermore, the significant interaction between photosynthetically active radiation (PAR) and sea level anomaly (SLA) indicated a higher probability of M. leidyi presence in low values of PAR and SLA. In the next step, the final GAM was applied in a prediction grid of mean monthly satellite values for June 2004–2006 in order to estimate probability of M. leidyi presence in the Hellenic Seas and the whole Mediterranean basin at a GIS resolution of 4 km. In the Aegean Sea, species potential habitat included areas influenced by the Black Sea Water (e.g. Thracian Sea, Limnos-Imvros plateau), gulfs that are affected by river runoffs, such as the Thermaikos, Strymonikos and Patraikos gulfs, or areas with strong anthropogenic influence such as the Saronikos gulf. Areas with the same environmental conditions as those in Aegean Sea have been indicated in certain spots of the Levantine Sea as well as in coastal waters of Egypt and Libya, although their spatial extent varied largely among years examined. However, the occurrence of conditions that are linked to high probability of M. leidyi presence does not necessarily mean that these areas can support successful reproduction, high population or bloom levels, since these depend on a combination of temperature, salinity, food availability and the abundance of predators. Guest editor: V. D. Valavanis Essential Fish Habitat Mapping in the Mediterranean     

Habitat discrimination of juvenile sardines in the Aegean Sea using remotely sensed environmental data

Despite the importance of the recruitment process for small pelagic fish and the high economic importance of European sardine (Sardina pilchardus, Walbaum 1792) in the Mediterranean Sea, knowledge on the distribution and environmental characteristics of its nursery grounds is very limited. In the present study, we used pelagic trawl data collected during 1995–2006 to explore the spatial distribution of sardine juveniles in the Aegean Sea in early summer. Based on sardine abundance per length class, a cluster analysis was initially used to define hauls dominated by juveniles. In a subsequent step, Discriminant Function Analysis (DFA) was applied to discriminate stations with high relative abundance of juveniles using satellite environmental data and bottom depth. The parameters contributing mostly to the discrimination of juvenile grounds were sea level anomaly, photosynthetically active radiation, sea surface temperature, chlorophyll-α and bottom depth. The classification functions of DFA were finally used to post classify unsampled areas in the Greek Seas and the Mediterranean Sea in order to map grounds that meet characteristic environmental conditions for young sardine. Such areas were mostly located inshore, in semi-closed productive areas and often in proximity to river mouths, a pattern that is generally supported by existing information. Guest editor: V. D. Valavanis Essential Fish Habitat Mapping in the Mediterranean     

Genetic structuring of European anchovy (Engraulis encrasicolus) populations through mitochondrial DNA sequences

Mitochondrial DNA sequence variation in 655?bpfragments of the cytochrome oxidase c subunit I gene, known as the DNA barcode, of European anchovy (Engraulis encrasicolus) was evaluated by analyzing 1529 individuals representing 16 populations from the Black Sea, through the Marmara Sea and the Aegean Sea to the Mediterranean Sea. A total of 19 (2.9%) variable sites were found among individuals, and these defined 10 genetically diverged populations with an overall mean distance of 1.2%. The highest nucleotide divergence was found between samples of eastern Mediterranean and northern Aegean (2.2%). Evolutionary history analysis among 16 populations clustered the Mediterranean Sea clades in one main branch and the other clades in another branch. Diverging pattern of the European anchovy populations correlated with geographic dispersion supports the genetic structuring through the Black Sea-Marmara Sea-Aegean Sea-Mediterranean Sea quad.     

Allozyme differentiation of bonito in the Mediterranean Sea

Bonito Sarda sarda samples collected in three areas from the Mediterranean Sea exhibited considerable genetic heterogeneity over all locations. This provided preliminary evidence for two different groups of bonito, one for the Ligurian and the Ionian Seas and the other for the Aegean Sea.     

Demosponge distribution in the eastern Mediterranean: a NW–SE gradient

The purpose of this paper was to investigate patterns of demosponge distribution along gradients of environmental conditions in the biogeographical subzones of the eastern Mediterranean (Aegean and Levantine Sea). The Aegean Sea was divided into six major areas on the basis of its geomorphology and bathymetry. Two areas of the Levantine Sea were additionally considered. All available data on demosponge species numbers and abundance in each area, as well as their vertical and general geographical distribution were taken from the literature. Multivariate analysis revealed a NW–SE faunal gradient, showing an apparent dissimilarity among the North Aegean, the South Aegean and the Levantine Sea, which agrees with the differences in the geographical, physicochemical and biological characteristics of the three areas. The majority of demosponge species has been recorded in the North Aegean, while the South Aegean is closer, in terms of demosponge diversity, to the oligotrophic Levantine Sea. The number of studies in the Aegean and Levantine subareas was positively correlated with the number of species recorded within each Aegean subarea. Demosponge species with an Altanto-Mediterranean distribution prevailed in the Aegean and the Levantine. The reduced contribution of the endemic component, as compared to the western Mediterranean, is consistent with the general NW–SE decrease in the number of endemic species in the Mediterranean. Demosponge distribution at the order level showed also a NW–SE gradient, similar to that observed in the entire Atlantic–Mediterranean region, suggesting a warm water affinity. Sublittoral, circalittoral and bathyal zones were clearly distinguishable in the Aegean Sea on the basis of their sponge fauna. The total number of species was an exponential negative function of depth.     

Mitochondrial DNA reveals a mosaic pattern of phylogeographical structure in Atlantic and Mediterranean populations of anchovy (Engraulis encrasicolus)

This study extends the geographic coverage of a previous study of mitochondrial DNA restriction fragment length polymorphism in European anchovy. Both studies together include 24 samples representing 17 localities extending from the Black Sea, through the Mediterranean Sea to the eastern Atlantic as far south as Dakar, Senegal. Eighty-eight haplotypes define two clades (A and B) separated by 3.2% sequence divergence. Clade A has a star-like genealogy indicative of a recent population expansion. Clade B has a more complex genealogy, consisting of several haplotypes at intermediate frequencies. The distributions of these clades consist of a mosaic with abrupt changes between some areas and gradients between other areas. Clade A predominates the Black and Aegean seas, but is present throughout the Mediterranean. Unexpectedly, new data show that clade A is also at a high frequency in the Atlantic, from Portugal to at least Senegal. Overall, the level of genetic differentiation among populations is high (F(ST)=0.148, p<0.0001), with the greatest differences between basins. AMOVA reveals four main geographical groups: Atlantic, central Mediterranean, Aegean Sea, and Black Sea. Mismatch distribution clearly indicates historical bottleneck and population expansion for clade A, while for clade B such evidence is equivocal. This difference may reflect a range expansion for both clades, but with higher gene flow (Nm values) between demes for clade A. Both contemporary and historical processes are important in shaping the complex genetic population structure of European anchovy.     

Mitochondrial DNA phylogeny and the reconstruction of the population history of a species: the case of the European anchovy (Engraulis encrasicolus)

Analysis of mitochondrial DNA restriction fragment length polymorphism in European anchovy (Engraulis encrasicolus) revealed a large number of mitotypes that form two distinct clusters (phylads). Phylad A consists of one common mitotype and many rare secondary mitotypes that are one mutational step removed from the main type. Nucleotide diversity and number of homoplasious changes are low. Phylad B has a complex pattern of mitotype connectedness, high nucleotide diversity, and a large number of homoplasious changes. It is suggested that the two phylads evolved in isolation from each other and that present coexistence is the result of a secondary contact. Moreover, phylad A has a "star" phylogeny, which suggests that it has evolved in a population that experienced a drastic bottleneck followed by an explosion of size. Phylad A is practically the only phylad present in the Black Sea, with its frequency dropping to 85% in the northern Aegean, and to 40% in the rest of Mediterranean and the Bay of Biscay. The Black Sea is, therefore, the most likely place of origin of phylad A. Molecular data are consistent with a population bottleneck in the Black Sea during the last glaciation event and a subsequent exit of phylad A with the outflow into the Aegean following the ice melting. Phylogenetic analysis of anchovy mtDNA provides a reconstruction of population history in the Mediterranean, which is consistent with the geological information.      

Application of a bioenergetics growth model for European anchovy (<Emphasis Type="Italic">Engraulis encrasicolus</Emphasis>) linked with a lower trophic level ecosystem model

A bioenergetics model is implemented for European anchovy (Engraulis encrasicolus) and applied to the north-eastern Aegean Sea (eastern Mediterranean Sea). The model reproduces the growth of anchovy in a one-way linked configuration with a lower trophic level (LTL) ecosystem model. The LTL model provides densities for three zooplankton functional groups (heterotrophic flagellates, microzooplankton and mesozooplankton) which serve as available energy via consumption for the anchovy model. Our model follows the basic structure of NEMURO.FISH type models (North Pacific Ecosystem Model for Understanding Regional Oceanography for Including Saury and Herring). Several model parameters were specific for the Mediterranean or the Black Sea anchovy and some others were adopted from related species and NEMURO.FISH due to lack of biological information on E. encrasicolus. Simulation results showed that the fastest growth rate occurs during spring and the slowest growth rate from August to December. Zooplankton abundance during autumn was low implying that decreased prey density lead to a reduction in anchovy weight, especially for the age-3 class. Feeding parameters were adjusted to adequately fit the model growth estimates to available weight-at-age data. A detailed sensitivity analyses is conducted to evaluate the importance of the biological processes (consumption, respiration, egestion, specific dynamic action, excretion and egg production) and their parameters to fish growth. The most sensitive parameters were the intercept and exponent slope of the weight-dependent consumption and respiration process equations. Fish weight was fairly sensitive to temperature-dependent parameters.     

Ecoregion-Based Conservation Planning in the Mediterranean: Dealing with Large-Scale Heterogeneity

Spatial priorities for the conservation of three key Mediterranean habitats, i.e. seagrass Posidonia oceanica meadows, coralligenous formations, and marine caves, were determined through a systematic planning approach. Available information on the distribution of these habitats across the entire Mediterranean Sea was compiled to produce basin-scale distribution maps. Conservation targets for each habitat type were set according to European Union guidelines. Surrogates were used to estimate the spatial variation of opportunity cost for commercial, non-commercial fishing, and aquaculture. Marxan conservation planning software was used to evaluate the comparative utility of two planning scenarios: (a) a whole-basin scenario, referring to selection of priority areas across the whole Mediterranean Sea, and (b) an ecoregional scenario, in which priority areas were selected within eight predefined ecoregions. Although both scenarios required approximately the same total area to be protected in order to achieve conservation targets, the opportunity cost differed between them. The whole-basin scenario yielded a lower opportunity cost, but the Alboran Sea ecoregion was not represented and priority areas were predominantly located in the Ionian, Aegean, and Adriatic Seas. In comparison, the ecoregional scenario resulted in a higher representation of ecoregions and a more even distribution of priority areas, albeit with a higher opportunity cost. We suggest that planning at the ecoregional level ensures better representativeness of the selected conservation features and adequate protection of species, functional, and genetic diversity across the basin. While there are several initiatives that identify priority areas in the Mediterranean Sea, our approach is novel as it combines three issues: (a) it is based on the distribution of habitats and not species, which was rarely the case in previous efforts, (b) it considers spatial variability of cost throughout this socioeconomically heterogeneous basin, and (c) it adopts ecoregions as the most appropriate level for large-scale planning.     

Habitat Selection Response of Small Pelagic Fish in Different Environments. Two Examples from the Oligotrophic Mediterranean Sea

A number of scientific papers in the last few years singled out the influence of environmental conditions on the spatial distribution of fish species, highlighting the need for the fisheries scientific community to investigate, besides biomass estimates, also the habitat selection of commercially important fish species. The Mediterranean Sea, although generally oligotrophic, is characterized by high habitat variability and represents an ideal study area to investigate the adaptive behavior of small pelagics under different environmental conditions. In this study the habitat selection of European anchovy Engraulis encrasicolus and European sardine Sardina pilchardus is analyzed in two areas of the Mediterranean Sea that largely differentiate in terms of environmental regimes: the Strait of Sicily and the North Aegean Sea. A number of environmental parameters were used to investigate factors influencing anchovy and sardine habitat selection. Acoustic surveys data, collected during the summer period 2002–2010, were used for this purpose. The quotient analysis was used to identify the association between high density values and environmental variables; it was applied to the entire dataset in each area in order to identify similarities or differences in the “mean” spatial behavioral pattern for each species. Principal component analysis was applied to selected environmental variables in order to identify those environmental regimes which drive each of the two ecosystems. The analysis revealed the effect of food availability along with bottom depth selection on the spatial distribution of both species. Furthermore PCA results highlighted that observed selectivity for shallower waters is mainly associated to specific environmental processes that locally increase productivity. The common trends in habitat selection of the two species, as observed in the two regions although they present marked differences in hydrodynamics, seem to be driven by the oligotrophic character of the study areas, highlighting the role of areas where the local environmental regimes meet ‘the ocean triad hypothesis’.     

The use of otolith shape and chemistry to determine stock structure of Mediterranean horse mackerel Trachurus mediterraneus (Steindachner)

Otolith shape and chemistry of Mediterranean horse mackerel Trachurus mediterraneus were simultaneously used to assess the feasibility of using these natural tags to discriminate populations throughout the Black, Marmara, Aegean and eastern Mediterranean Seas. Otolith shape and chemistry analyses showed a similar pattern of differentiation between T. mediterraneus stocks, revealing a clear discreteness of the middle Black Sea (Sinop) and Aegean Sea (Izmir) samples. Otolith upper side length and width, and Na, K, Mg and Ba, were the morphological traits and trace elements, respectively, differing most among groups. Overall assignment of individuals into their original sample was high for both otolith shape and chemistry. Highest reclassification rate was observed for the south-middle Black Sea and Aegean Sea samples for both analyses. Hierarchical cluster analyses also supported high differentiation of the south-middle Black Sea and Aegean Sea samples for both analyses. Mantel's test revealed that the Euclidean distance both for otolith shape ( r =−0·0917, P > 0·05) and chemistry ( r =−0·1248, P > 0·05) between these populations were not significantly associated with their geographical distances.     

Identification of deep-water pink shrimp abundance distribution patterns and nursery grounds in the eastern Mediterranean by means of generalized additive modelling

Generalized Additive Modelling (GAM) techniques were used to model the time series of abundance data of deep-water pink shrimp, collected during the MEDITS bottom trawl surveys carried out in the Greek seas from 1996 to 2006, as functions of the sampling location (longitude–latitude), depth and year. The life stages of the species were taken into account. All variables were highly significant, although latitude and depth had always the highest explanatory power. The total abundance was higher between 100 and 400 m of depth, with juveniles and adults being more abundant in the 100–200 m and 200–400 m depth strata, respectively. GAM prediction maps showed high abundance concentration areas for all life stages mainly in the Aegean Sea. The most important nursery ground identified was located in the Saronikos Gulf and a secondary in the Thracian Sea. Concerning the concentration areas of the adult specimens, they are located mainly in the Saronikos Gulf, the Thracian Sea, the Thermaikos Gulf, the Cretan Sea and the eastern part of the Aegean. Guest Editor: V. D. Valavanis Essential Fish Habitat Mapping in the Mediterranean     

Biogeographic zonation of the Black Sea and Caspian Sea basin based on Mysid fauna (Crustacea: Mysidacea)

A biogeographic analysis of the Black Sea and Caspian Sea basin and adjacent Aegean Sea and the Sea of Marmara was conducted based on the distribution of 55 mysid species. The Black and Caspian Sea basin proper is inhabited by 35 mysid species belonging to 12 genera, among them 26 species and 3 genera are endemic to the region. Ponto-Caspian and Caspian species are predominant. The explored areas were included into two provinces of the Mediterranean subrealm of the East-Atlantic Subtropical realm and three provinces of the Ponto-Caspian realm.     

Endemic and Indo-Pacific plankton in the Mediterranean Sea: a study based on dinoflagellate records

Aim To investigate biogeographical patterns based on published dinoflagellate records from the Mediterranean and Black Seas, and to provide a tentative list of endemic and Indo‐Pacific dinoflagellates in the Mediterranean Sea. Location Mediterranean Sea, Black Sea. Methods Checklists of dinoflagellates of the Mediterranean and Black Seas were compared with worldwide literature records. Only species reported in the Indo‐Pacific Ocean or exclusively known in the Mediterranean Sea were selected for biogeographical analysis. Results Dinoflagellates in the Mediterranean Sea comprised c. 43% of the world marine species and c. 88% of the dinoflagellate genera. Species richness among the Mediterranean sub‐basins showed marked differences due to the less reliable records of unarmoured (athecate) and rare dinoflagellates. These differences disappeared when only the more easily identifiable taxa were considered. Of the 673 dinoflagellates cited in the Mediterranean, 87% were also reported in the Atlantic Ocean. Only 40 taxa (6% of the total) were considered to be potential Indo‐Pacific species. Most were reported from the Ligurian Sea (21), and only two species from the Levantine basin. The other 48 taxa (7% of total) were known exclusively from the Mediterranean Sea, mainly from the Ligurian Sea. Half of these taxa were reported by a single author. Main conclusions Substantial dinoflagellates species richness can be attributed, in part, to the historical tradition of taxonomic studies in the Mediterranean Sea. The list of species of both Indo‐Pacific and exclusively Mediterranean species included taxa of dubious taxonomic validity or that were insufficiently known. The exclusion of these questionable taxa revealed the near absence of endemic dinoflagellates in the Mediterranean Sea compared with macroscopic organisms. This could be related to: (1) continuous replenishment of the plankton populations by the inflow of Atlantic water through the Strait of Gibraltar, (2) the possibility that species introduced during the Pliocenic flooding after the Messinian salinity crisis have not had enough time to diverge from their Atlantic ancestors, and/or (3) the reliance on traditional taxonomy based on morphological characters, which precludes the detection of cryptic speciation.     

Fistularia commersonii in the Mediterranean Sea: invasion history and distribution modeling based on presence-only records

The bluespotted cornetfish (Fistularia commersonii) (Osteichtyes, Fistulariidae) is considered to be one of the most invasive species of the Mediterranean Sea and Europe but only scattered information exists on its distribution and abundance. Here we collated the available species records, following its first detection in the Mediterranean Sea, in January 2000, until October 2011. A total of 191 observations were used to reconstruct the invasion sequence, to provide estimates of the rate of spread and to construct an environmental suitability model based on six biophysical variables and the maximum entropy approach. The results showed that colonization of the Mediterranean Sea proceeded in parallel along the southern and northern rim of the Basin at speeds that reached 1,000–1,500 km year^?1 with a clear decrease in the rate of spread at the Sicily Strait. The most important explanatory variables for describing the distribution of F. commersonii were mean depth (explaining 32.4 % of the data variance), chlorophyll-a (29.3 %), and salinity (18.4 %). Coastal areas with relatively low chlorophyll-a concentrations and high salinity were the preferred habitat of the bluespotted cornetfish in its invaded range. Conversely, extreme productivity (highly eutrophic or highly oligotrophic), low salinity and cold temperatures provided abiotic resistance to this invasion. Areas of high environmental suitability were identified along the northern coasts of the Levantine Sea, Dodecanese, Sicily Strait and Tyrrhenian Sea. In contrast, the north Aegean Sea, the Adriatic and the Alboran Sea, the Nile Delta, the western coasts of Egypt and Cyrenaica were unfavourable for the invasion. Despite some limits due to the model’s resolution scale, these general predictions provide new insights into the F. commersonii invasion, indicating abiotic factors of primary importance in shaping the distribution of this species in its invaded range.     

Westernmost record of the diamondback puffer,Lagocephalus guentheri (Tetraodontiformes: Tetraodontidae) in the Mediterranean Sea: First record from Greek waters

On 5th November 2017, a puffer fish specimen was caught in the Aegean Sea. Morphological and molecular analysis identified it as Lagocephalus guentheri. This first record from Greek waters represents the westernmost observation of the species in the Mediterranean Sea. The expansion and establishment of puffer fish species in the basin are discussed, with particular emphasis on the importance of the additional use of molecular analysis for increasing the efficiency in understanding species spread and dynamics.     

Fluctuating asymmetry and fitness correlations in two Engraulis encrasicolus populations

Correlations among several measures of fluctuating asymmetry (FA) and fitness‐related variables were assessed in two populations of the European anchovy Engraulis encrasicolus with fast growth (Aegean Sea) and slow growth (Ionian Sea), respectively. FA levels were borderline significantly higher in the Ionian than in the Aegean for some variables. Variation in otolith shape (deviation from population norm) was lower in the Ionian than the Aegean, contrary to expectation. Within the Aegean, there was no relation between any of the FA indexes and fitness estimators, while in the Ionian a composite otolith FA index was significantly negatively correlated to standard length at age only in 2 year‐old individuals. This difference between the Aegean and Ionian may have been related to the lower growth rate in the Ionian, as FA–fitness relations may be more apparent in less‐beneficial environments. The absence of significant correlations in the Aegean and the low correlation in one age group in the Ionian suggests that FA is not a sensitive indicator of individual fitness in adult E. encrasicolus.     

Elemental and biochemical composition of<Emphasis Type="Italic"> Nephrops norvegicus</Emphasis> (Linnaeus 1758) larvae from the Mediterranean and Irish Seas

The Norway lobster, Nephrops norvegicus, is a commercially exploited decapod which is widely distributed throughout the north-eastern Atlantic and the Mediterranean Sea. Ovigerous females originating from the Mediterranean and the Irish Seas were held in the laboratory until larvae hatched. Biomass and biochemical composition, as well as digestive gland structure, were examined in newly hatched larvae from these two regions. In addition, previously published data from a North Sea population were included in our comparison. Elemental analyses showed that the absolute quantities of dry mass (DM), carbon (C), nitrogen (N) and hydrogen (H) (collectively referred to as CHN) per individual, and the C:N mass ratios, were significantly lower, while the relative CHN, protein and lipid values (in % of DM) were higher in samples from the Irish Sea compared to larvae originating from either the Mediterranean or the North Sea. As in CHN, the absolute level of protein per individual was higher in larvae from the Mediterranean, while no significant differences were observed in the individual lipid contents. Likewise, the digestive gland structure at hatching did not show any differences between study areas. Intraspecific variability in biomass and chemical composition of newly hatched larvae from different regions may be related to differential patterns of reproduction in regions with different climatic conditions. Lobster larvae hatch in the Mediterranean Sea predominantly in winter when both water temperature and planktonic food availability are at a minimum, while hatching in the Irish Sea occurs under more favourable conditions in spring. Hence, significantly higher wet mass, dry mass and protein values in Mediterranean larvae may represent adaptive traits allowing for early posthatching survival and development under food-limited conditions in an oligotrophic environment.Communicated by H.-D. Franke