A High-Resolution Numerical Model of the North Aegean Sea Aimed at Climatological Studies

A new, high-resolution model for the northern part of the Aegean Sea, aimed primarily at climatological research (relaxation and data assimilation-free climate simulations), is hereby presented, along with the results of a 28-year-long simulation covering the period from 1986 to 2013. The model applied is the Regional Ocean Modelling System (ROMS). A significant improvement over previous models of the Aegean introduced in this work is the replacement of parameterizations of the Dardanelles exchange by a fully three-dimensional simulation of the flow in the Strait. The incorporation of part of the Marmara Sea in the model domain enables the interaction with other regional climate simulations, thus allowing climatic variability of the exchange of the Mediterranean and Black Seas. An extensive validation is carried out comparing the model output with all the available observations from several different platforms, i.e., satellite sea surface temperature and height, T/S profiles from R/V ships, and HF radar surface currents velocity. We focus on the model’s ability to reproduce, to some extent, the distinct thermohaline features and circulation patterns that characterize this specific area of the Mediterranean Sea. Our findings, after comparing simulation results with all the available observations, revealed the model’s sufficiency to simulate very adequately the complex hydrology of the North Aegean Sea, and the model’s ability to reproduce incidents of deep-water formation that took place in the region in previous decades during the Eastern Mediterranean Transient (EMT).

Biodiversity of freshwater macroinvertebrates on Gökçeada Island (North Aegean Sea, Turkey)

Island ecosystems have attracted the attention of scientists since the early days of science because of their formation and biodiversity. Despite their abundant freshwater resources, data on freshwater diversity of Gökçeada are incomplete, especially for macroinvertebrates. This study was conducted in a wide range of freshwater ecosystems on Gökçeada between May 2016 and September 2018. Sampling of macroinvertebrates was performed using a hand net from 34 pre-selected sampling sites on the island. A total of 78 aquatic macroinvertebrate taxa were found, 46 of which are new records for the island. The most common species during the study were Gammarus komareki and Bythinella gokceadaensis with a frequency of 25.93%, followed by Tubifex tubifex, Gammarus sp. and Caenis sp. with a frequency of 22.22%. Endemic and invasive species on the island were assessed according to their presence and frequency values. Based on the results of the current study and previous data, it can be concluded that Gökçeada is characterized by a remarkable species diversity compared to aquatic macroinvertebrates on other Aegean islands.

A three dimensional, full life cycle, anchovy and sardine model for the North Aegean Sea (Eastern Mediterranean): Validation, sensitivity and climatic scenario simulations

We present the development of a 3D full-lifecycle, individual-based model (IBM) for anchovy and sardine, online coupled to an existing hydrodynamic/biogeochemical low-trophic level (LTL) model for the North Aegean Sea. It was built upon an existing 1D model for the same species and area, with the addition of a horizontal movement scheme. In the model, both species evolve from the embryonic stage (egg+yolk sac larva) to the larval, juvenile, and adult stages. Somatic growth is simulated with the use of a “Wisconsin” type bioenergetics model and fish populations with an adaptation of the ‘super individuals’ (SI) approach. For the reference simulation and model calibration, in terms of fish growth and population biomass, the 2000-2010 period was selected. Interannual biomass variability of anchovy was successfully represented by the model, while the simulated biomass of sardine exhibited low variability and did not satisfactorily reproduce the observed interannual variability from acoustic surveys. The spatial distribution of both species’ biomass was in relatively good agreement with field data. Additional single-species simulations revealed that species compete for food resources. Temperature sensitivity experiments showed that both species reacted negatively to a temperature increase. Anchovy, in particular, was more affected since its spawning and larval growth periods largely overlap with the period of maximum yearly temperature and low prey concentration. Finally, simulation experiments using IPCC climatic scenarios showed that the predicted temperature increase and zooplankton concentration decrease in the future will negatively affect anchovy, resulting in sardine prevalence.

Examining the Vulnerability Procedure: Group-based Determinations at the EU Border

Between 2016 and 2019, almost all asylum seekers who managed to reach the Greek islands in the North Aegean Sea had to undergo an assessment of their vulnerability within the EU hotspot system. Those who were found vulnerable were exempted from return under the EU-Turkey Agreement and were free to leave for the Greek mainland. This article provides a detailed account of the vulnerability procedure, which classifies migrants through pre-established categories on account of externally distinguishable features rather than individual experiences. As is shown, this type of group-based management of refugees preceded the Refugee Convention, but has since the 1960s primarily been applied in the Global South. The use of this procedure in Europe reflects an exception from the European individualist human rights approach. In the context of EU hotspots, the vulnerability procedure provides a pathway to exemption from externalisation, for those who can live up to its requirements of documentable hardship.

Study of the climatic changes in the North Aegean Sea and their effects on the ecosystem dynamics

Η κλιματική αλλαγή έχει γίνει πιο αισθητή από ποτέ και ήδη επηρεάζει το θαλάσσιο οικοσύστημα σε παγκόσμια κλίμακα. Για τη μελέτη των μεταβαλλόμενων κλιματικών συνθηκών, εστιάσαμε στο Βόρειο Αιγαίο και στα πιο διαδεδομένα είδη μικρών πελαγικών ψαριών, τον γαύρο και τη σαρδέλα. Μελετήσαμε τις συνέπειες στα είδη από τις αλλαγές στη θερμοκρασία, την αλατότητα, την εισροή νερών από τη Μαύρη Θάλασσα και μεταβολή της εισροής θρεπτικών από τα γειτονικά ποτάμια. Για τις ανάγκες της μελέτης αναπτύχθηκε ένα πολυειδικό (γαύρος/σαρδέλα), πλήρους κύκλου ζωής μοντέλο για το Βόρειο Αιγαίο, αμφίδρομα συζευγμένο με υδροδυναμικό βιογεωχημικό μοντέλο. Η αρχική μονοδιάστατη εκδοχή του μοντέλου, προσομοίωσε επιτυχώς την ανάπτυξη, τις βιομάζες και τις περιόδους αναπαραγωγής και των δύο ειδών, ενώ επιπρόσθετα διαφάνηκε μία ξεκάθαρη εξάρτηση ανάμεσα στην αλίευση των ειδών και τη συγκέντρωση ζωοπλαγκτού. Το επόμενο βήμα στην ανάπτυξη του μοντέλου, απέδωσε την τρισδιάστατη εκδοχή αυτού. Τα κυριότερα χαρακτηριστικά και η χωρική κατανομή των βιομαζών των ειδών, ήταν για άλλη μια φορά σε συμφωνία με τα δεδομένα. Επιπρόσθετες προσομοιώσεις έδειξαν πως τοπικά τα είδη ανταγωνίζονται για την τροφή. Προσομοιώσεις με χρήση κλιματικών σεναρίων IPCC έδειξαν πως μια μελλοντική αύξηση της θερμοκρασίας θα οδηγήσει σε υποβάθμιση του οικοσυστήματος μέσω της μείωσης των πλαγκτονικών συγκεντρώσεων, επηρεάζοντας αρνητικά τον γαύρο και οδηγώντας σε επικράτηση της σαρδέλας. Τέλος, μεταβολές στα εισερχόμενα από τα ποτάμια θρεπτικά και στην εισροή υδάτων από τη Μαύρη Θάλασσα, επίσης υποβαθμίζουν τους πληθυσμούς των ειδών.

Biology of the thornback ray (Raja clavata Linnaeus, 1758) in the North Aegean Sea

The study deals with aspects of the population dynamics in the thornback ray (Raja clavata L., 1758), one of the most abundant cartilaginous fish caught in the North Aegean Sea. Females accounted for 73.08% and males 26.92% of all individuals. Total length of females and males ranged between 50.2 and 89.9 cm (disc width: 33.4–62.0 cm), and between 43.1 cm and 82.7 cm (disc width: 30.7–64.2 cm), respectively. Relationships between total length (TL) and total weight (TW), and between disc width (DW) and total weight (TW) were described by the equations: TW = 0.0041 TL3.10 and TW = 0.0178 DW3.03, respectively. Age data derived from vertebrae readings were used to estimate growth parameters using the von Bertalanffy function: L∞ = 101.71 cm, K = 0.18 y−1, t0 = −0.07 y for males and L∞ = 106.54 cm, K = 0.16 y−1, t0 = −0.28 y for females. The maximum age was 8 years for males and females. Total length at first maturity of males and females was 70.9 cm and 81.2 cm, respectively. Based on the gonadosomatic index and gonadal macroscopic observations, it was determined that the spawning period lasted throughout the year. Stomach content analysis showed that crustaceans (53.03% IRI) and teleosts (14.70% IRI) were the most preferred prey.

Length-weight relationships of fishes caught by stationary uncovered pound nets in the coastal waters of Saros Bay, North Aegean Sea (Turkey)

Stationary uncovered pound nets are passively fishing gears used for fish catching according to traditional methods in the coastal waters of Saros Bay, North Aegean Sea. The purpose of this study was to determine length-weight relations (LWRs) for a wide range of fish species collected from the traps in the coastal waters of Saros Bay. We are studied in the fish species caught by four traps between April and August (fishing season) in 2010 - 2012. A total of 23 fish species belonging to 12 families were caught individual 2.096. The mean value of the growth pattern (b) was 3.141±0.042, ranging from 2.711 to 3.577. Whereas they were negative allometric only for two species (Pomatomus saltatrix and Mugil cephalus), from other fish species were observed to be 13 positive allometric and 8 isometrics. The LWR parameters for the positive allometric and isometric fish species were significant (P<0.05). This study provides some general information about the status of the LWR parameters of the fish species caught from the traps.