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

Cabbar Koray, Yiğin Cahide Çiğdem

Oceanological and Hydrobiological Studies

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2021

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Vol. 50, No. 2

115--127

EN

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.

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Modeling the buoyancy-driven Black Sea Water outflow into the North Aegean Sea

Kokkos N., Sylaios G.

Oceanologia

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2016

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No. 58 (2)

103--116

EN

A three-dimensional numerical model was applied to simulate the Black Sea Water (BSW) outflux and spreading over the North Aegean Sea, and its impact on circulation and stratification–mixing dynamics. Model results were validated against satellite-derived sea surface temperature and in-situ temperature and salinity profiles. Further, the model results were post-processed in terms of the potential energy anomaly, ϕ, analyzing the factors contributing to its change. It occurs that BSW contributes significantly on the Thracian Sea water column stratification, but its signal reduces in the rest of the North Aegean Sea. The BSW buoyancy flux contributed to the change of ϕ in the Thracian Sea by 1.23 × 10-3 W m-3 in the winter and 7.9 × 10-4 W m-3 in the summer, significantly higher than the corresponding solar heat flux contribution (1.41 × 10-5 W m-3 and 7.4 × 10-5 W m-3, respectively). Quantification of the ϕ-advective term crossing the north-western BSW branch (to the north of Lemnos Island), depicted a strong non-linear relation to the relative vorticity of Samothraki Anticyclone. Similar analysis for the south-western branch illustrated a relationship between the ϕ-advective term sign and the relative vorticity in the Sporades system. The ϕ-mixing term increases its significance under strong winds (> 15 m s-1), tending to destroy surface meso-scale eddies.

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Meteorological influences on the surface hydrographic patterns of the North Aegean Sea

Sylaios G.

Oceanologia

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2011

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No. 53 (1)

57-80

EN

Hydrographic data from the North Aegean Sea were used to examine the summer variability of surface water masses during the period 1998-2001. Attention was placed on the surface hydrographic features of the area, such as the Black Sea Water (BSW) plume expansion, the frontal characteristics of the BSW with the Levantine Intermediate Water (LIW) and the variability of submesoscale hydrographic features (such as the Samothraki Anticyclone). Strong southerly wind stresses were found responsible for relaxing the horizontal density gradients across the BSW-LIW frontal zone and displacing this front to the north of Lemnos Island, thus suppressing the Samothraki Anticyclone towards the Thracian Sea continental shelf. Under northerly winds, the BSW-LIW front returns to its regular position (south of Lemnos Island), thus allowing the horizontal expansion of the Samothraki gyre up to the Athos Peninsula. Present results indicate the importance of medium-term wind stress effects on the generation of Samothraki Anticyclone suppression/expansion events.