The reproductive biology of Mediterranean horse mackerel Trachurus mediterraneus was investigated in the Sea of Marmara over a one-year period. Duration and timing of the spawning season, gonad histology, oocyte development and size at sexual maturity were determined. The presence of all stages of oocytes without any distinct size hiatus indicated that oocyte development of Mediterranean horse mackerel is asynchronous with indeterminate fecundity. Spawning starts in May, peaks in July–August and ends in September. Sizes at 50% maturity were found to be 12.2 and 12.5 cm (total length) for females and males, respectively. Mean batch fecundity was calculated as 10136 ± 703 eggs. Our results on the size at sexual maturity and batch fecundity for the Sea of Marmara differed significantly from published results from the Mediterranean Sea. We ascribe these differences to different stock structures in these seas. Our results form the basis for future reproductive biology studies on the under-studied Marmara–Black Sea stock.

This paper describes phytoplankton abundance data collected in the Sea of Marmara in 2000 and identifies new species to the region. The quantitative sampling was carried out monthly at 3 stations and seasonally at 6 stations at depths of 0.5, 5, 10 and 20 m. Additionally, vertical samplings by plankton net from 15–0 m were carried out in order to determine the species diversity. Simultaneously measured physical (salinity, temperature and Secchi disc) and chemical parameters (dissolved oxygen, nitrate + nitrite, chlorophyll-a, phosphate and silicate) were evaluated together with phytoplankton data. One hundred and fifteen species were recorded which are members of 55 dinoflagellates, 42 diatoms, 11 silicoflagellates and 7 other groups. Those species include toxic species with high reproduction rates and species that are accepted as carrying risk. Blooms of Anabaena sp., Nitzschia longissima (Brébisson in Kützing) Ralfs in Pritchard and Prorocentrum triestinum Schiller species were observed during the study. Dictyocha antarctica Lohmann 1919, Dictyocha crux Ehrenberg 1840 and Nitzschia rectilonga Takano 1983 species were recorded for the first time from the coastal waters of Turkey. The maximum phytoplankton abundance has been observed during the winter and autumn seasons in the 0.5 and 5 m depths; the concentration of the diatoms was higher during late winter and autumn, however, the concentration of the dinoflagellates was always high during the whole year and they did not show a seasonal variation. The phytoplankton density was lower at the 20 m depths because of the low light penetration property of the Sea of Marmara.