Reproductive biology and population structure of the Mediterranean killifish Aphanius fasciatus in a coastal lagoon of the Sea of Marmara

The Mediterranean killifish Aphanius fasciatus is a characteristic species of Mediterranean transitional water ecosystems and an important component of lagoon biodiversity. This study investigated the reproductive biology and population structure of A. fasciatus inhabiting Çardak Lagoon in the Sea of Marmara (Türkiye). Fish were sampled monthly between May 2024 and April 2025 using a beach seine net. A total of 489 individuals were analysed to determine population structure, length–weight relationships, reproductive parameters and condition factor. Total length ranged from 2.3–6.7 cm (mean: 4.3 cm). Length–weight analysis indicated positive allometric growth in females (b = 3.21) and in the combined population (b = 3.20), whereas males showed isometric growth (b = 3.01). The estimated size at first sexual maturity (L₅₀) was 3.1 cm for females and 4.2 cm for males. Gonadosomatic index values revealed a clear seasonal reproductive cycle with peak spawning occurring between April and May. Fecundity ranged from 10 to 326 oocytes (mean: 67 ± 6.3). Seasonal shifts in length–frequency distributions suggested recruitment following the spring–summer spawning period. The observed seasonal reproductive pattern coincided with temporal changes in temperature and salinity. This study provides site-specific baseline information for monitoring the A. fasciatus population and its habitat in Çardak Lagoon.