Genetic Identification of Ichthyoplankton in the Black Sea and Their Abundance and Community Assemblages

Accurate identification of fish species during the planktonic stages is vital for detecting spawning, foraging, and growth sites of fish species to provide data for stock assessment, environmental impact assessment, and ecological monitoring. In this study, seasonal ichthyoplankton abundance, community composition, and main possible environmental drivers that influence their community composition were investigated in the Southeastern Black Sea. Ichthyoplankton samples were seasonally collected for a year from four stations, two coastal and two offshore sites. DNA barcoding method was applied to identify ichthyoplankton specimens. Of 33 ichthyoplankton taxa identified using DNA barcoding, 31 taxa were identified at species level and 2 taxa were identified at genus level due to unavailability of reference sequences from the Black Sea in public databases. Engraulis encrasicolus, Sprattus sprattus, Mullus barbatus, and Trachurus mediterraneus were identified as the most abundant species. Engraulis encrasicolus alone accounted for nearly 91.5% of the total catch. Seasonality had a strong influence on the ichthyoplankton assemblages. The highest ichthyoplankton abundance was found in summer, followed by winter, autumn, and spring. While 30 fish taxa were identified in summer, 10 taxa were identified in spring, 3 taxa were identified in both autumn and winter. Ichthyoplankton communities were dominated by E. encrasicolus, M. barbatus, and T. mediterraneus specimens in summer, S. sprattus, and M. merlangus specimens in winter, S. sprattus and G. mediterraneus specimens in autumn, Alosa immaculata and Merlangius merlangus in spring. The ichthyoplankton abundance and richness was generally higher in coastal stations in each season. Engraulis encrasicolus, S. sprattus, M. merlangus, P. incognitus were the only species that detected in more than 4 hauls. On the other hand, M. batrachocephalus, P. flesus, S. abester, T. draco, U. cirrosa, and C. lucerna were only detected in a single haul. Overall, the dominant environmental variable affecting abundance of ichthyoplankton was temperature. The relationship between community composition and environmental variable based on nMDS analysis indicated that ichthyoplankton assemblage structure is also influenced by salinity, dissolved oxygen, chlorophyll-a, NO2, and NO3.