Current population structure of European anchovy Engraulis encrasicolus L. (Engraulidae: Pisces) in the Sea of Azov – Black Sea basin and history of its formation

European anchovy Engraulis encrasicolus L. is one of the most abundant fish species in the Sea of Azov – Black Sea basin. Historically it is the main commercial fish in all Black Sea countries. In modern conditions, the share of anchovy catches totals 80–85 %. In recent years (since 2001) the average annual catch was 250 tones. E. encrasicolus is a polytypic species. In the Black Sea and in the Sea of Azov there are two intraspecific forms – Black Sea anchovy (E. encrasicolus ponticus Alex.) and Sea of Azov anchovy (E. encrasicolus maeoticus Pusanov), and their taxonomic status is still unclear. An actual task is the assessment of the current population structure of E. encrasicolus and its possible changes under the influence of climatic and anthropogenic factors in order to develop the necessary measures to prevent negative impacts and to preserve the resource potential of the species. This work focuses on the study of European anchovy modern population structure in the Black Sea and the Sea of Azov, the geological and paleoclimatic conditions of its formation and assessment of possible changes taking into account real natural and anthropogenic risk factors. The work is based on the research results of intraspecific morphobiological, biochemical, genetic and ecological variability of E. encrasicolus, as well as spatial and reproductive relationships between Sea of Azov anchovy and Black Sea anchovy. Investigations concerned with the study of intraspecific heterogeneity problem of E. encrasicolus in the Black Sea and in the Sea of Azov were reviewed. Data obtained by domestic and foreign authors for a 100-year period (1913–2014) were analyzed; the list of publications includes more than 40 titles. Our own research results were also used in the work. The information about reproductive area boundaries of Sea of Azov anchovy and Black Sea anchovy was systematized and summarized. Their reproductive areas were found to be distributed over the entire water area of the Black Sea and the Sea of Azov and overlapped over the whole space. The absence of spatially separate reproductive areas and the presence of simultaneous spawning exclude belonging of Black Sea anchovy and Sea of Azov anchovy to different subspecies (geographical races). Population level of the differences between Black Sea anchovy and Sea of Azov anchovy was confirmed by the results of genetic and biochemical studies: the coefficients of genetic similarity and genetic distance between them were 0.9983–0.9985 and 0.0015–0.0017, respectively. Geological history of population structure formation of Engraulis encrasicolus was presented. Modern Sea of Azov anchovy and Black Sea anchovy in the past were spatially separated and came into contact again only after appearance of a number of differences between them in the course of evolution. Their isolation occurred in the newly formed Sarmatian Sea isolated from the Tethyan Ocean in the Miocene (23.0–5.3 million years ago). As a result, two geographically isolated and independent anchovy groups (western and eastern) appeared. Further development of these groups occurred at different rates. Faster rates of evolution of the western group led to the formation of more progressive Black Sea anchovy and Mediterranean anchovy, lower rates of evolution of the eastern group led to the formation of more primitive Sea of Azov anchovy. Only in the modern era, after the last glacial period had ended and the connection of the Black Sea with the Mediterranean Sea had been reconstituted (7–5 thousand years ago) Sea of Azov anchovy and Black Sea anchovy contacted, the process was followed by their subsequent hybridization, i. e. by the appearance of the zone of secondary intergradation. There is a real threat of Sea of Azov anchovy genofund destruction and its “genetic absorption” by Black Sea anchovy in conditions of introgressive hybridization in recent decades associated with salinity increase of the Sea of Azov caused by human economic activity. Nevertheless, the genetic uniqueness of Sea of Azov anchovy persists to the present, first of all, due to ecological isolation mechanisms – seasonal isolation (timing) and biotope isolation (site selection) during the reproductive period. А certain pattern of redistribution in the spawning population composition of Sea of Azov anchovy and Black Sea anchovy females was identified: the share of Sea of Azov anchovy females decreased while the share of Black Sea anchovy females increased. Thus, Sea of Azov anchovy and Black Sea anchovy populations are the “temperature” races adapted to different reproductive temperature conditions: Sea of Azov anchovy – to lower temperature conditions, Black Sea anchovy – to higher ones. In addition to seasonal isolation, the biotopic isolation of Black Sea anchovy and Sea of Azov anchovy was found. Black Sea anchovy prefers to spawn in open areas of the Black Sea with water salinity above 16 ‰: its share is here 55–60 %, whereas in the coastal waters its share does not exceed 5–40 %. Sea of Azov anchovy, on the contrary, in spawning period prevails in the coastal freshened waters with salinity below 15 ‰, with its share here reaching 60–90 %.