Modern state of the population of the European anchovy Engraulis encrasicolus (L.) (Pisces: Engraulidae) wintering off the coast of the Eastern Crimea and the North Caucasus

European anchovy (Engraulis encrasicolus L.) is one of the most abundant fish species in the Sea of Azov – Black Sea basin. Anchovy is the main commercial species in the region. In recent decades the share of anchovy catch has reached 80–85 %. The species is represented by two forms – Black Sea anchovy and Sea of Azov anchovy. One of the most important aims of modern research is to assess the state of anchovy population and to forecast its possible changes under the influence of various natural and anthropogenic factors. The results of studying the long-term (2010/11–2017/18) dynamics of the length-age structure and the intraspecific composition of anchovy wintering off the coast of the Eastern Crimea and the North Caucasus are presented. Totally 138 fish samples from trawl catches of fishing vessels were studied. Standard length of 44 202 specimens was measured with accuracy of 0.1 cm. The age of 1162 specimens was determined. Length-age key was compiled. The intraspecific identification of European anchovy was determined using otolith index (method of Skazkina). As indicators of the length-age structure, the average length and average age, distribution (number ratio of representatives of different length groups and age classes) were studied. Positive trends of the average length and the average age of anchovy were found, reflecting an increase of the proportion of large (> 9.5 cm) individuals in the population – three yearlings (2+) and four yearlings (3+). In the long-term plan the average length of the anchovy increased from 8.06 to 9.09 cm. At the same time the relative number of small (< 7.5 cm) individuals decreased almost 4 times (from 22.5 to 5.7 %), and the share of large individuals increased almost 6 times (from 5.7 to 33 %). The average age increased from 1.64 to 1.98 year. On the one hand, it was due to a nearly 3-fold reduction (from 7.1 to 2.6 %) in the relative number of young-of-the-year individuals (0+) and 1.5-fold reduction (from 72.6 to 47.7 %) of yearlings (1+). On the other hand, it was due to 2.4- and 4.3-fold increase in the relative number of three yearlings (2+) and four yearlings (3+), respectively. The appearance of abundant year class in 2013, 2014 and 2015 was the immediate reason of these changes. The intraspecific determination of the anchovy was found, Sea of Azov and Black Sea forms were identified. In the long-term plan their quantitative redistribution was shown. So, in 2010/11–2013/14 Sea of Azov form dominated in mixed wintering aggregations. Its average share was 58.5 %, with the share varying from 55 to 63 %. Average share of Black Sea form did not exceed 41.5 %, with the share varying from 37 to 45 %. However, in 2014/15 the number ratio of Sea of Azov and Black Sea forms changed to the opposite – with Black Sea anchovy dominating. Its share increased to 53 %, and in subsequent years ranged from 52 to 63 %, with average value of 56 %. The results obtained are in full accordance with the hypothesis of intraspecific structure reorganization of anchovy because of the change in climatic conditions (general regional warming). Earlier we suggested this hypothesis based on the results of research of long-term (1999–2010) structure dynamics of anchovy wintering off the western coast of Crimea. According to this hypothesis, in 1999–2004 Sea of Azov anchovy dominated in wintering aggregations with average share 66.7 % of the total number, with the share varying from 56 to 87 % in different years. The average share of Black Sea anchovy in that period did not exceed 33.3 %, with the share varying from 13 to 44 % in different years. In 2005 the ratio of two forms changed to the opposite. Black Sea anchovy dominated. In 2005–2010 its share was 76.7 % on average, varying in different years from 57 to 88 %. Apparently, the reorganization of the anchovy intraspecific structure should be considered as ecological adaptation of this species, ensuring more perfect adaptability to changing environmental conditions, in particular, to the water temperature. Sea of Azov anchovy and Black Sea anchovy are “temperature races”. Sea of Azov anchovy is resistant to lower reproductive temperature, Black Sea anchovy – to higher one. From the standpoint of the occurred changes in the length-age structure, the current state of anchovy population can be considered as quite good. However, taking into account the sharp decline of relative number of young-of-the-year individuals (0+) in 2016 and 2017, significant rejuvenescence of the population, and consequently anchovy length decrease should be expected in the nearest future.