Quantitative distribution and lipid reserves of the population of <i>Calanus euxinus</i> (Copepoda) in the Black Sea in late Autumn 2017

Field data on abundance, biomass, age structure and lipid reserves of the population of copepod Сalanus euxinus in deep pelagial and Crimean shelf regions of the Black Sea in November 2017 were analyzed. Abundance and biomass of this species in deep areas (10.2 ± 0.5×103 ind/m2 and 7.3 ± 0.5 g/m–2, respectively) were close to the average annual values for the seasons with moderate development of planktivorous jellyfish populations. Mean abundance and biomass of the copepod were similar to the analogous values for 2016 when the abundance of medusae was twice lower. However, in 2017 the dense aggregations of C. euxinus (12.8 ± 1.0×103 ind/m2 and 9.8 ± 0.5 g/m–2) were found in the central parts of cyclonic circulations while in 2016 they were recorded more frequently at the peripheries of the cyclonic gyres. The structure of C. euxinus population also changed. In November 2017 the percentage of I–IV copepodites, females and males increased whilst the share of V copepodites reduced. Probably, these changes may be a result of later sampling period of 2017 when the C. euxinus population is ready to active reproduction. Sufficient lipid amounts in V copepodites, decreased oil sac volumes in females and increased abundance of males in the anticyclonic regions indicate more favorite trophic conditions for this part of the C. euxinus population and its higher maturity.