Black-striped pipefish Syngnathus abaster Risso, 1827 (Actinopterygii: Syngnathidae) has successfully spread in freshwater ecosystems of Ponto-Caspian basin. Monitoring studies show a steady increase in the number and biomass of pipefish in waters of Dnipro region. It is highly adaptable and breeds in a fresh water. This fact indicates the physiological plasticity of the reproductive system and ability to adapt to new environmental conditions. In the Zaporizke Reservoir pipefish spawns at a

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... erature of 18.0–22.0°C in the coastal area of the reservoir. Pipefish with a body length of 8.9 to 12.5 cm had gonads with weight of about 0.13 g, and the number of eggs ranged from 26 to 68. The maximum fertility had female 155 mm long and it reached 69 eggs. Size of eggs reached about 1.2±0.05 mm. The histological analysis of the gonads of pipefish and research of histometric measurements of eggs of fish at various stages of development was first conducted. In the period before pipefish spawning, there were eggs in different stages of maturity in the ovaries: oocyte in the phase of primary follicle «С», oocyte in the phase of vacuolation «D3», oocyte in the phase of trophoplasmatic growth «Е», beginning phase of mature follicle «F». Research of pipefish reproduction has important theoretical and practical significance for resolving issues with the spread of this species and its adaptation to new conditions.