Ripening of the oocyte of the river lamprey (Lampetra fluviatilis L.) after river entry

River lampreys (Lampetra fluviatilis L.) enter rivers for spawning with their gonads in the final stages of maturation, however the oocytes continue to develop until the spawn. This study was undertaken to depict detailed changes in the oocytes in the period approaching the spawn, using metric analysis and to look for atresia to determine if the time period influences the final fecundity. The study was performed on 37 females caught between October and May in the Rega River, north‐western Poland. Mid‐part sections of fixed ovaries were stained with Heidenhain haematoxylin and PAS. Ten cell structures of the oocytes were measured under light microscope with the aid of a computer image analysis programme. In the autumn, when the first lampreys entered the river, the nuclei of the oocytes were in the polar position. With the approaching mating season many oocyte structures changed significantly. Statistically significant (P < 0.001, Mann–Whitney U‐test) was the increase in oocyte size (up to 1.06 × 0.78 mm in morphometric measurement), enlargement of the yolk platelets (f.a. 8.0 × 5.1 μm), elevation of theca over the chorion followed by the accumulation of glycoconjugates (f. a. 29.2 μm), growth of the zona granulosa at the vegetative pole (f. a. 24.2 μm), and increase in the thickness of the chorion at the animal pole (f. a. 11.1 μm). Other statistically significant changes (P < 0.05 and P < 0.01, Mann–Whitney U‐test) included a decrease in the width of the cortical cytoplasm band (f. a. 20.5 μm), decrease in the ratio of the distance between the nucleus and cell membrane and the long axis of the oocyte (f. a. 7.6 μm), and increase in the thickness of the chorion at the vegetative pole (f. a. 6.0 μm). No statistically significant changes in the area of the nucleus and diameter of the cortical alveoles were noted. Beginning with the lamprey entering the river until the spawn, the oocytes undergo significant growth and maturation. In this period the fecundity of the lamprey decreased only slightly, as atresia was observed sporadically in the ovaries.