As a typical river-sea migratory fish, the anadromous migration of
C. nasus is a seasonal reproductive activity [
19].
C. nasus mostly lives in coastal and nearby waters, aggregates in the offshore waters of the Yangtze River estuary during spawning migration [
15,
23]. The increase of water temperature induces the pre-spawning migratory behavior of
C. nasus and promotes its gonadal development and maturation [
27,
28]. The study analyzed the
C. nasus in April-July 2018 of AQ sectionshowed that the main developmental stage of the ovaries was stage Ⅲ, and the ovaries began to appear in June to stage Ⅴ of
C. nasus [
29]. He et al. [
30] investigated and studied
C. nasus in AQ section from April to August 2005, found that the maturity coefficient of ovaries in stage Ⅱ and Ⅲ in April was 1.36% on average, in June, the maturity coefficient of ovaries developing to stages Ⅳ and V reached 6.66%. In this study, in the early stage of migration from March to April, the ovarian development of
C. nasus was dominated by stage I and Ⅱ, the proportion of stage Ⅲ and Ⅳ increased in May. The number of
C. nasus individuals in stage Ⅳ and Ⅴ increased in June and July. Overall, the proportion of ovarian mature individuals and the coefficient of ovarian maturity increased with the passage of time and the increase of water temperature in all water segments. This was consistent with the results of historical studies. Yolk, as an energy-supplying substance for ovarian development in fish, has also attracted much attention in terms of changes in its content. Observation of tissue sections showed that the ovary in stage Ⅱ was dominated by the 2nd time-phase oocytes, which were smaller, and no yolk material was produced, which was consistent with the histological characteristics in stage Ⅱ of
Rhinobio ventralis [
31]. In addition, the ovary in stage Ⅲ was dominated by oocytes in the 3rd temporal phase, which enlarged about onefold and began to show nutrients such as yolk granules and oil droplets, which was consistent with the findings of
Girella leonina [
32], and
Xenocypris microlepis [
33]. Moreover, ovaries in stage Ⅳ were dominated by time-phase 4 oocytes, which were almost full of nutrients such as yolk granules and oil droplets, with yolk granules becoming fuller and the area of oil droplets fused and becoming larger. Similarly, a large number of oil droplets synthesized into oil globules occurred in stage Ⅳ of
Girella leonina [
32]. The results of ovarian development and ovarian maturity coefficients in the present study were consistent with those of previous studies on
C. nasus [
17,
34], reflecting the typical ecological characteristics of
C. nasus in fattening and long-distance reproductive migration in different waters.
In this study, the observation of ovarian tissue sections of stage Ⅳ in CM, TZ and AQ sections revealed no significant difference in oocyte size. However, the proportion of yolk material composition of oocytes was different. Concretely, the ovarian tissue sections of the CM and AQ sections showed that oil droplets occupied about half of the cell area, and yolk granules were slightly smaller than the proportion of oil droplets, which was consistent with the results of stage Ⅳ ovarian tissue of the pond culture of
C. nasus,
Parabramis pekinensis, and
Hippocampus erectus [
18,
35,
36]. In contrast, ovarian tissue sections from the TZ section showed that yolk granules accounted for a larger proportion of oocytes and fewer oil droplets. This suggests that the maturity level of
C. nasus collected in TZ section was lower than that in CM and AQ sections. Further development and maturation of
C. nasus collected in TZ section were needed, probably because the target spawning grounds had not been reached yet, and it was necessary to continue migrating upstream to complete the reproduction of spawning [
18,
37].