Objective To explore the gonadal development patterns and reproductive characteristics of Coilia nasus in the Dayang River, thereby providing a scientific basis for its artificial breeding and resource enhancement and protection.

Methods From April to October 2020, five wild male and female C. nasus were collected monthly from the Dayang River (N39°55′8″, E123°38′53″). The gonadal tissues were dissected and fixed in Bouin’s solution, dehydrated in gradient alcohol, embedded in paraffin, and continuously sectioned (with a thickness of 6 μm). Then, they were observed under a Nikon microscope, and images were collected and measured using CaseViewer software. The gonadosomatic index (GSI) was calculated. The development stages of oocytes and testes were classified according to existing standards, and the data were statistically analyzed using SPSS 17.0 software.

Results The ovary development of C. nasus in the Dayang River can be divided into six stages, and the oocyte development can be divided into five phases. The ovary development type is partially synchronous, belonging to the single-batch spawning type. The testis development is divided into six stages, and it is a fully mature lobular-type testis. The peak maturity of the ovaries of sexually mature C. nasus in the Dayang River Basin occurs from June to July each year, while that of the testes occurs from May to July. The body length of C. nasus shows no significant change in different months, and the body weights of both male and female fish are the lowest in April. The GSI of C. nasus exhibits variability across different months, with notably elevated indices in male fish from May to July and in female fish from June to July, compared to the other months. In addition, the gonadal development of male C. nasus is slightly earlier than that of female fish.

Conclusion Compared with the C. nasus in southern regions, the peak period of gonadal maturation of C. nasus in the Dayang River Basin is delayed and lasts for a shorter time. However, the gonadosomatic index tends to be marginally higher during the peak of maturity. Northern regions should make full use of the period from June to July to carry out artificial breeding of C. nasus, so as to improve the fertilization efficiency. At the same time, the capture of wild C. nasus in other months should be reduced to better protect the wild C. nasus resources.