Preliminary study of Coilia nasus spawning grounds at Sutong section in the lower reaches of the Yangtze River

The Sutong section in the lower reaches of the Yangtze River is located in the estuary section and close to the estuary of the Yangtze River, which is the migration channel for Coilia nasus. Recent studies have shown that C. nasus adult fish resources still indicate a trend downwards. It’s of great significance to conduct research on C. nasus early resources, which is a supplementary group of adult fish resources. In this study, early resource surveys were conducted in Sutong section for a total of 84 days from May to August each year in 2018-2020 to reveal the spatial and temporal distribution characteristics of C. nasus early resources. A total of 646 fertilized eggs of C. nasus were collected during the survey period, including 328 eggs collected in 2019 accounting for 50.77% of all. The mean abundance of eggs collected in May and June was significantly higher than that in the other months of collection (P<0.05), where as no fertilized eggs were collected in April. The peak of fertilized eggs collected in each year was on 15 May (4.96 ind./100 m³), 16 June (26.48 ind./100 m³) and 24 May (26.11 ind./100 m³), respectively. Pearson correlation analysis showed significant differences in the spatial distribution of the sampled sections (P<0.05), with mean abundance distribution on the left bank (1.42 ind./100 m³) > right bank (0.87 ind./100 m³) > centre of the river (0.69 ind./100 m³). The mean abundance and frequency of fertilized eggs of C. nasus collected in the downstream S1 (waters of Lvhua Town in Chongming Island) and S2 (waters of Xintonghaisha, Haimen City), midstream S5 (waters of Gangzha District in Nantong) and upstream S8 (waters of Changqing Sha Dian Yuan in Rugao) of the Sutong section were significantly higher than those of other collection sections (P<0.05). The analysis of environmental factors showed that the abundance of fertilized eggs of C. nasus was negatively correlated with temperature (P<0.01), and the abundance of fertilized eggs of C. nasus was higher in the range of 21.1-25.3 °C and 6.80-7.41 mg/L of dissolved oxygen. The results showed that May and June were the peak spawning months for C. nasus in Sutong section, and the temperature range of 21.1-25.3 °C and dissolved oxygen range of 6.80-7.41 mg/L were suitable for C. nasus spawning. It is speculated that the waters in Lvhua Town of Chongming Island, Xin Tonghaisha of Haimen, Nantong Gangzha District and Changqing Sha Diangyuan of Rugao are potential spawning grounds for C. nasus.