Study on early resources of four major Chinese carps in middle and lower reaches of Pearl River and its relationship with hydrological environment

The four major Chinese carps (FMCC) are the economically important freshwater fish species and crucial germplasm resources in China. However, their populations have been declining continuously due to the impacts of hydraulic engineering, human activities, and extreme weather events. The middle and lower reaches of the Pearl River serve as vital habitats for the FMCC. Investigating the relationship between the spawning activities of FMCC and hydrological factors in this region can provide a scientific basis for the fish resources conservation and ecological regulation in the Pearl River Basin. We applied the Generalized Additive Model (GAM) to analyze the early resources of the FMCC, along with hydrological factors such as average flow, duration of water level rise, and flow rate increase. The data were collected from Zhaoqing and Lubu from 2008 to 2021. The results indicate that the number of spawning batches and the peak value of each batch of the FMCC showed a declining trend with fluctuations. The increase in flow and the duration of water level rise were the most significant factors affecting the quantity of early resources, while the relationship between the quantity of early resources and average flow was not significant. The GAM model's cumulative explanatory rate was 60.4%, with the increase in flow contributing the most to the explained deviation (44.4%). The relative importance of hydrological factors was ranked as: increase in flow>duration of water level rise>average flow. The residuals of the model conformed to a normal distribution (p>0.05). Based on the model results, it is recommended to artificially create a rapid water rise process in the middle and lower reaches of the Pearl River from May to August each year, with the duration of water level rise about 6 d and the average flow controlled at 5 000−15 000 m³·s⁻¹. This can serve as an ecological dispatching target in the middle and lower reaches of the Pearl River to promote the early resources replenishment for the FMCC.