Understanding fish migration patterns and habitat use is essential for fisheries management. We conducted an acoustic fine-scale tracking experiment from March to June 2017 to determine the common carp movement pattern and habitat use in the downstream area of the Pearl River. The eight tagged common carp were detected for 39.38 ± 29.57 d, on average, with a total average detection period of 42.12 ± 28.02 d. A general linear mixed model suggested they were detected more during the night than during the day. Common carp preferred habitats near the sandbar and riparian shallow habitats, as revealed by their movement tracks and habitat selection indexes. The general additive mixed model (GAMM) analysis of their shortest distances to riverbanks and depth indicated that they migrated to shallower riparian habitat more frequently during the night than during the day. The continuous wavelet spectrum further confirmed the significant diel pattern of their vertical movement. Their habitat use was also influenced by the water temperature, water level, and river discharge, as suggested by the GAMM. Our results provide novel information on common carp spatiotemporal movement and habitat use patterns, and have important implications for its native population restoration and the invasive population control/exclusion.

Silver carp (Hypophthalmichthys molitrix) is a world-wide species and its ecosystem level influence is controversial. Therefore, understanding the feeding behaviour of silver carp is important for assessing ecosystem functioning in many freshwater habitats. In this study, we used 18S rRNA sequencing to identify the spatiotemporal taxonomic composition of water and fish gut samples. Significant seasonal and spatial variations were observed in the taxonomic compositions of water and fish gut samples (p < 0.05). Five selected environmental factors (temperature, total phosphorus, chlorophyll a, ammonia, and distance from the first dam downstream) significantly affected the taxonomic composition of water (p < 0.05). Additionally, the temperature was the most important factor affecting seasonal (winter and summer) variations in the prey community composition of water, whereas the distance from the dam was the key factor contributing to spatial differences in the community. Significant differences were observed in the taxonomic composition between the water and fish gut samples (p < 0.05). Feeding selectivity analysis revealed that the selection ratios of the top 20 prey taxa underwent slight spatial and temporal changes, but silver carp always preferred to feed on Peridiniales, unidentified Streptophyta, unidentified Trebouxiophyceae, and unidentified Chlorophyceae. These results indicate that changes in the prey community in the environment affect the taxonomic composition of the fish gut but do not affect feeding preferences.

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