Stable nitrogen and carbon isotope ratios of fish tissues, in combination with stomach content analysis, provide insights into how energy flows through marine food webs. Using these techniques, we investigated the diets and trophic niches of four important fish species of the Yellow Sea: the Spotty‐bellied Greenling Hexagrammos agrammus, Fat Greenling H. otakii, Korean Rockfish Sebastes schlegelii, and Japanese Seaperch Lateolabrax japonicus. The stomach content analysis revealed that Spotty‐bellied Greenlings mainly fed on crabs and fish; Fat Greenlings mainly preyed on fish, crabs, and shrimps; Korean Rockfish primarily consumed fish and shrimps; and Japanese Seaperch mainly fed on fish. Korean Rockfish had the highest dietary breadth index among the four fish species. Dietary composition of the four species did not overlap significantly, as indicated by Schoener's index. Prey category composition varied across fish species and differed between species pairs (except Korean Rockfish and Japanese Seaperch), as suggested by permutational ANOVA tests. The stable isotope analysis suggested that Japanese Seaperch had significantly higher δ¹³C and δ¹⁵N and had a significantly different centroid position, both of which indicated a higher trophic position. The corrected standard ellipse area on the δ¹⁵N–δ¹³C (‰) biplot varied in size among species and significantly overlapped in the three rockfishes, with the largest overlap area observed between Fat Greenlings and Korean Rockfish. The results, in general, suggested that the three rockfish species, especially Korean Rockfish and Fat Greenlings, had similar trophic niches, but they were partitioned by prey resources according to the stomach content composition. Our study provides novel insights into the dietary niches and trophic dynamics of these four exploited species, which could help to improve ecosystem‐based fisheries management in the Yellow Sea.

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.

This study aimed to determine the age, growth, mortality, and population structure of the economically important cyprinid silver carpHypophthalmichthys molitrix(Valenciennes, 1844) in the middle and lower reaches of the Pearl River. A total of 297 silver carp were sampled quarterly from the catches of gillnet fishermen, at six sites, between June 2019 and September 2020. Standard length of the specimens ranged from 130 to 585 mm, and body weight ranged from 45.5 to 3930 g. The length–weight relationship parameterbvalues reached 3.015. Age was determined through examination and measurements of fish scales, and the age composition of the sampled silver carp varied from 0+ to 4+. Fitting the new data to the von Bertalanffy growth model, we obtained an asymptotic size (L_∞) of 1107 mm,kof 0.135, andt₀of −0.666 for silver carp in this stretch of the river. The calculated growth performance indexϕand estimated longevityt_maxwere 5.22 and 21.56, respectively. The rates of total mortality, natural mortality, and fishing mortality were calculated as 0.4997, 0.1621, and 0.3377, respectively, while the exploitation ratio was evaluated as 0.6757. The overall results confirm overexploitation of this resource in the middle and lower reaches of the Pearl River. It was concluded that this species should be protected from capture until at least 790 mm in standard length, representing an optimal minimum size for capture to benefit conservation of the species and to sustainably develop this valuable fishery.

The Pearl River is a global hotspot of fish biodiversity, yet has the most threatened endemic fish species in China. Since the establishment of the Changzhou Dam in the lower reach, changes in hydrological rhythm have negatively impacted fish downstream of the dam, but their spatiotemporal distribution in response to flood alteration has received little attention. In this study, hydroacoustic surveys were undertaken monthly in 2016 to monitor the distribution and behavior of fish. Fish densities were higher during the water discharge rising stage than during the falling stage, indicating that the fish aggregate during flooding (coefficient of variation [CV] > 100%) and depart after flooding (CV < 100%), especially aggregations of large fish. The target strength (TS) was allocated to two groups as per their frequency distributions, defined as small fish (−55 dB < TS < −40 dB) and large fish (TS > −40 dB). The sizes of both groups were significantly larger during the rising stage when compared to those during the falling stage (p < 0.01). Comparatively more fish were present with a greater average TS, and a substantially greater proportion of large fish was detected during rising stages. Hydrological variation importantly influences fish aggregations, including the numbers and sizes present, with the differences being particularly pronounced between the rising and falling stages. Combined with relevant studies, it is suggested that water releases from the Changzhou Dam should be regulated to satisfy fish spawning and migration demands during the main breeding season.

Understanding the mechanisms that promote coexistence between functionally analogous non-native and native species that share similar prey is important for predicting the ecological consequences of their competitive interactions. Theory predicts that species coexistence is facilitated by stabilizing mechanisms that enhance inter-specific trophic niche partitioning, and/ or the equalizing mechanisms that reduce differences in the average fitness. Inter-specific niche partitioning, influenced by the ecological opportunity in prey resources, also affects intra-specific niche partitioning and individual niche expansion. These theories were tested using the non-native mrigal carp Cirrhinus mrigala and its congener native mud carp C. molitorella that coexist across the Pearl River basin, Southern China, with their trophic interactions analysed using DNA metabarcoding on fish gut contents sampled across the basin in both the dry and wet seasons of 2019 and 2021. The results revealed that the inter-specific trophic niche overlap was lower than their intra-specific trophic niche overlap, suggesting their coexistence was supported by stabilizing mechanisms. The ecological opportunity in prey resources was driving inter-specific niche partitioning and this partitioning then positively influenced intra-specific niche partitioning and individual niche expansion, especially in mrigal carp that had larger body sizes and occupied higher trophic positions at constricted individual niches. Fish body size also affected inter-specific niche partitioning and the relationship between inter- and intra-specific niche partitioning, with positive effects on both in mrigal carp but negative effects on both in mud carp. The results thus provide a mechanistic explanation on how closely related non-native and native species coexist and highlight the importance of dietary plasticity in facilitating the invasion of non-native species.