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Fish represent a large part of the taxonomic diversity of vertebrates and are of high commercial value. However, the factors influencing the gut microbiota composition of freshwater fish over large spatial scales remain unclear. Therefore, this study explored gut microbiome diversity in 24 fish species from the Yellow River, which spans over 1500 km across China. The results showed that geographical distance, host phylogeny and diet significantly influenced gut microbial community diversity, whereas sex, body length and body weight had minimal influence. Geographical distance was the primary factor shaping gut microbiota, and dissimilarity in microbial community structure increased with an increase in geographical distance, which was mainly driven by dispersal limitation. The microbial communities were more homogeneous at higher host taxonomic resolutions due to the dominant role of homogeneous selection in community convergence. Phylosymbiosis was observed across all host species, with a stronger pattern in Cypriniformes, which harbour host‐specific microbial taxa. Host diet explained little variation in gut microbiome diversity, although it was significant for all diversity metrics tested. These findings collectively suggest that the geographical and host‐based patterns of fish gut microbiota tend to be shaped by different ecological forces across the Yellow River. The present work provides a robust assessment of multiple factors driving fish gut microbial community assembly and offers insight into the mechanisms underlying shifts in fish gut microbiota in rivers across large spatial scales.

Organochlorine contaminants (OCs) in fish were determined to evaluate the potential risk to humans consuming fish originating in Baiyangdian Lake, North China. Relatively low levels of PCBs, HCHs and DDTs were observed, with mean concentrations ranging from 0.28 to 3.28 ng/g, wet weight. Among various fish species tested, the highest burden of OCs was recorded in northern snakehead (7.39 ng/g, wet weight) and the lowest was in grass carp (2.04 ng/g, wet weight). The hazard ratios (HRs), based on noncancer risk were all less than 1.0, while the HRs based on cancer risk exceeded 1.0 only for PCBs based on the 90th percentile concentration.

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