Research advances in diversity and conservation science of freshwater fish in China

In China, as elsewhere, freshwater fishes are facing considerable threats due to climate change and anthropogenic activities, including habitat modification, water pollution, over-fishing, and non-native fish introductions (Best, 2019; Feio et al., 2023). As a result, the taxonomic, genetic, and functional diversities of freshwater fishes in China have been negatively impacted. To fulfill China's Biodiversity Conservation Strategy and Action Plan (2011–2030), it is important to have a comprehensive understanding of freshwater fishes and aquatic ecosystems in China, especially with regard to threatened species and protected areas. This is essential to inform management strategies to mitigate impacts on ecosystems critical to fish fauna and to protect and conserve endemic and threatened native fish species. This special issue includes two papers dealing with the fish diversity at a national and regional scale, four papers regarding new taxa or new records of fishes, five studies about the life history and the general biology of fishes, and two papers focused on technologies in fisheries and fish conservation. The third Red List assessment of Chinese freshwater fishes was conducted recently with the results published by Zhang and Cao (2021); the checklist was updated but with limited case studies and no recommendations for the conservation of freshwater fishes. Cao et al. (2024) provided an overview of species diversity and their status based on the work of Zhang and Cao (2021), and they highlighted the challenges in attempts to conserve Chinese freshwater fishes using case studies and suggestions to improve the conservation of freshwater ecosystems, thereby fishes. Functional biodiversity and the impact of exotic species in local ecosystems have rarely been evaluated thoroughly in China at a riverine scale. Lin et al. (2024) conducted a case study using a schemed mechanistic model to reveal the impacts of exotic species on functional diversity in the Minjiang River, the largest river in southeastern China, and the study suggested that exotic species altered the endemic fish community significantly if they accounted for 2% of the total species in the ecosystem. About 1591 freshwater fish species were recorded in China up until the end of 2020, and about 20 new species have been described annually (Cao et al., 2024). However, both numbers must be an underestimate, especially for the rare endemic species in upstream areas of rivers that are the most remote. This special issue includes four papers describing new records, new taxa, and phylogeny in China (Guo, Cao, and Zhang (2024); Hu et al., 2024; Li et al., 2024; Luo et al., 2024). Guo, Cao, and Zhang (2024) described two new Leptobotia spp. in the upper Qiantang-Jiang River where the fish diversity is very much underestimated. Morphological and molecular approaches were used to describe a new cave species, Sinocyclocheilus xingyiensis in the southwest part of China (Luo et al., 2024), and to confirm a new record of Schizothorax richardsonii on the southern slope of the Central Himalayas where fish surveys have rarely been conducted (Li et al., 2024). Hu et al. (2024) highlighted that the quaternary climatic oscillation, in conjunction with ecological traits and lineage diversification time, shaped contemporary phylogenetic patterns of stone loaches in the Himalayas. Five studies in this special issue focus on the biology and ecotoxicology of fishes in China. Life-history patterns of Japanese eels Anguilla japonica remain poorly understood. Shuai et al. (2024) conducted a 6-year study, expanding on the knowledge of life-history patterns and habitat preferences of Japanese eels in the Pearl River, China, with implications for the conservation of the eel habitat. Qin et al. (2024) reported on the interpopulation and ontogenetic trophic niche shifts of a mangrove fish predator Acentrogobius viridipunctatus and highlighted the important role of trophic plasticity in dynamic ecosystems. The environmental and biological condition of the Chang-Jiang River is of great concern in China. Yimer et al. (2024) studied a commercially important species Coilia brachygnathus in the Lake Honghu and found that it had a short life span, early sexual maturity, and a relatively fast growth rate, which had important implications for the management of fish resources in rivers and lakes situated in the middle and lower reaches of the Chang-Jiang River. He et al. (2024) investigated the key life-history traits of a critically endangered freshwater fish Ptychidio jordani, which had not been known previously and was essential for conservation strategy and policy development. Guo, Chu, et al. (2024) provided the safe concentrations of the common biocides, including copper, zinc, and methylene blue to treat the disease of the threatened fish species that are endemic to China such as Brachymystax tsinlingensis, which benefited from resource protection and restoration measures. New technologies are important to develop sustainable fisheries and effective fish conservation strategies. Shen et al. (2024) developed an automatic image-processing programme to automatically track and count fish using a technique that fuses K-nearest neighbor background subtraction with DeepSort target tracking, which could replace time-consuming semi-automatic approaches and improve the efficiency of imaging sonar data processing. To mitigate the negative effect of dam construction, Song and Xie (2024) investigated the effect of substratum roughening on the swimming capacity, oxygen consumption, and behavior of Schizothorax wangchiachii, and the results indicated that swimming performance was improved in the presence of a rough substratum with results in lower energetic costs to the fish.

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