Species-Specific Response of Fish Health Condition to Nutrient Enrichment in Subtropical Rivers

Fish growth is closely related to environmental variables; however, how nutrient enrichment affects fish growth conditions remains unclear in subtropical polder rivers. We investigated the growth status of six fish species and nutrient concentrations at 40 sites in the subtropical polder rivers of the Lake Chaohu Basin, with the aim of evaluating how the fish condition factor (K) responds to nutrient enrichment across different feeding groups. We observed species-specific responses of growth conditions to nutrient enrichment. The omni-zooplanktivorous Hemiculter leucisculus and zooplanktivorous Toxabramis swinhonis exhibit better growth in nutrient-enriched rivers, with their K values showing a significant positive correlation with both total nitrogen (TN) and total phosphorus concentrations. However, the K values of the omni-benthivorous Carassius gibelio and Pseudobrama simoni exhibited no significant correlation with nutrient enrichment. Interestingly, the condition factor of the piscivorous Culter species also increased with rising TN levels. The growth patterns of H. leucisculus and C. gibelio were positive allometric, where the fish body weight increased at a faster rate than length. T. swinhonis and P. simoni showed isometric growth, in which both the body weight and length of them increased at approximately the same rates. These findings highlight the importance of considering fish functional traits (e.g., feeding guilds) when assessing the ecological impacts of eutrophication and provide insights for the management of subtropical polder river ecosystems under nutrient enrichment, such as predicting changes in fish community structure and developing targeted conservation strategies.