Integrated physiological and molecular insights into grass carp adaptation to eutrophic environments

In recent years, freshwater fisheries have experienced rapid development worldwide. However, poor water management practices associated with intensive, high-density aquaculture systems may accelerate eutrophication within aquaculture systems—posing a major challenge to the sustainable development of the finfish aquaculture industry. Grass carp ( Ctenopharyngodon idella ), one of the most widely cultivated herbivorous fish species characterized by rapid growth and high environmental adaptability, plays a pivotal role in the field of national aquaculture. To investigate the adaptive mechanisms of grass carp in eutrophic environments, a 30-day controlled culture experiment was conducted using lightly eutrophic water as the baseline rearing environment [Eutrophication levels were defined as follows: Lightly eutrophic: total nitrogen (TN) = 1.00 mg/L, total phosphorus (TP) = 0.100 mg/L; Moderate eutrophic: TN = 2.00–6.00 mg/L, TP = 0.200–0.600 mg/L; Severe eutrophic: TN = 9.00–16.00 mg/L, TP = 0.900–1.300 mg/L]. Two grass carp strains—the common strain and the gynogenetic disease-resistant strain—were reared under controlled eutrophic conditions. Using a multi-faceted analytical approach that included transmission electron microscopy, histological paraffin sectioning, blood smear analysis, quantitative real-time reverse transcription PCR (qRT-PCR), and antioxidant enzyme activity assays, this study revealed that eutrophic conditions induce structural damage to multiple tissues and organs in grass carp. Furthermore, beneficial responses of the antioxidant system to environmental stress and immune activation were observed. Notably, the gynogenetic disease-resistant strain exhibited superior adaptability and more efficient inflammatory regulation than the common strain. Integrated transcriptomic analysis revealed that grass carp mitigate the adverse effects of eutrophication by upregulating metabolic processes and activating immune signaling pathways. Specifically, the peroxisome proliferator-activated receptor (PPAR) and vascular endothelial growth factor (VEGF) signaling pathways were found to function synergistically, thereby promoting the adaptation of grass carp to eutrophic environments. This study establishes a molecular basis for comprehensively elucidating the environmental adaptation mechanisms of fish, providing significant insights for the selective breeding of stress-tolerant aquatic species and the advancement of sustainable freshwater aquaculture practices.