Abstract
Abstract The gills of teleost fish are multifunctional organs involved in respiration, osmoregulation, and immune defense, and are highly sensitive to environmental stressors. This study evaluated the histological and ultrastructural condition of the gill epithelium of common carp (Cyprinus carpio Linnaeus, 1758) inhabiting the middle reaches of the Ile River (Kazakhstan) under chronic anthropogenic pressure. Although hydrochemical parameters remained within permissible limits for fishery waters, all examined specimens (n = 10) exhibited pronounced structural alterations. Histological analysis revealed edema, epithelial hyperplasia, lamellar fusion, necrosis, and desquamation, indicating substantial impairment of gill integrity. Ultrastructural examination demonstrated advanced developmental stages of rodlet cells, mast cell degranulation, and increased activity of mucous and chloride cells, accompanied by mitochondrial alterations and autophagosome formation. Alongside destructive changes, moderate compensatory reactions were observed, reflecting tissue-level adaptation to prolonged environmental stress. These findings demonstrate that gill histopathology provides a sensitive tool for detecting sublethal effects of chronic anthropogenic impact, even when conventional hydrochemical indicators suggest acceptable water quality. The results support the use of cellular and tissue-level biomarkers in freshwater environmental monitoring programs.