Cage-Farming Causes Histopathological Alterations in the Renal Tissues of the Rainbow Trout Oncorhynchus mykiss (Walbaum, 1792).

Fish are widely recognized as effective bioindicators in ecotoxicological studies due to their repeated exposure to aquatic pollutants that accumulate in metabolically active organs, often leading to histopathological changes. In aquaculture, cage-farmed fish experience continuous environmental and culture-related stress, which can affect renal integrity. The kidney, a central osmoregulatory organ, is particularly sensitive to such conditions. Renal tissues were collected from different growth stages of cage-farmed rainbow trout. Hematoxylin and eosin staining was performed to detect morphological alterations, while transmission electron microscopy was used to assess cellular damage at the ultrastructural level. The expression of fibronectin and caspase-3, markers of extracellular matrix remodeling and apoptosis, respectively, was also evaluated. TEM examination showed pronounced alterations in both the glomeruli and renal tubules, accompanied by increased expression of fibronectin and caspase-3, indicating ongoing tissue remodeling and cellular stress. This study demonstrates that cage-farmed rainbow trout exhibit progressive ultrastructural kidney alterations that appear to be associated with environmental confinement, nutritional practices, and prophylactic treatments. These conditions collectively contribute to renal stress and the onset of nephropathic changes in aquaculture settings. Further research should focus on molecular marker expression to better understand renal adaptation and injury progression under intensive farming conditions.