Characterisation of Urocystolithiasis in Farmed Atlantic Salmon ( <i>Salmo salar</i> L.): Histopathological Features and Pathophysiological Considerations

While nephrocalcinosis has received considerable attention in Atlantic salmon aquaculture, mineralisation of the urinary bladder remains largely overlooked. Obstruction of the urinary bladder, well recognised in terrestrials as a potentially fatal condition, is often missed in fish due to its exclusion from routine histological assessments. An increasing number of field cases, colloquially referred to as 'urinary plugs', are now being observed, frequently occurring without or with only slight, nephron mineralisation. This study describes histopathological changes associated with bladder mineral deposits, urocystolithiasis, in farmed Atlantic salmon, including obstruction, urothelial degeneration and intra-renal lesions. The findings highlight urocystolithiasis as an underdiagnosed condition with potential fish health and welfare implications. Functional and histopathological studies targeting the entire urinary system are warranted to elucidate its pathophysiological nature and to support the development of preventative and therapeutic approaches.