Infectious diseases are a leading cause of suffering and mortality in farmed rainbow trout. Effective biosecurity and vaccination programmes protect both fish welfare and farm viability.
Disease outbreaks in trout farms cause mass mortality with significant acute suffering before death — haemorrhagic diseases cause internal bleeding, while bacterial infections cause systemic illness, lethargy, and loss of schooling behaviour indicating distress. The vaccine injection process itself causes acute pain and persistent localised inflammation from oil adjuvants, particularly in the peritoneum. However, the welfare benefit of preventing disease outbreaks substantially outweighs the cost of vaccination in most cases. Immersion vaccines, which avoid injection entirely, represent a welfare improvement for species where they are effective. Biosecurity protocols that prevent disease introduction — including UV water treatment, fallowing between production cycles, and careful stock movement controls — prevent outbreaks and their associated welfare harms from occurring at all.