Rainbow trout are one of the world's most important farmed fish species. Disease management is central to trout welfare, as outbreaks can cause mass suffering and mortality in intensive production systems.
Disease in farmed trout represents welfare harm across two dimensions: the suffering of individual fish experiencing illness and death, and the systemic conditions that make outbreaks more likely. High stocking densities, optimal production temperatures for growth that also favor pathogen replication, and limited vaccination availability compared to salmon create a disease-prone environment for many trout farms. When outbreaks occur, mass mortality events involve millions of fish experiencing physiological distress before death. Prevention through biosecurity - limiting pathogen entry, maintaining appropriate stocking densities, and developing effective vaccines - is the most welfare-positive approach. The economic case for disease prevention also aligns with the welfare case, making this an area of natural interest convergence.